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LIFE OF BIRDS

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THE STRUCTURE

A N I >

LIFE OF BIRDS

PA'

F. W. HEADLEY, M.A., F.Z.S.

ASSISTANT MASTER AT HAILEYBVRY COLLEGE

WITH SE VENTY-EIGHT ILL USTRA TIONS

Hotitron

MAC MI LEAN AND CO.

AND NEW YORK
1895

The Right of Translation and Reproduction is Reserved

Richard Clay and Sons, Limited,
london and bungay.

TO

F. E. BEDDARD, F.R.S.

THIS BOOK
IS GRATEFULLY DEDICATED

PREFACE

The aim of this book is an ambitious one. It
attempts to give good evidence of the development
of birds from reptilian ancestors, to show what
modifications in their anatomy have accompanied
their advance to a more vigorous life, and, after ex-
plaining, as far as possible, their physiology, to make
clear the main principles of their noble accomplish-
ment, flight, the visible proof and expression of their
high vitality. After this it deals, principally, with
the subjects of colour and song, instinct and reason,
migration, and the principles of classification, and,
lastly, gives some hints as to the best methods of
studying birds. The field is a wide one. but this is
not in every way a drawback. Specialisation has its
evils as well as its advantages. Many of our best
ornithologists write admirably of the life and habits
of birds, but leave out of sight their anatomy and
physiology. Thus they separate things which in
nature are mutually dependent. The anatomists
and physiologists, on their side, limit themselves
sternly to their own departments. Their works,

x PREFACE

too, are often over-elaborate for those to whom
ornithology is not the main business of life. Many-
people, however devoted to the subject, have not
the time for the proper study of Dr. Gadow's great
work, which, moreover, being written in German, is
a sealed book to numbers of English readers. For
the study of bird anatomy they are often thrown
back upon the good but insufficient treatises written
for the use of medical students. Natural History
books, describing the lives of birds, are many, and,
not a few of them, first-rate, though, curiously, there
was not, to my knowledge, any English book in
which a general review of the known facts and of
the problems of migration had been successfully un-
dertaken, till the appearance of the second volume
of Professor Newton's Dictionary of Birds, containing
a very able article on the subject. There are several
books in English treating of flight. Of these
Professor Marey's Animal Mechanism seems to me
the best. But some of his most instructive experi-
ments have been made since its publication, and an
account of them is to be found only in his later and
larger work, Le Vol des Oiseaux. It has been my
wish to bring the work of the anatomist and physio-
logist into connection with that of the student of
flight and that of the outdoor naturalist. With this

o

object I have made some investigations and observa-
tions which, I believe, may lay claim to originality,
and I have consulted the best English, French, and
German books upon ornithology, as well as a number

PREFACE xi

of papers to be found in the Proceedings and Trans-
actions of learned societies. Wherever it has been
possible, I have verified what I have learnt from
books by the methods explained in the concluding
chapter. Discussions with mathematicians, in par-
ticular with Mr. T. J. Bowlker and Mr. R. C. Gilson,
to whom my most cordial thanks are due, have
greatly helped me in dealing with the difficult
problems of flight. Mr. A. H. Macpherson has
very kindly put at my service the results of some
of his observations on the song and on the migration
of birds. I have also to thank Mr. F. E. Beddard,
without whose encouragement I should not have
begun this work. In important cases I have
mentioned in footnotes the source to which I am
indebted for facts or theories, and at the end of each
chapter I have given a list of some of the best books
and papers on the subject treated of. Those of my
readers who wish to proceed to a more special study
of any of the different branches of ornithology will,
I hope, find these references an assistance.

On many parts of the subject I have delivered
lectures to the Hailevbury Natural Science Society.
In this book I have followed much the same lines, in
the hope that it may prove useful to lovers of birds
here and elsewhere. Technical terms, except such as
are unavoidable, I have dispensed with, and, as far as
possible, I have explained difficulties, never, inten-
tionally, wrapped them in the obscurity of big words.
There is only one species of ornithologists with whom

PREFACE

I have no sympathy — the species represented by a
boy famous in fiction, whose fondness for birds
always led him to throw stones at them. It includes
many varieties and sub-varieties, but all its repre-
sentatives are either random slaughterers or cold-
blooded exterminators. I would urge my readers
to oppose them by joining the Society for the Pro-
tection of Birds}

The illustrations are, nearly all of them, by Mr.
Prendergast Parker, who has taken great pains to
make them accurate and really illustrative. Professor
Howes has very kindly watched their progress and in a
good many cases provided the specimens from which
they were drawn. I am much indebted to him for
this great assistance. Dr. Hans Gadow, Mr. W. B.
Hardy, Mr. Gilson, and Mr. Macpherson have been
very kind in reading proof-sheets, and I wish most
warmly to thank them for the great service they
have thus rendered me.

1 For particulars about the Society address the Hon,
Secretary, Mrs. F. E. Lemon Hillcrest, Redhill, Surrey.

CONTENTS

CHAPTER I.

PAGE

Introductory l

CHAPTER II

The Skeleton of Bird and Reptile 6

The fore limb— The breastbone and the bones that meet at
the shoulder joint — The ribs— The hind limb— The skull
— The vertebral column.

CHAPTER III.

Evidence of Relationship to Reptiles

CHAPTER IV.
Connecting Links 4*

CHAPTER V.
The Process of Change from a Reptile to a Bird .... 53

CONTENTS

CHAPTER VI.

PAGE

Form and function 60

Digestive apparatus — The heart and circulation — The valves
of the heart— The blood — Breathing apparatus and pneu-
matic bones — The process of breathing — Lungs of lower
vertebrate animals — Regulation of temperature — Problems
connected with the hollow bones of birds — The kidneys
— The nerves — The brain — The eye — The ear — The organ
of voice — Muscles and tendons — Bones — Ligaments —
Feathers, their structure and development — Varieties of
contour feathers — Feather tracts — Moulting — Change of
colour without moulting — Spurs — The beak — The foot-
Perching— Swimming.

CHAPTER VII.

Flight 173

The wings as levers, the air as fulcrum — Horizontal flight — The
bird in motion, support given by the air — The general shape
of the bird — Passive machinery — Active machinery — The
spreading of the wing — Active and passive machinery
(summary) — Weight of the breast muscles — Movements of
the wing partly due to the action of the air — The tail —
Rate of stroke — Phases of the stroke — Figures described
by the different parts of the wing — The bird's trajectory —
Long distance flight — Upward flight — Horizontal and
gliding flight — Flight in troops — Wind and flight — Rising
with the help of the wind— Soaring — Steering — Stopping,
use of the bastard wing — Force exerted in flight — Propor-
tion of wing area to weight in large and small birds —
Velocity — A note on flying machines — Conclusion.

CHAPTER VIII.
The Bird Within the Egg 275

CHAPTER IX.
Youth, Maturity and Age 2S6

CONTENTS

CHAPTER X.

PAGF.

Bird Population 294

CHAPTER XI.
Colour, Song and Allied Phenomena 297

' Nature of colours — Objective colours — Subjective colours —
Variety of colours in the same bird — Protective coloration.
— Sexual coloration, song, antics, combats — Theories to
explain these phenomena — Summary

CHAPTER XII.
Coloration of Eggs 3 21

CHAPTER XIII.
Instinct and Reason ■ . . . 327

Song — Nest-building— The cuckoo instinct — Piracy — The
death-feigning instinct— Origin of instincts.

CHAPTER XIV.

Migration 347

The distances covered— The east and west migration— The
return route — Return to the same spot — The time occupied
in migration — The beam wind theory— Wings shaped for
long flight— From far south to far north— The height at
which migrants fly— Order of departure— Partial migrants
—The nesting-places of migrants— The cause of migration
—How migrants find their way— Exceptional migrations
— Stray wanderers— A note on geographical distribution.

xvi CONTENTS

CHAPTER XV.

PAGE

Classification 379

CHAPTER XVI.
The History of the Ostrich . ... 391

CHAPTER XVII.
Outdoor and Indoor Ornithology 394

INDEX 407

LIST OF ILLUSTRATIONS

fig. PAGK

Gulls on the Wing Frontispiece

i. Skeleton of Hatteria Lizard 7

2. Skeleton of Fowl 8

3. Tibia and Fibula of Fowl 9

4. Hand of (a) Lizard, (b) Chick 10

5. (a) Sternum of Iguana, {6) Coracoid, Scapula, and

Clavicle of Fowl 13

6. Hind Foot of Hatteria Lizard *7

7. (a) Two Vertebrae of Snake, {b) Two Neck-Verte-

brae of Eagle 2 3

8. Pelvis of {a) Lizard, (b) Bird 27

9. Skull of Rhea viewed from below 3 2

10. Side View of Skull of Rhea 33

11. Part of Vertebral Column of Rabbit; showing

Epiphyses 33

12. Coracoid of Rhea with Rudimentary Precoracoid 34

13. Head of Arch/eopteryx 35

14. Wing of Arch/eopteryx 35

15. Tail of Arch/eopteryx 3 6

16. Vertebra of Hatteria Lizard 3 8

17. (1) Pterodactyl, (2) Wing of Pterodactyl .... 44, 45

18. Pterodactyl restored 47

19. Dinosaur 5 1

b

xviii LIST OF ILLUSTRATIONS

FIG PAGE

20. Diagram illustrating Circulation of Blood .... 70

21. (a) Bird's Heart, (b) Human Heart, opened to show

Valves 73

22. {a) and (b) Red and White Corpuscles of Man, (c)

and (d) Red Corpuscles of Humming Bird and
Ostrich, (e) Amozb^e 75

23. Air-sacks— Diagram 80

24. Section of (a) Femur of Ostrich, (b) of Skull of

Carinate Bird 82

25 Diaphragm of Duck 85

26. Diagram illustrating Process of Breathing .... 87

27. Do. do. do. .... 91

28. Humerus of (p) Skua, (b) Hornbill, (c) Eagle .... 111

29. Two Cubes 113

30. Brain of Bird 118

31. Eye of (a) Man, (b) Typical Bird, (<■) Owl 122

32. Section of Retina of Bird 125

33. (a) Human Ear, (b) Ear of Owl, (c) of Thrush ... 132

34. Syrinx of Raven 137

35. Convolutions of Crane's Trachea 140

36. Striated and Unstriated Muscle 142

37. (a) Feather carrying Nestling Down-feather, (b)

Nestling "Down" of Thrush, (c) of Pigeon, (d)
Thread Feather 145

38. Contour Feathers, (a) Plumelike, (b) Flight-Feather 147

39. Barbules and Barbicels to illustrate interlock-

ing 148

40. Cassowary's Feather 150

41. Illustration of Development of Feather 151

42. Beak of (i) Falcon, (2) Duck 162

43. Foot of (i) Woodpecker, (2) Grebe . . . . • .... 165

44. Flexor Tendons of Toes of {a) Fowl, {b) Passerine

Bird 167

LIST OF ILLUSTRATIONS

xix

FIG. PAGE

45. Ambiens Muscle 169

46. Diagram illustrating the Velocity of the Wing's

Descent 177

47. Diagram illustrating Combined Action of Wings 180

48. Parallelogram of Forces 181

49. Boat sailing at an Angle to the Wind — Diagram . 182

50. Diagram illustrating the Action of the Air upon

the Wing 183

51. Diagram explanatory of the Amount of Support

afforded by the Air to a Bird in Motion . . . 187

52. Birds Gliding : (a) Tern with Wings partly flexed ;

(6) Gull with Wings outstretched 191

53. A Paper Toy illustrative of Gliding Flight . . . 192

54. Wing, showing Elastic Ligaments and the Muscle

that rotates the Secondary Feathers 199

55. Primary Wing-feather of Heron, showing Narrow

Outer Web 203

56. Humerus, showing Attachment of Important Flight-

muscles 204

57. Gulls Flying, showing various Positions of Wing 218

58. Do. do. do. 218

59. Gulls Flying 219

60. Figure described by Tip of Crow's Wing 221

61. Gull Flying— to illustrate the Question of Tra-

jectory 224

62. Breastbone of (a) Frigate Bird, {&) Duck 225

63. Flight in Troops • 230

64. Diagram to illustrate Flight at Right Angles to

the Wind 241

65. Pigeons Flying — to show the Use of the Bastard

Wing 254

66. An Egg and its Contents 277

67. Embryo Chick, Sixty-eight Hours Old 280

xx LIST OF ILLUSTRATIONS

FIG. PAGE

68. (a) Transverse Section through Embryo during

Third Day, (6) Diagram of the Circulation of
the Yolk-sack 28 1

69. Hyoid Bone of Crow 282

70. Diagrams of Aortic Arches {a) on Third Day, (b) on

Fifth or Sixth Day 283

71. Diagram showing Aortic Arches in (a) Lizard, {b)

Bird, (c) Man 284

72. Wing of Hoatzin (a) Young, (/>) Mature 289

73. Cone characteristic of the Structure of Blue

Feathers 299

74. Snipe's Tail-feather 309

75. Sternum of Ostrich 383

76. Spinal Feather Tracts of Snipe and Blackbird . . 384

77. Albatrosses on Nest 399

THE

STRUCTURE AND LIFE OF BIRDS

CHAPTER I

INTRODUCTORY

A BIRD seems to have more life in him than any
other living creature. A Swift will outpace the
fastest racehorse. Migratory birds arrive unexhausted
after a flight of hundreds of miles. An Ostrich will
leave behind the horseman who pursues him. The
Penguin swims as if water were the native element of
birds. And this vitality shows itself in many other
ways, notably in the brilliant colours of the plumage,
as in the Bird of Paradise or the Peacock. Sometimes,
as in the Argus Pheasant, there is great richness of
colouring without any gaudiness. Even when there
is neither brilliancy nor richness of hue, but the
plumage is of the very soberest, there is often such a
brightness about the eye and the general air of our
birds that their plainness passes unnoticed. Their
leading characteristic is not their dull colouring but

3R B

2 THE STRUCTURE AND LIFE OF BIRDS CHAP.

their grace and sprightliness. Not even the most
dazzling and ethereal of Humming-birds surpasses
the Wheatear in beauty. High spirits are another
sign of vitality in most if not in all birds. The Thrush
will sing by the hour together for pure jollity. With
our English birds this is the common way of giving
expression to hilarity. Some foreign species hold
most elaborate dances, and in all lands the fights
among the cock birds in spring are signs of exuber-
ant life. To find food and meet the actual needs
of the day seems easy enough, so that there is a
large surplus of energy to devote to pleasure or
rivalry.

There is nothing in nature more wonderful than the
instincts of birds — for the present we must use this
much-debated word without explanation— nothing,
perhaps, that presents more interesting problems.
Take, for instance, the migratory instinct. The
Swallow travels to the south of Africa to spend the
winter and returns in spring to build her nest, often
in the very chimney where she reared her brood the
previous year.

The nests are in many ways interesting, from their
beauty, from their wonderful variety — birds belonging
to the same family often building quite differently — and
from the fact that young birds have to build their first
nest without any instruction. Take again the extra-
ordinary instinct of the Cuckoo, and the still more
extraordinary instinct of her infant progeny. Then,
too, there is the death-feigning and wound-feigning
instinct. All these, however much they may be
studied, can never lose their interest

I INTRODUCTORY 3

The bodies and brains of birds arc suited to their
full and varied life. Their circulation is rapid, their
temperature higher than that of mammals, their lungs,
though small, are probably the most efficient in the
animal kingdom, their bones and their muscles have
been adapted for purposes of flight, running, or swim-
ming, they have brains that are, comparatively
speaking, highly developed.

And yet the bird is a near relation of the lizard.
He is descended, unless nearly all our great authori-
ties are at fault, though not from any existing reptile,
yet from ancestors that were definitely reptilian.
And yet how enormous is the difference between
these descendants from the same stock ! For the two
lines must meet if we trace them upward far enough.

A lizard is limited to earth, and even there his gait
is an undignified shuffle. True, he slips with un-
surpassed nimbleness into his hiding-place on a hot
day when the sun has warmed his sluggish blood.
But though he be the fastest of lizards, many a bird of
no great size is a better runner, and could, without
having recourse to the magic of wings, easily distance
him in a race of more than a few yards. His forehead
is low, hardly rising above his nose, showing, if
other evidence were wanting, that his intellect is feeble.
Indeed, in this respect, he is hardly above the boa-
constrictor who mistakes his blanket for a rabbit
and swallows it. His blood is cold, and when the
thermometer sinks a little below the freezing-point
he torpifies. In the winter he merely exists, while the
bird lives. He eats but little and digests slowly —
a sign of the sluggishness of his whole life. Both

B 2

4 THE STRUCTURE AND LIFE OF BIRDS chap

in body and in brain he presents a striking contrast
to the bird. But the evidence of anatomy is irresist-
ible. From lizard-like ancestors the bird is de-
scended. A reptilian fore limb has been modified and
adapted for flight, two of the five fingers having
disappeared during the process, and one of the re-
maining three having shrunk almost to a mere
rudiment. The breast bone has attained wonderful
dimensions for the attachment of the muscles of
flight. The hind leg has been much strengthened so
as to make the quadruped a biped and keep the wings
from dragging in the mire. The back has been
stiffened, since an approach to rigidity is required
for flight, and, perhaps still more, for the nearly
horizontal carriage of the body in walking. A long
neck, snakelike but more supple than a snake, brings
the ground within easy reach of the longest-legged of
the race. The scales have become feathers. The
part of the brain in which the higher faculties reside
has grown, so that the forehead rises high. The eye
has become large and has acquired a wonderful keen-
ness of sight. The three-chambered reptile's heart
has become a four-chambered heart, and untold advan-
tages have thereby been gained. The lungs have not
been enlarged, but their effectiveness has been many
times multiplied. In many cases the heavy bones
have become hollow and are filled with air. The
teeth are gone, but a gizzard or muscular stomach,
better placed and equally efficient, has taken over
their work. The gastric juices are stronger and equal
to dealing with hearty and frequently recurring meals.
All this— better heart, better lungs, better digestive

i INTRODUCTORY 5

apparatus— means higher temperature, comparative,
superiority to external conditions, greater vitality,
and greater enjoyment of life. And the changes from
first to last have not been additions of new organs, but
adaptations of existing materials.

The differences between a bird and a reptile are so
striking, they lie so much on the surface, that we are
in danger of seeing the differences only and passing
over the resemblances, some of which arc obscure and
require a minute knowledge of anatomy. But there is
no reason why the pedigree should be taken on trust.
Much of the evidence that enables us to trace it is
easily intelligible.

After trying to make clear the development of the bird
from reptile ancestors, I shall go on to describe (1) his
structure in more detail and the work done by the
different organs, (2) what cannot be dissociated from
his structure, his life and habits.

CHAPTER II

THE SKELETONS OF BIRD AND REPTILE

The Fore Limb

I WILL now put the skeletons of a bird and of a
lizard side by side and compare them. Everywhere
there are striking contrasts, but since the power of
flight is the prominent characteristic of birds, or of
most of them, the fore limb shall be the starting-
point. At the outset it will be necessary to learn the
names of the bones that compose it, taking them first
in the lizard, since there they are more easily made
out. The first bone is the Humerus, or upper-arm
bone (HU, fig. i). Then follow two which lie side by
side, one slender and one stout. The former of these
is the Radius (R), and is described as being pneaxial
in position — i.e., in front of the axis, the axis of any
part of a limb being an imaginary central line drawn
through it lengthwise ; the other is the Ulna (U),
and is postaxial or behind the axis. These terms,
praeaxial and postaxial, should be thoroughly under-
stood, because, in all the transformations which they

en. ii SKELETONS OF BIRD AND REPTILE

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THE STRUCTURE AND LIFE OF BIRDS CHAP.

Fig. 2.— Skeleton of Fowl (after Marshall and Hurst).
a, acetabulum ; at, atlas vertebra ; ax, axis vertebra ; cl, clavicle ; cm, carpo-
metacarpus ; co, coracoid ; cr, cervical rib ; n i, 2, 3, 4, digits ; dr, dorsal rib;
he, femur; hu, humerus; hy, hyoid bone that supports the tongue ; il, ilium;
is, ischium ; k, keel of sternum ; mc, 1,2, 3, metacarpals; MT, first metatarsal;
pb, pubis ; py, pygostyle ; r, radius ; rc, radial carpal ; sc, scapula ; si», spine ; sr,
sternal piece of rib; tm, tarso-metatarsus ; tt, tibio-tarsus ; u, ulna; uc, ulnar
carpal ; up, uncinate processes.

SKELETONS OF BIRD AND REPTILE

undergo, bones remain, relatively to each other, in the
same position. Hence it often happens that to observe
carefully the position of a bone is the best way to
discover what bone it is. The wrist now follows, con-
sisting of two rows of bones called Carpals (C, fig. 4a),

with a central one (CE) wedged
after these the five Metacarpals,
the bones of the hand (MC).
Next to them come the finger
bones, each division being
called a Phalanx (D I, 2, 3,
4, 5). The thumb has two
phalanges, the second digit
three, the third four, the fourth
five, and the fifth three. And
at the end of each digit is a
claw. Apart from its being
featherless, nothing less suited
for flying can be imagined.

If we turn now to the bird
we shall find that the Hu-

111

between the two,

Fig. 3-

-Tibia and Fibula of

Fowl,
fibula ; T, tibia.

merus (HU, fig. 2) has broad-
ened, especially at the nearer
end, and is covered with great

protuberances, good evidence that powerful muscles
spring from it and are attached to it. Till we come
to the fingers, there will be a striking increase in the
length of the various bones. A bird's wing would be
an outrageously long leg for a lizard of equal weight
and bulk. When the long radius and ulna are ex-
tended, the elbow-joint allows of no turning motion.
As is essential for flight, they are held stiff whatever

IO

THE STRUCTURE AND LIFE OF BIRDS chap.

strain is put upon them. The Radius is a very
slender bone, the Ulna much thicker, with small but
well-marked projections at the points where the great
feathers grow. Of the nearer row of carpal bones
there are only two (RC and UC), whereas there are
three found in the lizard ; in the bird the small inter-
mediate one has disappeared, and also the central bone

Fig. 4.— Hand of (a) Haiteria Lizard; {b) Chick,
ci, near row of carpals; C2. farther row of carpals ; ce, central bones — there
are often two ; d i, 2, 3, 4, 5, digits ; mc, metacarpals ; p, pisiform bone, originally a
tendon ; r, radius ; u, ulna.

beyond it. Of the more distant row there is not a
sign in the mature bird, but, if we examine the skeleton
of an embryo, it may be made out. In a young chick-
there are still two free bones to represent it (C 2, fig. 4).
In the adult these have been fused with the Meta-
carpals beyond. The tendency to fusion, or, as it is
technically called, ankylosis, is found in many parts of

ii SKELETONS OF BIRD AND REPTILE u

the bird's skeleton, and may be regarded as a most
marked characteristic. The bone formed by the fusion
of the farther row of carpals with the metacarpals
forms a broad slab on which rest many of the most
powerful feathers of the wing. Its compound nature
is described by its name of Carpo-metacarpus, or
wrist-hand bone (CM fig. 2). Of the five metacarpals
only three remain, and these three are fused together
at their bases. Farther on they separate and can be
easily distinguished. The first is only a slight pro-
jection from which the thumb 1 springs, the second is
long, strong, and nearly straight, the third after de-
scribing a curve fuses again with the second at its
farther end. The very short thumb consists of two
phalanges only, the last being very small. It fre-
quently has attached to it a claw suggestive of
reptilian ancestry. The second and third digits are
far away at the ends of their long metacarpals,
attached firmly one to the other so that neither can
move separately. The second is formed of three
phalanges, of which the first is broad and plate-like,
the last very minute ; the third digit is insignificant,
with only a single phalanx. In digits I and II the
final phalanx is often missing. The two united
fingers have some slight power of movement, which
many birds turn to account ; but in no part of
the hand is there what can in any ordinary sense
be called a joint. The wrist-joint, also, has much
changed its character. It allows the hand to move
freely towards the place where the fifth digit (or

1 I am taking it for granted for the nresent that this digit
corresponds to our thumb. See p. 42.

12 THE STRUCTURE AND LIFE OF BIRDS chap.

"little finger") should be. The up-and-down move-
ment that the lizard has, and that we have in our
wrists, has almost disappeared : when the wing is
extended, it does not exist at all ; when it is flexed,
some movement of the kind becomes possible. The
fusion of one row of carpal bones with the meta-
carpals has no doubt helped towards this rigidity
which is so important to the wing. At the shoulder-
joint there is always the utmost free play.

A bird's bones combine in a most remarkable way
lightness and strength. It is popularly supposed that
all birds, or at any rate those which fly much, have
all their chief bones hollow and marrowless. This is,
however, a fallacy ; some of the best flyers — e.g., the
Swallow — having even the Humerus solid. But whether
pneumatic or not, the bones are always fine in the
grain and strong.

The chief results of the changes that the fore limb has
undergone may now be summed up. ( I ) It is of most re-
markable length. (2) It is at one time rigid, at another
flexible, according as rigidity or flexibility is required.
Contrast with a bird's hand a lizard's with its waggly
fingers. And how neatly and comfortably the wing
folds, when it is to be put to rest upon the body, in the
form of a Z- (3) There are broad surfaces of bone
to support the feathers. (4) Strength is combined
with lightness. (5) The loss of two metacarpal bones
and two fingers has been a gain, since the present
hand formed of three united metacarpals and two
united fingers (I am disregarding the insignificant
" thumb ") is more efficient for purposes of flight than
a hand with five fingers could well be.

SKELETONS OF BIRD AND REPTILE

13

The Breastbone and the Bones that meet at the shoulder-
joint.

A powerful wing would be of no use without
powerful machinery for moving it, and a lizard with
a bird's wings would be no more able to fly than any

Fig. 5. — (a) Sternum of Iguana, with interolavicle, coracoid, precoracoid, and
clavicle.
cl, clavicle ; co, coracoid ; icl, interclavicle ; pco, precoracoid ; st, sternum ;
(b) Coracoid, scapula, and clavicle of fowl,
co, coracoid ; sc, scapula ; cl, clavicle.

ordinary lizard. In the bird's skeleton the enormous
breastbone suggests a great deal. They must be
strong muscles which have so strong and big a bone
to which to attach themselves. No two things can
be more unlike than the breastbones of the bird and
the lizard, and the same may be said of the associated
bones. In the lizard the whole apparatus is flat and

i 4 THE STRUCTURE AND LIFE OF BIRDS chap.

weak. Its chief component, the Sternum (ST, fig.
5a) or breastbone, is a level expanse with ribs
attached to its margin. Down the middle may be
seen running a thicker and stronger bone, which throws
out a branch on either side, so that the whole makes
something of a T shape. This bone is called the Inter-
clavicle (ICL), and it will be seen that the Clavicles
(CL) or collar-bones converge upon it. This inter-
clavicle was formerly thought to correspond to the
keel, the high, projecting ridge upon a bird's breast.
But this is not the case, for the interclavicle is a
membrane bone — i.e., a hardened ossified membrane
— while the keel of a bird's breastbone originates from
cartilage or gristle. There is also a bone called the
Coracoid (CO) running from the shoulder-joint to the
fore part of the sternum, and this bone in the lizard
divides into two, the fore part being called the Pre-
coracoid (PCO). Both parts are thin and weak. At
the shoulder-joint the Coracoid and Clavicle are met
by another bone, the Scapula (SC) or shoulder-blade,
which with the Coracoid forms the socket. The
Scapula is a thin expanse of bone approaching close
to the back-bone, to which it is attached by muscles.
The upper part, the Supra-scapula, consists of gristle
or cartilage not completely ossified. In the bird all
this has been metamorphosed, though the materials are
in the main the same. There is the sternum with a
high ridge added which ossifies — i.e., becomes bone —
from a different centre. In the young bird the keel is
at first mere gristle, which at a certain point begins to
ossify, and the process continues till the bone of the
keel meets and joins the bone of the sternum proper.

ii SKELETONS OF BIRD AN 1 1) REPTILE 15

The Coracoids have become much stronger, while the
Precoracoids and the Interclaviclc have disappeared.
The Scapula is formed of sound bone from end to end.
In shape it has grown long and blade-like — in the
chapter on Flight I shall explain what advantage has
thus been gained — and it is fastened to the back-bone
by muscles firmly, but in a way that allows a great
deal of free play. And while maintaining their posi-
tions relatively to one another the various bones have
much changed their attitudes. If we look upon the
shoulder-joint as the fixed point, the sternum begins
farther back, and, consequently, the Coracoids slope
forwards instead of backwards, in order, with their
other ends, to reach the joint, towards the formation
of which they contribute so much. And the bird is
much deeper-chested than the lizard : therefore they
must slope not only forwards but upwards. With all
these changes they retain their slope outward. Sca-
pula and Coracoid form an acute angle, opening
towards the tail, whereas, in the lizard, the angle
formed opens towards the head, or, sometimes, one
bone continues the line of the other. The clavicles,
moreover, have not the slope backward towards the
shoulder-joint that is so marked in many lizards ;
they point upward and outward. The most im-
portant result of all these changes is that a firm
pivot has been found on which the wing can turn —
a firm pivot (1) because of the great strength of
# the Coracoid ; (2) because both it and the Clavicle
have a marked outward slope ; and (3) because
they buttress each other.

16 THE STRUCTURE AND LIFE OF BIRDS chap.

The Ribs.

The ribs have very remarkable cross-pieces, called
Uncinate — i.e., hooked — processes (UP, fig. 2) that
spring from about the middle of their upper part and
slope backwards and upwards. These uncinate pro-
cesses, no doubt, serve to strengthen the chest, and,
apart from this, they possess a singular interest.
They are common to all birds, but absent in nearly
all reptiles. In a very few lizards they are found —
e.g., in the Hatteria, a native of some of the islands of
the New Zealand group, and now so rare that it may
be in danger of extermination (fig. 1). It will be seen
that each rib has a joint at a point considerably
nearer to the sternum than to the backbone, the two
parts being spoken of as the dorsal ribs (Latin
dorsum, the back) and the sternal pieces respectively.

The Hind Limb.

In naming the bones of the leg, the genius of the
older anatomists for seeing resemblances where it is
difficult for us to see any has run riot. The larger of
trie two leg bones is the Tibia or flute, the smaller one
is the Fibula or brooch (F, fig. 1, 3). The Acetabulum,
as they called the socket of the thigh-bone, is a happier
name. Strictly the word means a vinegar-pot, and
it is used of any cup-shaped vessel. A bird's Ace-
tabulum (A, fig. 8) is remarkable. If you look at a
skeleton in a museum you will see that the cup has no
bottom to it. The bottom was formed of membrane,

ii SKELETONS OF BIRD AND REPTILE 17

not, as in mammals and most reptiles, entirely of
bone, and it has vanished. But ankylosis or fusion is
perhaps the most marked characteristic of a bird's
Leg. There are the same bones as in the lizard's leg,
if we could only see them — viz. : Femur (FE, fig. 2)
or thigh-bone, Tibia (T), Fibula (F), two rows of
Tarsals or ankle-bones (TA), four of the lizard's five
Metatarsals (MT), though of one of the four only the
farther end (MT P fig. 2) remains, and four of his five

Fig. 6.— Hatteria Lizard's left hind foot.
4, 5, digits ; f, fibula; MT, metatarsals ; t, tibia; taj, two bones fixed,
represents near row of tarsals ; ta-_>, distant row.

digits. The Femur has not undergone so much
change, but the Tibia and Fibula (fig. 3) are very
different from the corresponding bones in reptiles.
The latter has nearly vanished ; it is a slender, almost
needle-like bone, attached to the side of the Tibia and
not reaching to its farther end. In many mammals
too the Fibula is but a remnant. The way to make
certain, in the skeleton of any animal whatever, which
bone is the Tibia and which the Fibula, is to imagine
the limb extended, as it is in the lizard, outwards from
the body ; then the Tibia is praeaxial and the Fibula

C

18 THE STRUCTURE AND LIFE OF BIRDS chap.

postaxial. But if we look for the bird's tarsals they
are not to be seen. The disappearance of the nearer
row is to be accounted for in this way : the bone we
have just called the Tibia is really the Tibia plus the
nearer row of tarsals which have been fused with it, and
its proper name is Tibio-tarsus (TT, fig. 2). This has
been made out clearly in the leg of the embryo bird.
The farther row of tarsals has also no separate ex-
istence. They have been fused with the Metatarsals.
In the young chick each row of tarsals has one large
separate bone to represent it. In the mature bird,
directly below the Tibio-tarsus comes another long
compound bone, the Tarso-metatarsus. At the farther
end of this, deep grooves show that it is made up of
three bones — the second, third, and fourth metatarsals.
Of the first metatarsal there is only the afore-mentioned
remnant (MTi). All these things are very difficult to
remember. One plan is to go over them again and
again till in time they become familiar. A better plan
is to remember the names Tibio-tarsus and Tarso-
metatarsus, which explain the most difficult points.

The four digits or toes possessed by most birds are
the first, second, third, and fourth. The " great toe "
is dwarfed by the others, and has only two phalanges ;
the second has three, the third four, the fourth five.
Thus the numbers run in regular progression — 2, 3, 4,
5. In lizards the five toes, each attached to its
independent metatarsal, are always present, and they
have respectively 2, 3, 4, 5, 3 or 4 phalanges. The
correspondence in numbers is very curious. No bird
has a fifth toe. Domestic fowls, Dorkings especially,
often have a supernumerary " toe," which is really a

ii SKELETONS OF BIRD AND REPTILE 19

skin formation, and no more a toe them a caterpillar's
hind " lei; " is a leg.

The whole limb is very different from a lizard's ; it
is longer and stronger and fitted for an upright car-
riage. The strong Tibia is well able to do its own
work as well as that of the Fibula, which has almost
disappeared. The bird stands on his toes, which are
strong and springy, and jumps lightly into the air in
order to start his flight. Length of leg is to many
species of vital importance, and the elongation has been
to a great extent effected by the large development
of the metatarsals. The fusion of the two rows of
ankle-bones with the longer bones above and below
was, I think, necessary for the effective working of
the machinery by which a bird is enabled, even during
sleep, to grip his perch firmly (see p. 166). Moreover,
without all this fusion of bones, would the luxury of
standing on one leg be a possibility for him ?

The horse's leg presents remarkable points of
resemblance to the bird's. In both, the Tibia relieves
the Fibula of all its work. In both the Femur is
short, so that the knee-joint is high and easily remains
unnoticed ; in the bird it is hidden among the feathers.
In both the ankle-joint is raised high above the
ground.

The Skull

In a way, the skull is the most bird-like part of the
whole skeleton. It is light, not only because of the
thinness of its walls, but because of its many air-
cavities. Even birds which have their long bones
solid have the skull pneumatic. Lightness is of the

C 2

20 THE STRUCTURE AND LIFE OF BIRDS chap.

utmost importance. Otherwise, how could the head
be supported at the end of so long a neck ? But
muscles get tired with prolonged exertion, however
slight the exertion may be, and to provide against
this there are, between the spines of the vertebrae of
the neck (SP, fig. 2), elastic ligaments similar to that
which is so enormously developed in the horse's neck
to support his ponderous head. These ligaments hold
the neck in position when it forms an S. In the Swan
they are but slightly developed, hence perhaps the
ease with which he erects his neck straight as a flag-
staff. Even when there are ligaments to relieve the
muscles, the skull is the place where aeration of bones
is desirable if anywhere. Its great size compared with
that of the lizard, and, consequently, the great size of
the brain, I have already pointed out. The skull, too,
illustrates better than any other part of the skeleton
the tendency to ankylosis, or fusion of bones. Even
in a very young bird this has already proceeded a long
way. The skull seems to be made up of a shell of
bone almost without suture. It is really composed of
scores of different bones, the boundary lines between
which may be seen in the embryo. And how are these
to be studied ? It is possible to go through them and
learn them up as one does for an examination. But •
for such studies it is usually the imminence of the
examination, not the interest of the subject, which
supplies the stimulus. Without this stimulus, to a
mind that has not as yet the patience wanted for
scientific investigation — the patience to collect facts ?
even if the clue to them and the interest of them may
not be found tiii years after — there is something barren

ii SKELETONS OF BIRD AND REPTILE 21

and unsatisfying in, for instance, the isolated fact that
a certain bone in a certain part of the skull is called
the Squamosal (SQ, fig.- 10). Something is wanted to
give point and interest to such dry fragments of ana-
tomical knowledge, some strange variation in the bone
in question in different classes of animals, or a theory
as to the origin of the skull, so that the memory may
not have to deal with what abhors it more than anything
else — viz., isolated meaningless facts. There is much
pleasure to be got by putting the wing of a bird and the
fore leg of a lizard side by side, and observing the
changes that have made a fuller and more vigorous
life possible to the bird. The same kind of interest
may be found in a general comparison of the skulls.
But since it is difficult for human weakness to main-
tain this during the slow groping progress through
the labyrinth of bones, I shall pass over this part
of the subject without going into any detail. There
is, however, something of a clue to the labyrinth.
Gothe discovered one which will lead us some way,
though not nearly so far as he imagined. The skull,
according to him, was simply an expanded vertebral
column, all its chief bones being vertebra?, the name
given to the series of bones which combine to form
the neck, backbone, and the bony skeleton of the
tail, all included in the term vertebral column.
This is one of the great ideas which advance science.
Even if it had turned out entirely unfounded, still
something would have been learnt in the process of
testing it. But this theory has not proved to be with-
out foundation. There is no doubt that the skull is
made up partly of vertebra?. But, so far, the dimcul-

22 THE STRUCTURE AND LIFE OF BIRDS chap.

ties have been insurmountable when it has been
attempted to show how many vertebrae go to the
skull, or to find in a bone of the skull parts that
correspond to the easily distinguishable parts of a
vertebra. The skull as yet presents many unsolved
problems. For the present we must make a guarded
statement that vertebrae to some extent enter into the
formation of it. The bones of the jaw and those con-
nected with them present problems of hardly inferior
interest. But it will be time to give some account of
them when we come to the subject of the embryo bird.
The articulation of the skull with the neck will be
described in the next chapter.

The Vertebral Column.

The bird's neck bends with greater ease and freedom
than the lizard's. Indeed it outdoes even the supple-
ness of the snake. If you hold up a snake by his
tail, he tries to get at your hand, with a view to
biting you if he is of a poisonous kind, by bending
upward sideways. He has not much power of
hollowing his back, so as to rise without a curve to
the side. True, some snakes have much more supple-
ness in an upward and downward direction than others.
If the Hooded Snake is irritated, he raises the fore part
of his body so that it forms a double curve or S shape.
But even he cannot make by a long way so decided
an 3 as a long-necked bird, and the reason is that in
the bird the bones which form the neck articulate
with each other differently, by a joint which is a
marked improvement on the reptile's joint. The

SKELETONS OF BIRD AND REPTILE

23

latter is a ball-and-socket. At the hinder end of eacli
vertebra there is a protuberance, rounded on its upper
side but nearly flat below, which fits into a hollow in
the vertebra behind. The fact that the protuberance is

Fig. 7.— (A) Two vertebrae of Snake. (/?) Two neck vertebr.x of Eagle.

I. Anterior end. II. Posterior.

b, ball ; cr, cervical rib; sd. "saddle" ; sk, socket ; sf, spine ; va, tunnel through

which vertebral artery passes.

not a perfect ball but has its underside flattened limits
the freedom of movement, and, in addition to this, each
vertebra bears a spine (SP, fig. 1 ) upon the top, the spine
of one being very near to that of the next, and thus
a further limit is put to movement up and down. The

24 THE STRUCTURE AND LIFE OF BIRDS CHAP.

joints in a bird's neck resemble two saddles laid
crosswise one upon the other, so that the pommels
of one face at right angles to those of the other, the
upper saddle being, of course, upside down. In the
bird's neck the " saddles " are so arranged that at the
hinder end of each vertebra the pommels are at the
top and bottom, while at the front end they are at
the sides. Thus the two vertebrae will slide over
one another sideways, or up and down, in the same
way that two saddles will slide when they are laid
upon one another in the way which I have described.
And the upright spines in a bird's neck are small and
far from one another, so that they do not hinder the
movement (SP, fig. 2).

The backbone of the bird has been so much modified
for purposes of flying and walking that it presents a
difficult study. The vertebral columns of reptiles and
mammals are divided into well-defined regions — the
regions of the neck, breast, loins, pelvis, and tail. In the
bird, fusion or ankylosis has gone on to such an extent,
and the pelvis has extended so far backwards and
forwards, that it is a most perplexing problem how to
map out the backbone. I shall not attempt this here.
The subject does not seem to be of first-rate importance;
in studying the views held by various great authorities,
not very much of the principles of anatomy would be
learnt, since it is admitted that there is no essential, no
morphological difference, to use the technical term, be-
tween a neck vertebra and a thoracic or breast vertebra.
They are corresponding organs, in slightly different
places and slightly modified, a breast vertebra being
defined as one which has attached to it a rib that

ii SKELETONS OF BIRD AND REPTILE 25

unites with the sternum, and as the neck vertebra
bear small undeveloped ribs this is not an important
distinction. These neck ribs, short thin straight bones
pointing backward, can be seen in fig. 2 (CR, cervical
ribs) ; the two bases of each arc fused with the verte-
bra, and between them runs a tunnel through which
the vertebral artery passes. Besides this, the fore
limb has in some cases, very possibly, moved back-
ward, since the neck varies very greatly, far more
than the backbone, in the number of vertebrae that
compose it. 1 Where, then, is our fixed point ?

I have already described the way in which the neck
vertebrae articulate. The next point to notice is their
large number, sixteen or seventeen being not un-
commonly found ; the Ostrich and the Swan having
considerably more : even small song-birds have not
less than ten. With mammals seven is the almost
invariable number, the neck of a Giraffe and of a
Hippopotamus being alike in this. A bird's neck,
to be supple and more than snake-like, must clearly
have a great many vertebrae. In the lizard eight is
the normal number.

By far the most noticeable feature about the remainder
of a bird's vertebral column is its stiffness, due to the
fact that the vertebrae have become ankylosed together.
But it is quite erroneous to describe the bird's back-
bone as being throughout its length a rigid rod. In
all the specimens I have examined it bends, at a point
just in front of the pelvis, with some freedom to either

1 The question is discussed by Max Fiirbringer in his
Morphologie und Systematik der Vogel, of which there is a good
summary in Nature, 1888-89.

26 THE STRUCTURE AND LIFE OF BIRDS chap.

side, and in all there is some flexibility upwards and
downwards. But the amount varies much in different
species. The tail vertebrae are very different, and, in
the freedom with which they move upon one another,
approximate to those of the neck. Were it not so,
birds could not do what they may easily be seen to do
while flying — move their tails for purposes of steering
or to check themselves suddenly. The Pygostyle,
the large bone which supports the tail, consists of a
number of vertebrae fused together (PY, fig. 2).

The Pelvis.

The bird's Pelvis, at its anterior end, roofs over
the backbone. It is formed of three bones, which
in different classes of animals assume forms so
different that they are often difficult to recognise.
The difficulty, however, will be got over, if we bear in
mind what I have already explained, that bones,
however much they may change their form, yet keep
the same position relatively to each other. One of
these bones, the Ilium (IL, fig. 8) attaches to the back-
bone, and by that it may be recognised. Its peculiar-
ity in the bird is that it unites with so many vertebrae
both before and behind the hip-joint, fusing with
them and making this part of the backbone absolutely
rigid. The two remaining bones assist the Ilium to
form the socket of the hip-joint, and they must be
distinguished by their positions relatively to it. The
Pubis forms the lower front of the socket (PB), the
Ischium (IS) the hinder part. The former projects a

SKELETONS OF BIRD AND REPTILE

27

very little way forward and a long way backward, a
slender bar of bone, united for part of its length with
the Ischium. To discuss here whether this is all

IL

--W-

~D^ V

Fig. 8.— Left side of Pelvis (a) of Lizard ; (/>) of Bird.

A, acetabulum ; 11., ilium ; is, ischium ; PB, pubis ; v, vertebrae fused.

In (a) the ischium and pubis slope inwards to meet their fellows from the opposite

side. In (/>) all three bones are fused where they meet. The ilium extends forwards

as well as backwards from the acetabulum.

Pubis proper, or whether a pubic " process " has
been added, would be out of place. The term
process, meaning an out-growth, is a favourite one
with anatomists, and is often useful as a name,
though, it must be owned, it does not explain much.

28 THE STRUCTURE AND LIFE OF BIRDS ch. ii

Its remarkable form of pelvis is of great advantage
to the bird. It has helped in the stiffening of the
backbone ; it gives room for the attachment of the
large muscles necessary now the quadruped has be-
come a biped ; its backward extension is useful for
the attachment of the muscles that move the tail. The
lizard's pelvis, hanging downward from the backbone,
looks like a different organ. The bones, however,
are the same. The Ilium attaches to the vertebral
column, and the other two can be made out by their
position relatively to it and to each other ; the Ischium
forming the hinder part of the socket in which the
thigh-bone moves, the Pubis the anterior part.

Some Books on the Subject.

Marshall and Hurst's Practical Zoology.

Parker's Zootomy.

Huxley's Vertebrate Anatomy.

Gegenbaur's Comparative Anatomy.

Alix's Essai sur Vappareil locomoteur des Oiseaitx.

CHAPTER III

EVIDENCE OF RELATIONSHIP TO REPTILES

AFTER all that has been said about the great differ-
ences between birds and reptiles, the reader may begin
to think that the points of resemblance are few and
small, and that the relationship after all may be only
a distant one. In reality the evidence of a compar-
atively near relationship is convincing. But it must
not be expected that the resemblances should be
as striking as the differences. The latter are due
mainly, perhaps entirely, to natural selection working
during long ages and gradually suiting the bird's
structure to new conditions of life and changing habits.
The metamorphosis produced is so great that to the
untrained eye the bird has been altered almost beyond
recognition. The points of resemblance are ancestral
peculiarities that have survived all changes of habit.
Not being connected, as a rule, with the new and
more brilliant life of the ennobled race, it is only to be
expected that they should be comparatively incon-
spicuous or of the nature of mere rudiments. Before
mentioning these marks of reptilian origin it will be

3o THE STRUCTURE AND LIFE OF BIRDS chap.

well first to explain what is meant by the anatomical
terms rudiment, and homology and analogy.

A rudiment is an organ which survives, though it has
become wholly or almost functionless. Often it is much
reduced in size. The Python has hind legs, the claws
of which are sometimes just perceptible through the
skin. Men have a muscle for moving the ear forward,
but very few are able to use it. The Sea-lion has claws
at the end of his toes, but the skin projects far beyond
them, so that the claws are absolutely useless. Crabs
which live in caves to which no light penetrates have
eye-stalks with no eyes on the top of them. Perhaps
the most startling rudiment of all is the Pineal body
found in lizards, birds, and mammals, and believed to
represent a central eye.

Two organs are said to be analogous when their
functions are the same ; they are homologous when
they are the same in nature and origin. The wing
of a bird and the wing of an insect are analogous
to one another because they do the same work.
The origin of an insect's wing is not known for
certain, but there is no doubt that it is not a fore-
limb. The relationship, therefore, is one of analogy
only. The wing of a bat is only analogous to the
wing of a bird ; it is not homologous, for, besides
the fact that all five fingers are found in it, it is
supported by the leg as well as the arm. There is
a small fish called Periophthalmus Kolreuteri, which
suns itself upon rocks with only its tail in the water. 1

1 See Professor Haddon's paper, Nature, January 17, 1889.
The fish soon died when its caudal fin was coated over with gold-
size. See also Professor Hickson's Naturalist in Celebes, p. 30.

in RELATIONSHIP TO REPTILES 31

The organ of respiration it then uses is in the tail,
and cannot, of course, be a gill, though it is doing the
work of one. In one and the same animal we some-
times have analogous organs. For instance, a cater-
pillar has only three pairs of legs properly so called.
The hind " legs " before mentioned are only growths
of the skin, and do not survive beyond the caterpillar
stage. On the other hand the tails of all vertebrates
are homologous, however different the purposes for
which they are used : the new-world monkey's for
climbing, the porpoise's for swimming, the kangaroo's
as a leg, the giraffe's and many others as fly-flappers,
the bird's for guiding his flight. Many fish have a
swim-bladder, which is filled with air and gives them
buoyancy. This organ was thought by Darwin to be
the same organ as the lungs of mammals. If so, it
would have been a paragon example of homology.
Unfortunately, it is an outgrowth from the back of
the alimentary canal, whereas the opening to the
lungs is from the front. By an extension two organs
in the same animal are said to be homologous ; for
instance, the Humerus is homologous to the Femur,
the corresponding bone in the hind limb. To prove
relationship we must look for true homologies, as
mere analogies prove nothing.

Here are some of the most striking features common
to birds and reptiles.

(1) A single condyle or rounded projection in the
skull fits into a cup-like hollow in the centrum or
thickened base of the first or Atlas vertebra, which
is so short as to be hardly more than a ring of bone.
At one point in the rim of the cup there is a notch, and

32 THE STRUCTURE AND LIFE OF BIRDS CHAP.

this is filled by a projecting tongue from the second
or Axis vertebra, called the odontoid process, which
thus completes the cup. All mammals have two
condyles. The great freedom with which a bird
moves its head is due to the way in which, by its
single condyle, it articulates with the vertebra.

\

r A °

J

Fig. 9. — Skull of bird (Rhea) viewed from below.
c, condyle ; sp.C, entrance of spinal cord.

(2) The lower jaw articulates with a bone called
the Quadrate, which may be easily recognised. It
roughly resembles a St. Andrew's cross. To the two
lower and shorter arms the lower jaw is hinged. To
the outside corner of the outer of these is attached a
long thin bone, which connects with the upper jaw.
The outer of the two upper arms fits into a hollow
in the bone called the Squamosal. In mammals the
quadrate is represented by an insignificant bone, the
Annulus of the ear (fig. 10, see p. 135).

(3) In mammals the centra, the strong bases from
which spring the arches of the vertebrae, have between
them plates of bone, called Epiphyses, which are

Ill

RELATIONSHIP TO REPTILES

33

easily distinguishable. These are absent both in
birds and reptiles (fig. n).

n- — — 3d

i

v

Fig. io. — Side view of head of Rhea.

q, quadrate ; qj, quadrato-jugal, hindmost component of bone connecting with

upper jaw ; sq, squamosal.

(4) The coracoid bone is present. . In man there is
only a small remnant of its upper end. It is well

Fig. 11. — Part of vertical column of Rabbit.
E, Epiphyses, applied to anterior and posterior end of centrum of each vertebra.

developed in the Duck-billed Platypus, the lowest of
mammals, with a decidedly reptilian anatomy.

D

34 THE STRUCTURE AND LIFE OF BIRDS chap.

PCO

(5) Birds have a single coracoid ; in reptiles it is
divided into two, the coracoid proper and precoracoid.
But in the Rhea we find a very conspicuous, though
rudimentary, survival of the latter, and in the Ostrich
the bone is complete.

(6) The bird's feather
corresponds to the horny
coating of the reptile's
scale. The snake moults,
when, as we say, he
" sheds his skin."

In 1 861 there was
found in the Lithographic
Stone at Solenhofen in
Bavaria the form of an
animal very different from
any that had ever been
seen either alive or as
fossils. This stone be-
longs to the Jurassic sys-
tem and, consequently,
was deposited in the
Secondary or Mesozoic
period, and long, too,
before that period was
concluded. Here was a
feathered creature pre-
served in the form of a
bas-relief, with the detail standing out so distinct and
clear, that something even of the minute structure of
the feathers might be seen. As Sir Richard Owen
showed, it was a bird of a very primitive form. The

CO

%

Fig.

-Coracoid of Rhea.

coracoid ; pco, precoracoid ; sc,
capula.

in RELATIONSHIP TO REPTILES 35

fossil is now at South Kensington. In 1877 a more
perfect example of a bird of the same species was
found and is to be seen at Berlin.

Head of Archseoptery:

This ancient bird was about the size of a rook. His
tail was long, consisting of twenty vertebrae, at least
the twelve hindmost bearing a pair of well-developed

Fig. 14. — Part of wing of Archaeopteryx.
c, carpal bone ; d, i, 2, 3, digits ; m, metacarpal ; r, radius ; u, ulna.

feathers. His breast-bone seems to have been keeled.
His wing was strong and well-developed, the humerus
remarkably big at the near end, the bones of the fore-

D 2

36 THE STRUCTURE AND LIFE OF BIRDS chap

Fig. 15.— Tail of Archaeopteryx.
(All three figures after Dames.)

in RELATIONSHIP TO REPTILES 37

arm were long and strong - , the hand had three fingers,
each bearing a large claw, and it and the ulna sup-
ported a number of large feathers, which seem well
suited for flight. The bill was short. The jaws were
furnished with teeth, the upper one with many, the
lower one with three on each side.

Our first thoughts on looking at this creature might
well be, " It must be a feathered lizard." The long
tail and the teeth at once suggest this. But there are
many things which prove it to be a bird. ( I ) The three-
fingered hand. 1 True, the three metacarpals have
not become fused, and the second and third digits are
separate. Besides this, each of the fingers bears a
big claw. But in many existing birds a claw is found
on No. I, in a fair number on No. 2 as well, in the
young Ostrich on all three. The third digit has not
been reduced to a single phalanx. But this is no great
barrier. In birds of our own day the final phalanx
is often lost on digit I. (2) The length and strength
of the humerus and forearm remind one much of
existing birds. (3) The acetabulum or socket of the
thigh joint, seems to have been closed only with
membrane. (4) Scales, not feathers, are found on all
known lizards.

There are some interesting points which, if rep-
tilian, are also avian. The vertebrae seem to have

1 Dr. Hurst (Natural Science, October, 1893) has boldly tried
to show that archaeopteryx had really more than three fingers,
and that one or two with larger stronger bones have left no
impression on the stone. But when even the delicate forms of
the feathers are preserved, it is wonderful that there should be
no trace of these bones either in the Berlin Archaeopteryx, or in
the one at the British Museum.

38 THE STRUCTURE AND LIFE OF BIRDS chap.

been bi-concave, i.e., the centra presented hollows
at either end. This is a form of vertebra found
in very primitive reptiles, e.g., in the Hatteria lizard.
It is also found in Ichthyornis, a
fossil bird of more recent date than
Archaeopteryx. And what is far more
strange, the Gull, a highly special-
ised, a thoroughly modernised bird,
has some of its dorsal vertebrae con-
cave behind, thus conforming to an
old reptilian type, "and one almost
fig. 16.— Vertebra of bi-concave, which thus carries us back

Hatteria Lizard. . .1 r ,mi •,•

. . to reptiles oi a still more primitive

C, centrum ; it is am- L L

phicoeious, i.e. con- form. 1 In teeth, as I hope to show

cave at each end. x

presently, there is nothing unavian.

It would be very interesting to know how this
bird lived. Of one thing we may be certain — he
was a poor flyer. With its three long unconnected
fingers the wing must have been a weak one. Prob-
ably he fluttered, rather than flew, from bough to
bough, his long tail serving as a parachute, and his
claws may have been used when he was young,
as Mr. Pycraft has suggested, and also when he was
moulting, to aid him in climbing, as the young
Hoatzin uses his now.

Since the discovery of Archaeopteryx the fossil
bones of many birds of far later date have been found
in the cretaceous rocks, and also in the rocks of the
most recent geological periods, the Tertiary and

1 Sec \V. K. Parker on Opisthocomus Cristatus, Proc. Zool.
Society, vol. xiii. part 2.

in RELATIONSHIP TO REPTILES 39

Quaternary. Some of these were of gigantic size,
larger even than the Ostrich. Like Archaeoptcryx,
they had teeth. In Hesperornis, to take one example,
these are very reptilian, in the way they are set in a
long hollow in the jaw, in the absence of root and of
cement on the neck of the tooth, in the way they
were changed, a young tooth being formed on the
inner side of the base of the old one. But by this
time birds had lost their great length of tail.

Here it will be instructive to mention the discovery
in existing birds of what were supposed to be the
rudiments of teeth. If you take the beak of a Parrot
and macerate it well, you can separate the horny beak
from the bone beneath. The horn is only a form of
epidermis and, therefore, we should expect to find
skin underneath. Skin is found and on it papilla:,
small pimple-like elevations, similar to those found
beneath a horse's hoof. They nourish the growing
and quickly wasting beak. And these were the
" rudimentary teeth " which so much interested the
zoological world. But here, as often, a false theory by
stimulating investigation has led the way to the true
one. 1

There are some very remarkable points of agreement
between crocodiles and birds. The ordinary reptile
has only three chambers to his heart. The crocodile's
heart, though still a very imperfect organ, has four
see p. 284). He has a gizzard and habitually
swallows stones to aid in digestion. His lungs are
far more elaborate structures than those of a lizard.

1 See Bronn's Thier- Reich, vol. " Aves," p. 501.

40 THE STRUCTURE AND LIFE OF BIRDS CH. Ill

The socket of his thigh-joint is not completely
closed with bone : in the skeleton of the crocodile as
in that of the bird, the cup of the socket has a hole at
the bottom, the membrane which was there during
life having disappeared. Several of the ribs have
uncinate processes.

These resemblances, to which others might be added,
do not prove that the crocodile is the ancestor of the bird.
The heart and the gizzard were probably developed
independently after birds and crocodiles had arisen
from some common and more primitive stock. It
would be unwise to say that birds are descended from
any existing class of reptiles. But the facts justify us
in drawing the inference that birds and reptiles are
related; that they had common ancestors with less of
specialised character than either of themselves, and
that from these ancestors each class has developed in
accordance with its own mode of life. In the same
way Englishmen and Chinamen come from the same
stock ; neither race is descended from the other. The
Darwinian theory of the descent of man is that if
you trace upward the pedigrees of men and monkeys,
the lines will meet, not that men are descended from
monkeys.

Books on the Subject.
See at the end of Chapter II.

Also Newton's Dictionary of Birds, " Fossil Birds " ; Pycraft,
Nat. Science, November and December, 1894, " Archaeopteryx " ;
Owen, Philosophical Transactions, 1863, "Archaeopteryx."

CHAPTER IV

CONNECTING LINKS

The supply of connecting links can never equal the
demand. The discovery of the Ornithorhyncus brought
to light a connecting link between mammals on the
one side and birds and reptiles on the other. It lays
eggs ; it has a beak like a bird's ; its anatomy is highly
reptilian, and it suckles its young. Geology shows us
an animal, evidently akin to the Horse, with four
toes, and thus we are able to put the Horse down
as a near relation of the Rhinoceros and- the Tapir.
But the mending of one gap does not prevent the
existence of others. It often seems even to call
attention to them. Remains of extinct animals have
been found which certainly to some extent bridge the
gulf between birds and reptiles. Such evidence of
relationship is very valuable, but it is easy to mistake
its nature. These fossil reptiles, in so many ways
birdlike, must not be looked upon as the ancestors of
birds. Nor do they, like the Ornithorhyncus, carry us
back to a low unspecialised type. They are only con-
necting links in this sense, that they show that some
undoubted reptiles much resemble birds, that reptiles

42 THE STRUCTURE AND LIFE OF BIRDS chap.

may develop into very birdlike creatures, and so, that
birds themselves may have had a reptilian origin.

In the Secondary or Mesozoic period, there were
upon the earth Pterodactyls or wing-fingered animals,
also known by the name of Ornithosaurians or Bird-
lizards. Some very perfect specimens of these have
been found in the lithographic-stone at Solenhofen
in Bavaria. England and America have produced
pterodactyl bones and almost complete skeletons,
the latter country some of enormous size, and thanks
to the labours of great anatomists — among them Pro-
fessors Huxley and H. G. Seeley and Sir Richard Owen
— we now understand a great deal about these flying
reptiles, and can form a fair notion of how they lived.

On looking at a restoration of one of these ptero-
dactyls, one's first thought is, that it is their wings
which prove them to be nearly related to birds. This
requires to be closely looked into, and what was said
above about analogy and homology must be borne in
mind. The wings of birds and pterodactyls are similar
in function, but in their structure they are very different.
They are analogous but not homologous. A bird's
wing contains three fingers. The first is very small,
the second is far the biggest and strongest, and to it the
third is immovably attached. Dr. Hurst l has tried to
show from the evidence of the Berlin Archseopteryx
that these three digits are not the first three, but that
the two united ones are the fourth and fifth. For this
view, as far as I can see, no evidence is to be found in
the Berlin Archaeopteryx or anywhere else. But when
he maintains that the generally accepted view, that the
1 Natural Science, October, 1893.

iv CONNECTING LINKS 43

little digit is the thumb, and that the other two are Nos.
2 and 3 respectively, is unsupported by evidence, he
seems to me to be stating what is undeniable. When
he goes further and argues that they are Nos. 3, 4, 5,
he is flying in the face of facts. In the embryos of
the Swift and Tern several good observers have seen
a fourth unmistakable metacarpal on the ulnar side
{i.e. the side on which in our hand the little finger is)
and in the embryo of that extraordinary South
American bird, the Hoatzin, there is a remnant on
the same side of a fourth finger though the meta-
carpal has disappeared. There are, then, only two
alternatives : the surviving digits are either I, 2, 3, or
2, 3, 4. Now the Emeu has only one, the central one
of the three, and all analogy would lead us to believe
that this is the third of the original five and not the
second, since, when reduction proceeds very far with
the digits of birds' feet, or with those of the fore or
hind feet of mammals, they are lost, as far as can be,
symmetrically, not in lopsided fashion. And since it
is the ulnar side of the wing on which mainly the
strain falls in flight, it is not likely that all the weak-
ening would go on on this side and all the strength-
ening on the other. Moreover, in the embryo Hoatzin
there has been found beyond the so-called " thumb,"
besides vaguely suggestive cartilage, a bone, small yet
solid and well defined, that may be a trace of the true
thumb that has disappeared. 1 In any case, the bird's

1 See Leighton, Tuffs College Studies TIL, on " The Develop-
ment of the Wing of -Sterna Wilsonii," and W. K. Parker,
Trans. Zool. Soc, Part 2, April, 1891, on "The Morphology of
Opisthocomus Cristatus "

44 THE STRUCTURE AND LIFE OF BIRDS chap.

wing is very different from the pterodactyl's. To the
support of the latter only one finger, often called the
ulnar finger since it articulates with the postaxial side

Fig. 17.— (1) Pterodactylus Spectabilis, from lithographic-stone, Bavaria.

of the arm, contributes, and this one may be either No.
4 or No. 5. The settlement of the question depends
upon the nature of the small bone which can be seen

IV

CONNECTING LINKS

45

' M

-

s -1

O Q.

t
II

o 2
bo i»

1 Q.

46 THE STRUCTURE AND LIFE OF BIRDS chap.

projecting from the wrist, and which may be a remnant
of the first metacarpal or only a sesamoid bone [see Fig.
17 (1)]. The wing is a great sheet of membrane sup-
ported by this ulnar finger, which was of enormous,
length, and also by the leg and tail. Thus, whereas
two fingers united help in the formation of the bird's
wing, only one forms part of the pterodactyl's, for the
other three are little clawed appendages of no use in
flight. Whether these fingers in the bird be Nos. 3 and
4, or 2 and 3 makes little difference. There are two,
not one only, and there is no sign that the smaller one
is likely to disappear, and the larger one being No. 2
or 3 does not correspond to the pterodactyl's ulnar
finger which is No. 4 or 5. The two together form a
short stout bone that contrasts forcibly with the ptero-
dactyl's finger of which the one striking characteristic
is its length. Besides the question of fingers the whole
build of the pterodactyl's wing is different. It gets
its expanse from its great membrane. The bird ob-
tains from its feathers its spread of canvas, while the
pterodactyl has no feathers at all. Its wing, formed by
a membrane stretched from the arm to the leg and to
the tail, was more like a bat's wing than a bird's. But
here again there is an important difference. All the
bat's digits except the thumb help to support the wing,
in the pterodactyl only this one ulnar finger.

We must, therefore, look for other evidence of the
kinship of pterodactyls to birds. Take the head first.
The pterodactyl had a large brain case and, for a
reptile, an extraordinarily high forehead. The orbits
of his eyes were large. The bones of his skull were
light and became fused together at an early age.

iv CONNECTING LINKS

47

His teeth, as I have explained above, do not separate
him from the bird. In fact, it is far more a bird's
head than a reptile's. Proceeding now to the long
bones we find that many of them have a very
remarkable feature ; they have undoubted air cavities.
These two points — the birdlike character of the skull,
and the aeration of the bones — are, I think, the most
important of all. When they are combined with power
of flight we can infer from them other characters of

Fig. 18.— Pterodactyl, Rhamphorhyncus phyllurus restored (after Marsh).

which no direct evidence is obtainable, and principally
this — the pterodactyl must have been a warm-blooded
animal. Flight requires great vigour such as is not
found in any cold-blooded creature. Flying-fish can
hardly be said to fly, nor can the so-called Flying
Dragon. Its wings are merely parachutes. Moreover
no animal that we know of combines a highly-developed
brain with cold blood. Among existing animals, birds,
who on the average have a decidedly higher tem-
perature than mammals, have, very many of them,
pneumatic bones. It is true that the same tendency
to pneumaticity is found in the bones of the Dinosaurs

4 8 THE STRUCTURE AND LIFE OF BIRDS chap.

of which I shall speak soon. But these also may have
been to some extent warm-blooded.

The fact that existing reptiles are cold-blooded,
while birds have high temperatures, is really no barrier
between the two classes. There was once a Python
at the Zoological Gardens which laid eggs, and for
six weeks sat upon them, at the end of which time
they were addled, or, at any rate, the young snakes
in them were dead. 1 But the python had not " sat "
in vain, for every day her temperature was taken by
experts, and also that of a male python hard by who
was subjected to the same cpnditions. The female had
an average temperature of 89-07° F., the male of
86-03° F. The maximum temperatures were for the
female 92-8° R, for the male 89-8°. But the great
difference between warm and cold-blooded animals
is that the former do not change their temperature
as that of the air changes. The female python was
once 16-7° warmer than the surrounding air, the male
never more than 1 1 -6°. In a similar case observed in the
Jardin des Plantes in 1861 the female's temperature
once rose 38-7° F. above that of her surroundings. She-
pythons, therefore, when incubating are not altogether
the sport of atmospheric changes. Even in mammals
temperature varies very much, the small ones having
as a rule the hottest blood. Those of large bulk
range between 97^° and 98!°. The sheep is said to
have a temperature of 104°. The Echidna sends the
thermometer only to 82 1°, and the Ornithorhyncus
only to 76 f. In birds the range is considerable,

1 Proceedings of the Zoological Society, 1 881, "The Incubating
Python," by W. A. Forbes.

iv CONNECTING LINKS 49

from slightly over ioo° F. to 1 12°. This highest figure
is attained by some Passerine birds : Hawks and their
allies arc never much above 109 , and Gulls rise only a
little above 104 . Whether these recorded temperatures
arc in every case quite exact may be doubted. When
the subject is a wild animal, the use of a clinical
thermometer is difficult. In the case of the python the
results arc quite dependable, though the thermometer,
laid between the folds, no doubt registered a lower
temperature than it would have in the mouth. With
other animals fresh experiments are needed. But
even as it is we may be quite certain that the figures
given are nearly right, though there may be an error
of a half or even a whole degree. The conclusions
we arrive at, then, are — (1) that there is no hard and
fast line to be drawn between warm and cold-blooded
animals, and, consequently, our warm-blooded birds
may be related to our cold-blooded reptiles ; (2) that
pterodactyls, in respect of their temperature, were
birdlike rather than reptilian. Before leaving this
subject, an objection raised by Sir Richard Owen
must be met. He maintained that pterodactyls must
have been cold-blooded since they had no feathers to
prevent the escape of heat from their bodies. But
the temperature of the body has comparatively little
to do with external coverings, and depends mainly
upon its power of generating heat and upon the
regulating apparatus by which it adapts itself to
changing conditions.

We will now go on to other points which prove the re-
lationship of pterodactyls to birds, or prove, at any rate,
that they have developed separately on very similar lines

E

50 THE STRUCTURE AND LIFE OF BIRDS chap.

(i) The head of a pterodactyl is put on at right
angles to its neck like a bird's. The head in most
reptiles continues the line of the neck.

(2) The sternum or breastbone is broad and has
a crest in the middle. This is just what might be
expected where great strength is required in the
fore limbs and the parts connected. The mole also
shows a great development of breastbone : for bur-
rowing this is as necessary as it is for flight.

(3) The scapula is thin and blade-like, and the
angle it makes with the coracoid is less than a right
angle.

(4) The ilium, the bone of the pelvis that unites
with the backbone, is produced both ways, in front
of and behind the thigh-joint. This is eminently
characteristic of birds.

Altogether the pterodactyl is so near to being a
bird that we must, before leaving the subject, briefly
show why he is after all a reptile. (1) The hand has
at least four fingers, all but the last of these bear-
ing claws ; (2) there are no feathers ; (3) the ischium
and pubis are at right angles to the ilium, instead of
running parallel as in birds ; (4) the pelvis is weak,
so that it is extremely unlikely that he could walk
upright. In spite of their presumable intelligence and
high temperature, in spite of their power of flight,
pterodactyls were still reptiles. Many species of them,
some not larger than sparrows, others with a span
of twenty-five feet from wing tip to wing tip, lived and
throve when reptiles were the dominant class upon
the earth, and, no doubt, they preyed upon lizards,
birds, and mammals.

[V

CONNECTING LINKS

51

At the same time there were Dinosaurs, or, as
Professor I luxlcy has called them, Ornithoscclidaj — i.e.,
bird-legged animals. With wonderful enterprise and
zeal, skeletons of these enormous animals, weighing

Fig. 19. — Dinosaur, Iguanodon ManteIli(from a photograph by DJlo of the specimen
in the Brussels Museum).

tons, have been collected by Professor Marsh and
his assistants in North America. Almost certainly
they had the power of walking upon their hind
legs, their tails helping to support them. In some
species, the nearer row of tarsals or ankle bones is

E 2

52 THE STRUCTURE AND LIFE OF BIRDS ch. iv

fused with the tibia, the stronger of the two leg bones.
All species seem to have been tending towards this
birdlike fusion. The pelvis is very like that of birds
in its form and in its strength. The ilium extends
far in front of and behind the thigh joint, and the two
other pelvic bones, the ischium and pubis, extend
downwards and backwards. If the pelvis of a dino-
saur and an emeu be put side by side, the resemblance
is most striking.

Had the pterodactyl had the legs and hind-quarters
of the dinosaur, it would have been still more birdlike
than it is.

Some of the Literature of the Subject.

Besides books mentioned at the end of Chapter II, a number
of papers by Professor H. G. Seeley, Hutchinson's Extinct
Monsters, and Huxley's Vertebrate Anatomy.

CHAPTER V

THE PROCESS OF CHANGE FROM A REPTILE TO A
BIRD

WE have now decided that birds have sprung from
some reptilian stock, though not from any existing
order of reptiles, and unless we are prepared to differ
from the great majority of biologists, we must hold that
this has been brought about mainly by the struggle for
existence. All animals multiply rapidly, and, if there
were no check, this would continue in geometrical
ratio, till there would be enough of a single species to
people all the earth. Thus, if one pair left two pairs
of young, these two would leave four pairs, and
those four eight, and so forth. Linnaeus calculated
that an annual plant producing two seeds in a year
would after twenty years have a million descendants.
And as every plant produces many more than two
seeds— a horse-chestnut tree, for instance, many thou-
sands—every real instance would be far more telling
than this imaginary one. The elephant has very few
young. Darwin's estimate allows it six in all, born
while it is between the ages of thirty and ninety,

54 THE STRUCTURE AND LIFE OF BIRDS chap.

its term of life being estimated at one hundred years.
Yet with these data he calculates that " after a period
of from 740 to 750 years there would be nearly
19,000,000 elephants alive, descended from the first
pair."

This marvellously rapid increase of all species was one
of the two cardinal facts on which his theory of the
origin of species was based. The other was the constant
tendency to variation. The progeny are very like,
but never exactly like their parents. He took his
instances mainly from the domesticated animals,
because sufficient evidence had not then been collected
from wild nature. All the domestic pigeons — the
Fantail, the Pouter, the Dragon, the Carrier, the
Homer, the Runt, etc. — had been derived from one
wild stock, the Rock Dove. The breeder had per-
formed equal wonders with cattle and horses, and
during the many thousands of years that the world had
been peopled with animals and plants, nature had been
doing what the breeder had begun to do only some
centuries ago. She had been constantly weeding out
those that were less fitted to live. The rocks bear
records of thousands of extinct species that have made
room for others. Darwin, as I have said, assumed that
variation occurred in wild species as among domestic
animals. But until this assumption had been proved
true, clearly the theory rested upon an insecure
foundation. Many observations have now been made,
and any one who wishes for a detailed account of them
may find it in Dr. Russel Wallace's Darwinism.
He shows conclusively that in wild species there are
two principles working side by side : the principles of

v CHANGE FROM A REPTILE TO A BIRD 55

heredity and variation. In form and character the off-
spring take after their parents, but in almost every case
there is some slight discernible difference. The in-
dividuals that have variations that fit them better for
life survive, those that have injurious, or, in some
cases, those that have only useless variations or none
at all, perish. Thus are produced new species, one
useful variation after another being accumulated by
Natural Selection. The sickly and those who are
unsuited to their surroundings have no chance, for
the law is mercifully ruthless.

The facts that I have stated seem to prove much.
The struggle for existence is indisputable, and evolu-
tion through Natural Selection seems almost beyond
dispute. But when we come to investigate more in
detail how it has acted, we are met with great diffi-
culties. A living man of science has said that for the
explanation of the brilliant colours of the butterfly
Darwin's theory is but a barren formula. It may
be that only his own imagination is barren. Later
on, in a chapter on colour and song, I hope to
show that colour is, at any rate, connected with
Natural Selection. The particular difficulty that
confronts us when we try to trace the evolution of
birds is that the development of wings would have
been useless, and worse than useless, unless accom-
panied by other changes. And just as he who builds
a Latin verse conceives a master stroke and puts into
his line, that before was tame and commonplace, a
piirpurens pannus, then suddenly, to his dismay, finds
that his brilliant emendation has ruined the grammar
and the sense, so we may imagine a reptile, that

56 THE STRUCTURE AND LIFE OF BIRDS chap.

hitherto had passed muster in a mediocre world, ruined
by the splendid acquirement of flight, unaccompanied
by the other variations without which it would be
indubitably fatal. What if wings had been fully
developed so that the fore limbs could no longer be
used in walking, while, as yet, the hind limbs
had not grown strong so as to make the quadruped
a biped ? What if the long legs, so necessary to
many wading birds, had not been matched by the
length of the neck ? How would such a Tantalus on
stilts have reached to the ground to get his food ?
What would the large expanse of wings have availed,
if the muscles to work the wings had not been de-
veloped in a corresponding degree ? How would even
the fully developed muscles have been equal to a strain
so hard, and often so prolonged, had not the heart
been so improved that the arterial and venous blood,
the fresh and the exhausted, could be kept separate ?
And what would have been the use of a first-rate heart
without first-rate lungs to aerate the blood that was
to feed the whole body ? And without an excellent
digestive apparatus how could any other part of the
system be vigorous ? And without the power of
sitting firmly on an elevated perch when asleep, the
newly-gained power of flight, the dismay of all enemies
during the clay, might only have put off the hour of
capture and destruction till the night. This is one of
the greatest difficulties which the theory of evolution
presents, but it is not insurmountable. To begin with,
variations are almost always small. We must not
imagine the sudden development of a perfect wing.
Moreover, slight variations are perpetually occurring ;

v CHANGE FROM A REPTILE TO A BIRD 57

it cannot be too strongly insisted upon that they arc
not occasional but unceasing, so that it is highly
probable that two, that would be useless unless they
appeared simultaneously, might occur together in the
same individual. It is only reasonable to suppose
that some of the reptiles from which our birds have
sprung were born not only with a fore limb that,
fringed with scale-like feathers, might act as a make-
shift for a wing, but also, by a happy coincidence, with
hind legs stronger than those of their contemporaries.
Quite apart from such coincidences, a very slight
power of flight, due to a modification of the fore limb
not sufficient to incapacitate it for walking, would be
highly advantageous to the birdlike reptile, or reptile-
like bird. When menaced by a snake as he sat upon
a tree, he would flutter to another tree, perhaps feebly,
descending much as he went, his wings acting as a
clumsy parachute. Still, he would save his life. Here,
then, is a stage in advance accomplished. Why
not after this a development of the hind limbs, making
an upright posture possible ? And when this was
attained, why not a further development of wings ?
And why should not an improvement in the internal
organs follow closely upon the visible changes ?
The fact is that in this case it does not seem necessary
to assume an absolute simultaneity of variations.

But supposing it is held by any one that modifica-
tions arising singly could not have advanced the
reptile to a bird, and that for simultaneous variations
we must not trust to mere coincidence, we must appeal
to what is called correlated variation. Many instances
of this are given by Darwin, Cats, which are entirely

58 THE STRUCTURE AND LIFE OF BIRDS chap.

white and have blue eyes, are generally deaf. Pigeons
with short beaks have small feet, and those with long
beaks large feet. In wild animals the right and left
sides always vary very nearly in the same way. The
front and hind limbs often vary together, and even the
jaws and limbs. He maintains that parts which are
homologous — for instance, arms and legs — often show
similar tendencies. Length of arm in men generally
goes with height, and if a child has long hands, people
infer that it will be tall. Possibly, then, through corre-
lation the lengthening and strengthening of the fore limb
might be accompanied by a lengthening and strengthen-
ing of the hind limb. Of course, it cannot be looked upon
as an invariable law that variation follows the lines of
homology. The fore legs of a giraffe have altogether
dwarfed the hind legs. The tendency of such varia-
tion has been to lift the giraffe's head higher, so that
in times of drought, when other cattle were dying of
hunger, he might browse on the higher branches of
the trees. A corresponding growth of the hind legs
would have been waste of material, and possibly this
may have caused the weeding out of those whose hind
legs developed pari passu with their fore legs.

The difficulty of tracing back the course of develop-
ment is undeniably great in the case of any animal, and
he who attempts it is apt to lay himself open to ridi-
cule. The cautious exponent of evolution takes refuge
in generalities. It is more ingenuous, and in every way
better, to face the difficulties, while at the same time
confessing that much more evidence is wanted from
fossil remains before we can fill in the blanks. At
present the position of the evolutionist is somewhat,

v CHANGE FROM A REPTILE TO A BIRD 59

but by no means entirely, like that of a man who, know-
ing nothing of the facts of English history, should
attempt to infer them from the character and peculiar-
ities of our existing institutions. Many and ludicrous
would be his errors. The evolutionist is saved from
many mistakes by the geological record, which, how-
ever fragmentary, is a safe guide as far as it goes.
And if Sir Richard Owen, when presented with a
single bone from New Zealand, was able to some
extent to describe the giant bird of which it had
once formed part, is it not possible that, by the help
of animals, fossil or still existing, evolutionists have
drawn a picture of the primitive ancestors of our
present species that is at any rate not far removed
from the truth ?

Books on the Subject.

Darwin's Origin of Species.

Wallace's Darwinism.

Weissmann's Essays on Her edify, and Romanes Lecture.

Miss Buckley's Wiiiners in Lifers Race.

(The literature of the subject is endless.)

CHAPTER VI

FORM AND FUNCTION

Digestive Apparatus

WHAT is the cause of the wonderful vitality of
birds ? How is it that the Golden-crested Wren,
apparently so weak and helpless, can fly all across
the North Sea from Norway ? What are the pro-
cesses of life that go on within the bird and make
it so different from its lethargic reptilian ancestors ?
To these questions I hope to give some answer in
the present chapter.

To begin with, a bird has a very large appetite, and
a reptile a very small one. I have found twenty-two
acorns in the crop of an unusually small wood-pigeon,
and this was probably quite an ordinary meal to him. 1
They had not made him torpid, like a boa -constrictor
after his weekly rabbit. He was flying with all his
usual vigour when the shot brought him down. To
speak of an animal as an engine, the supplies of fuel

1 As many as sixty-three have been found. See Badminton
Library, Shootings p. 229.

CH. vi FORM AND FUNCTION 61

must be large and pretty constant if much work is to
be done. Little appetite, little energy, is a rule that
holds throughout nature. In his book on the Crayfish,
Professor Huxley has a very instructive illustration of
what life is. Pie compares a living creature to a wave
in a river which remains always in the same place,
being caused by a rock, or something of the kind,
near the surface. A still more striking illustration of
the same thing is a jet of water in a cataract which,
except for slight variations, always keeps the same
shape. The wave and the jet of water arc at no two
moments that you look at them made up of the same
materials. Every moment a fresh supply of water as
it reaches the same point assumes the same shape and
appearance. So it is with the living creature : he may
look the same from year to year, but the atoms of
which he is built up are not the same. And if he is
to be vigorous, an animal must change his constituent
atoms rapidly. The large appetite, therefore, of a
bird is to be looked upon as a proof of strength and
energy. Of course, the appetite alone, without pro-
portionate digestive power, would be worse than
useless. The apparatus of digestion must be first-rate,
and to the investigation of this apparatus we must
now proceed.

In man the saliva plays an important part. In
birds, however, the glands which secrete it are small,
and the secretion from them, probably, has but little
chemical effect upon the food, only helping to
soften it. Though the small development of the
saliva glands is their chief feature, they vary in
size in different birds, those of the Woodpecker being

62 THE STRUCTURE AND LIFE OF BIRDS chap.

comparatively big. 1 In all birds the gullet or
oesophagus is large. In many, especially in seed-
eaters, it opens out into a great expansion, with
thickened walls, called the crop, which reaches a
high development in the pigeon. In this bird it is
marked by irregular ridges, and in the breeding season
the cells of the mucous membrane that line it give off
the peculiar cheesy substance known as " pigeon's
milk," with which the young are fed. The crop secretes
no special digestive fluid : it is mainly a storehouse
in which the food is kept till the stomach is ready to
deal with it. The glands found there are only the
ordinary glands of the mucous membrane. Still it
must not be supposed that the food which passes
from the crop is in the same condition as that which
enters it. During its stay there it is acted on by what
saliva has been shot upon it, by water, by the watery
secretion of the mucous membrane, and by the warmth
of the body. Though the crop is not nearly so much
developed in birds of pre\ , yet in some of them it has
been found equal to hard work. In owls, for instance,
the contraction of the walls strips the skin off their
prey after the under-skin has been weakened by the
secretions, and then the well-known pellets consisting
of hair, feathers, and bones are thrown up. 2 The
South American bird, the Hoatzin, so remarkable for
the two claws on each of its wings and for having the

1 His tongue, which he can shoot out almost as far as a
chameleon shoots his, is armed with backward-pointing bristles,
and the sticky saliva poured upon it adapts it still further for
fishing out insects from under bark.

2 See Bronn"s Thier-Reich, vol. Ai'tS, p. 672.

vi FORM AND FUNCTION 63

keel developed only on the hinder part of its breast-
bone, is remarkable also for its highly muscular crop
with furrows and ridges 3 by means of which it squeezes
out the juices of leaves. In some birds the crop is
altogether wanting. In Cormorants, Flamingoes, and
Pelicans only a very small expansion, that might
easily escape notice, has been found. In all fish-
eaters it is either, as in those just mentioned, very
slightly developed or non-existent. The two most
striking points about a Cormorant's gullet are its
great size and its elasticity. Just below the mouth it
opens out to form a spacious pouch with very thin
walls. Below that it narrows but very slightly before
there comes the very small expansion representing the
crop, and its walls are there just a trifle thicker.
When the bird is lucky enough to secure a long and
thick fish this great tube of nearly uniform size is
ready to receive it. There is no contraction at any
point sufficient to hinder its downward progress. A
crop narrowing down at its lower end to a small tube
formed of strong walls would be out of the question in
a fish-eater. In diving-birds, fish have been found
with their heads partly digested, while upon their
bodies, which had not yet reached the stomach, the
process had not yet begun. This must inevitably be
so, since nothing is more remarkable than the narrow-
ness of the band within the area of which lie the
digestive glands : a fish of any length cannot possibly
come under the influence of the juices all at once.
Nearly all the stowage room for the cormorant's
large meals is in the ample gullet, and great demands
are made upon it. One of these huge feeders was

64 THE STRUCTURE AND LIFE OF BIRDS chap.

once watched by an apparently trustworthy observer,
at a repast which lasted for an hour and a half. Each
time that the bird rose after diving, he saw the flash
of a small fish and the jerk of the neck with which it
was swallowed. And the total number of fish dis-
posed of he estimated at one hundred and eighty.
However small they may have been, it must have
required a very large gullet to accommodate them.

The stomach has two compartments, very different
in their structure and function. There is the fore part,
or Proventriculus, which is highly glandular, and the
hinder part, which has no glands, and to which, when
it is very muscular, as it is in many birds, the name
of gizzard is given. The proventriculus secretes
strong acid juices from its glands, and some kinds
of food, such as meat, may, certainly in mammals.
probably in birds, be partly absorbed here, the
peptone, as it is called, that is formed from them,
passing into the blood vessels that are separated only
by a very thin membrane from the cavity of the
stomach. It has been thought that the process is that
called Osmosis, which is as easy to illustrate as it is
difficult to explain. If a bladder containing peptone
be held under water, a large quantity of the peptone
will make its way out into the water, while the bladder
will be distended by the water which has made its
way in. Peptone is quite exceptional among solu-
tions of organic matter in the readiness with which it
passes through the walls of a bladder. White of egg,
for instance, has been tried in the same way, and
hardly any of it has escaped.

The absorption of food into the blood-vessels is a

vi FORM AND FUNCTION 6$

process quite different from Osmosis. The living
membrane has a power of selection : it is like a sieve
which can let big molecules pass, while it can reject
smaller ones. Each cell seems, like an Amoeba (of
which more presently), to have the power of choosing
out and swallowing what it wants. In the same way
plants select their food from the ground. Much of
the everyday work of nature is too subtle for science
to explain.

When the food has penetrated into the blood-vessels
it is no longer a foreign substance, but having been
thoroughly assimilated has become part of the bird
itself. As a rule, however, the process of assimilation
is not completed in the proventriculus. The food
passes on to the second compartment of the stomach,
the walls of which, in seed-eating birds especially, are
very thick and strong, being formed of muscular fibres
which radiate out from two tendons running down the
centre of each side. No less powerful mill would be
equal to the grinding of acorns, and even this would
be insufficient did not the bird swallow stones which,
like molar teeth, break up the food as the muscles
contract and relax. So necessary are such molars,
that where no stones arc to be had birds have been
known to swallow hard stonelike seeds, for instance
those of the wild prairie rose (Rosa Blanda) which
fulfil the same purpose. I have seen stones of porten-
tous size which had been taken from the gizzard of an
Emeu. In birds which live on flesh the walls of the
stomach are very weak, so that it does not deserve the
name of a gizzard and, moreover, no stones are
swallowed, nothing of the nature of teeth being

66 THE STRUCTURE AND LIFE OF BIRDS chap.

necessary. Even- one is familiar with the toughness
and solidity of a chicken's gizzard. When the
stomach of a Hawk or Cormorant is set beside it, the
contrast is very striking.

In some birds, all of them fish-eaters, the stomach
has a small third compartment posterior to the
gizzard. The entrance to it is guarded by a flap of
skin or, in the case of the Darter, it is furnished with
thick hairlike formations which at the entrance are
especially long. The purpose both of the flap of skin
and of the hairlike growths seems to be to shut out
all food that has not become thoroughly fluid. 1

On leaving the second compartment of the stomach,
or the third compartment if there is one, the food
passes into the smaller intestine, where the process of
digestion is completed. The duodenum, the first part
of the smaller intestine, is a U-shaped loop, and in it
lies a great whitish gland called the Pancreas. The
liver is a much greater gland within the two lobes of
which lie the hinder part of the proventriculus and the
fore part of the gizzard. The liver and the pancreas
both pour upon the food in the duodenum the juices of
digestion. The work of the pancreatic juice is, mainly, to
break up starch and convert it into sugar, since starch
as long as it remains starch is of no use to the body as
food, and to emulsify fat, i.e. to dissolve it into fine
globules. The bile — the juice secreted by the liver —
is slightly alkaline and extremely bitter. It is im-
possible here to describe its exact working. When the
bile and the pancreatic juice have together done their

1 See Bronn's Thier-Reich^ vol. " Aves," p. 609. There Dr.
Gadow speaks of it as the Pylorus Magen

vi FORM AND FUNCTION 67

work, the process of digestion is complete, and the
food goes to build up the living animal. The wonder-
ful process mentioned above is in full swing here ;
the chyle, that part of the food which consists of
emulsified fats, passes into the lacteals (to be described
later on) through the villi, small creases in the coat of
the intestine, the rest into the blood-vessels, and so, in
either case, on to the heart. All refuse is carried into
the large intestine, any return from which is prevented
by a valve.

Before leaving the subject of digestion, I wish
to show how important an organ the liver is to birds.
For purposes of flight their weight is reduced as much
as possible, but in some good flyers the liver is ex-
traordinarily heavy. In the Tern it is ^ of that of
the whole bird, in the Swallow T V, in Yanellus
Cristatus, a kind of Lapwing, ^, in the Smew Jy. In
all these it forms a far larger fraction of the whole
than in man. And how account for the great differ-
ences ? The Smew is mainly a fish-eater and also the
Tern. But in the fish-eating Heron the liver is said
to be remarkably small. In the common Fowl it
contributes rather less than Y \ of the total weight,
suggesting that seed-eaters depend comparatively-
little on the liver and more upon the gizzard. But
in corn-eating pigeons we find both large gizzards and
large livers. And the liver of the flesh-eating gizzard-
less Kestrel is lighter than that of the Fowl, only T .V, in
fact, of his total weight, while in the Tawny Owl it is
less than J^.. These facts are very perplexing. Dr.
Gadow in quoting them remarks that they are un-
trustworthy, since thev must be affected by the con-

j '

68 THE STRUCTURE AND LIFE OF BIRDS chap.

dition and recent diet of the bird. But, even when we
allow for inaccuracies due to this, the differences are
startling. 1

The Heart and Circulation.

Every part of the body is nourished by the blood.
Only through the agency of the blood can food and air
make good what is lost by wear and tear.

The heart is a force-pump which drives the blood
to all parts of the body, and, when it returns impure
and loaded with used-up material, sends it to the
lungs to be purified, after which it is despatched all
over the body again. On the voyage much of it
passes through the kidneys, which help the lungs to
purge it of the waste of the tissues. The essentials of
an efficient heart are that it should be strong, and that
it should keep the pure blood separate from the impure.
These two essentials are found combined in the hearts
of mammals and birds. They are strong muscles :
that part at least of them which forces the blood
through the arteries is remarkable for its strong
thick walls. And, thanks to the perfection of the
machinery, the blood which has been purified in the
lungs is never mixed with the impure blood which is
coming from the body.

The heart is divided into right and left chambers
by a division through which there are no doorways.
The right and left chambers are each divided into two,

1 See Bronn's Thier-Reich, vol. "Aves," p. 68 1. The figures,
as I have quoted them, are very nearly exact. For simplicity I
have disregarded small fractions.

vi FORM AND FUNCTION 69

but there arc openings from the upper into the lower
which may be closed by valves. The two lower
chambers are called ventricles and the two upper ones
auricles. Before explaining the working of the
valves, I shall trace the circulation of the blood. The
left ventricle, which is the strongest and most muscular
part of the heart, opens into the aorta, the largest of
all the arteries. Thence it is distributed into branch
arteries and from these into smaller branches : these,
in turn, lead into smaller channels called capillaries,
varying in diameter, in man, from ^Vo to T gVo °f an
inch. It is when it reaches these extremely minute
vessels that the blood does its work of nourishing all
the tissues of the body. The capillaries unite to form
larger vessels called veins, and these finally form two
great trunk veins which carry the blood into the right
auricle. From the right auricle it passes to the right
ventricle. Thence it is driven into the lungs, from the
lungs it passes into the left auricle, and thence into
the left ventricle where the same process begins
again. Thus the blood in , the right chambers of
the heart can reach the left only through the
lungs : that in the left can find its way to the
right only through the arteries and veins of the
body. The pure arterial blood is all on the left side,
the impure venous blood on the right. The former
may be known by its bright red colour, the latter is
blue-black. The following diagram will make clear
the course of circulation.

When the blood has passed through the arteries
into the capillaries and from them into the veins, it
finds a new contrivance to assist in driving it on. In

jo THE STRUCTURE AND LIFE OF BIRDS chap.

vi FORM AND FUNCTION 71

the veins of the limbs arc valves which prevent any
backward flow. Every movement must tend by
pressure to move the blood forward or backward, and
it will be urged forward since no other course is open.
The places where these valves are in the human arm or
hand can be seen if a finger be pressed upon a vein
and then passed downwards along it in the direction
of the capillaries, thus tending to cause a backward
current. Little knots will be seen at intervals,
marking the places where the passage of the blood
is checked by the pouchlike valves. Birds have
fewer of these valves than mammals, but more than
reptiles.

Besides the veins there are other channels in all
parts of the body along which a current is setting
towards the heart. These are the lymphatics, so
called because they contain a pale watery fluid. They
differ from veins (1) in that the capillaries from which
they spring end blindly, i.e. do not connect with
arteries, (2) in having in their course numbers of
glands through which their contents must pass. Their
main function seems to be to assist the veins in
carrying on the drainage of the body. Some of the
lymphatics, however, have a special function and a
special name. They are called lacteals from the
milky nature of their contents, due to food containing
fat, and their duty is to carry the chyle to the heart
from the smaller intestine round which their capillaries
form a network. Like the veins they are provided
with frequent valves preventing any backward flow*
and eventually they pour their contents, in birds, into
the two trunk veins which bring; the blood from the

72 THE STRUCTURE AND LIFE OF BIRDS chap.

right and left sides of the head, in man into that from
the left side only.

The great difference (its significance will be made
clear in chap, viii.) between the circulatory systems of
birds and mammals is this — in mammals the aorta
arches over to the left, in birds to the right.

TJie Valves of the Heart.

When the black blood is discharged into the heart,
it has to be sent to the lungs to be purified and to be
recharged with oxygen, and the heart by contracting
drives it into the pulmonary artery, i.e. the artery
which leads to the lungs. But unless there is some
means of preventing it, obviously the blood will be
driven not only to the lungs, but back into the vein
which has just carried it to the heart. The right
side of the heart, therefore, is divided into two
chambers, the passage between them being guarded
by a valve which allows the blood to pass from the
upper chamber to the lower, but not from the lower
to the upper. In the bird this valve is simply a flap
of muscle which projects into the ventricle, and which
closes the aperture when it is lifted by a rush of blood
upwards. In man and other mammals the valve is
formed of thin membrane instead of muscle, and
consists of three flaps connected with one another and
fastened by strings of tendon to the walls of the heart
below. This is called the tricuspid valve. It is very
curious that the bird's heart should be in most respects
so similar to that of man, in this respect so different.
These valves remain to show that the two highest

VI

FORM AND FUNCTION

73

classes of animals, mammals and birds, have each
separately developed a perfect type of heart from
some lower form which allowed the pure and impure
blood to mix. On the left side of the heart also the
passage between the upper and lower cavities is
guarded by a valve. Both in birds and mammals it
is formed of two membranous flaps fastened to the
walls below by strong cords of the nature of tendons.

■ M^

TV -

Fig. 21. — (it) Bird's heart showing valve between right auricle and right ventricle.
(b) — modified from Quain — Man's heart showing the same, lv, wall of left ventricle ;
RV, right ventricle ; TV, tricuspid valve ; v, valve.

In the human heart it is called the mitral valve from
its fancied resemblance to a bishop's mitre. There
are other valves as well without which the heart would
be very imperfect. There must be some means of pre-
venting the blood when it is driven into the two great
arteries, the aorta and the pulmonary, from returning
to the heart. The entrance to each, therefore, is
guarded by three " semilunar valves," little pockets
which look outwards, away from the heart, and, con-

74 THE STRUCTURE AND LIFE OF BIRDS chap.

sequently, close against an inrush of blood, but allow
an outrush to pass. In a bird of any size they are
easy to see.

Some account of a reptile's heart will be found in
the chapter on " The Bird within the Egg."

The Blood.

Cut off the supply of blood from a limb, and all its
power goes. The muscles lose their sensitiveness to
stimulus, and eventually rigor mortis, the stiffness of
death, sets in. The life of the whole organism and
of each of its parts depends upon the blood. The
Jew, who will not eat blood " because it is the life,"
has dimly seen an important physiological fact.

Blood consists of corpuscles of two kinds, the red and
the colourless, commonly called white, and the liquid
plasma in which these float. The corpuscles are very
minute. It is said that 10,000,000 of the red ones will
lie on a space of one square inch. Among birds the
Cassowary has the largest, the Humming Bird the
smallest. The colourless ones also are always ex-
tremely small, though they vary much in size. In
shape the red ones, seen under the microscope, are,
in birds and reptiles, oval ; in man, round. And
the shape is not the only difference. In birds, reptiles,
and fishes there is. a nucleus, a small roundish body in
the middle. In mammals, except in the case of em-
bryos, no sign of this is, as a rule, to be found. Why
this nucleus has disappeared, no one has been able to
show. It certainly cannot be maintained that there
is any superior vitality which we can associate with its

VI

FORM AND FUNCTION

75

disappearance. It is easy to connect the reptile's low
temperature with his poverty in corpuscles.

The white blood corpuscle is an object of extreme
interest. It is simply a cell of granular appearance
with a nucleus. In shape it is roundish, but when
alive it is a perfect Proteus. If a drop of blood be

b c

Fig 22.— («) and (b) White and red corpuscles of man from Quain after Schafer : (c)
and (J) red corpuscles of Humming Bird and Ostrich after Gulliver; (*) Amoeba
after Moore. All to same scale.

taken from the finger and put under the microscope
the white corpuscles are easy to make out, scattered
in comparatively small numbers among the red, but,
unfortunately, they have ceased to move, the change
of temperature, and conditions generally, having
killed them. But there are one-celled creatures to
be found in stagnant water which closely resemble

76 THE STRUCTURE AND LIFE OF BIRDS chap.

them. These are called Amoebae from their incessant
restlessness. At one moment they may be round,
the next they throw out a limb on one side, the next
that limb is withdrawn and another thrown out else-
where. Wherever food comes in contact with them
they make a mouth and swallow it. The colourless
corpuscle is one of these simplest of creatures, lead-
ing a life of its own within the blood-vessels, but
dependent on the body for the conditions which make
life possible to it. There are several forms of
them, and some, it is believed, do us the priceless
service of swallowing the germs of diseases that find
their way into the blood. The bacillus that has
survived immersion in the strong acid juices of the
stomach is killed, so it is believed, by these small and
half independent organisms. Whether this is so or
not, it is certain that when the blood is thick with
corpuscles, red and white, there is less liability to
disease.

The red corpuscles carry a great deal of oxygen,
and thus they are able to oxidise the tissues, i.e. burn
them, for ordinary burning is only a rapid process of
oxidation. If the supply of oxygen is cut off from
it, a fire at once goes out. The oxygen in the blood
keeps up the warmth of the body by slowly burning it.
And in birds, with their very high temperature, the
process is more rapid than in other warm-blooded
animals. The redness of arterial blood is due entirely
to the pigment haemoglobin in the red corpuscles.
When they lose their oxygen it can be proved by
experiment that they become black, the colour of
venous blood.

vi FORM AND FUNCTION 77

The duty, almost the sole duty, of the red corpuscles,
is to carry oxygen. It is the work of the colourless
plasma to bring food to each part and to carry off the
used-up material. The carbonic acid, which is pro-
duced by the burning of the tissues, is probably
removed, not by the corpuscles, but by the fluid
in which they float. At the same time the plasma is
busy with other work which falls mainly upon it, the
work of carrying in all directions the food-materials
which have entered the body, and thus what has
been destroyed is rebuilt. It is probable that the
various components of the blood divide their functions
in the way I have described, but it is quite possible
that further investigation may show that the foregoing
account requires some modification.

In what part of the body do the corpuscles originate ?
In the lymphatic glands corpuscles very similar to, if
not identical with, ordinary white corpuscles have
been found in process of dividing into two. And it is
thought that it is in these glands that the white or
colourless kind are produced.

In embryos, and, on occasion, in adults, the spleen, a
small red body which can be found in birds attached to
the right side of the fore-stomach (Proventriculus)
certainly gives birth to many red corpuscles. In the
marrow of human bones corpuscles are found inter-
mediate in character between the white and the red,
like the former possessing a nucleus, but like the
latter having a little of the red haemoglobin, and these
it seems are somehow transformed into ordinary red
corpuscles.

78 THE STRUCTURE AND LIFE OF BIRDS chap.

Breathing Apparatus and Pneumatic Bones.

A bird's breathing apparatus is of the first order.
When a lark is rising, his wings are beating at the
rate of quite two hundred strokes per minute, probably
much faster. And yet all the while he sings as if he
were making no great muscular effort. I recommend
any one who does not appreciate the marvel of this to
try to run up hill and sing or shout at the same time.
Of all those who make this experiment we may quote
what Vergil says of the Greek ghosts in Hades, who
try to raise their war-cry when yEneas appears : —

Inceptus clamor frustratur hiantes.

On the floor of the mouth just behind the tongue is
the glottis or opening into the trachea or windpipe,
a tube formed of rings of bone and gristle which runs
along the neck to its base, where it divides into two
bronchi, which lead, one to the right, the other to the
left lung. The epiglottis, which springs from the
anterior end of the glottis, and the function of which
in mammals is to close the opening during the process
of swallowing, is very little, if at all, developed in
birds. Apparently the edges of the larynx, the name
given to the upper end of the trachea, meet so exactly
that no epiglottis or lid to the glottis is necessary.
The larynx has no vocal chords as in mammals, hence
it cannot produce voice, but only raise or lower a note
by bringing together or separating the stiff margins of
the glottis. The organ of voice is the lower larynx
or syrinx, an organ found in no other class of animals,

v , FORM AND FUNCTION 79

situated where the trachea divides to form the bronchi.
I shall describe it later on.

Though a bird has such a splendid "wind," his lungs
are small. They will be found lying close against the
back, and, if the body is laid down breast uppermost,
under the heart and liver. They extend from the
first rib to where the kidneys begin, and may easily
be known by their sponginess and their scarlet colour.
It is difficult to measure them exactly, but these are
the measurements as nearly as I could take them
in a common domestic pigeon: length if inch, depth
§ inch, breadth | inch. This gives for cubic content
| inch, for the two lungs together \ inch. There is
no reason to suppose that in a Homer pigeon the
dimensions are appreciably larger. These small lungs
are a wonderful feature in a bird, to whom, under
favourable conditions, a flight of fifty miles in an
hour is no great exertion. In all birds we find
the same striking contrast between the excellence
and the small size of the lungs. Though they are
spongy, they have but little elasticity. When a man
expands his chest, the lungs are distended and the air
rushes in to fill the vacuum caused. A bird's lungs
vary little in size. They are prolonged into spacious
air-sacks, the most characteristic part of the breath-
ing apparatus, which renders elasticity of lungs
unnecessary. These air-sacks are extensions of the
membrane which forms the walls of the bronchi.
The two bronchi we have already described as
leading to the lungs. They run through the lungs
dividing as they go, and end in these great ex-
pansions by the help of which a bird is able to get

8o THE STRUCTURE AND LIFE OF BIRDS chap.

more work, in proportion to the size of his body, out
of his exceedingly small and light lungs than a man
can out of his far heavier apparatus. Any one who

ICS

Fig. 23 — Diagram after Heider. Air-sacks excepting the cervical. The lungs are
shaded dark. Ahs, abdominal sack ; as, anterior thoracic ; B, entrance of bronchial
membrane; H, humerus; ics, interclavicular sack, surrounding trachea; and 1, 2,
3, 4, its extensions ; 2 opens between the pectoral muscles ; rs, posterior thoracic
sack ; t, trachea. See Fig. 25.

wishes to see the air-sacks — and to see them is much
better than only to read of them — should take some
bird of moderate size, such as a pigeon, cut through
the windpipe somewhere in the neck, insert a blowing-

vi FORM AND FUNCTION Si

tube and tic the windpipe round it with a piece of
fine string or cotton, then inflate them. The whole
breast and abdomen will be seen to rise and expand.
The windpipe should then be tied up and the air-
sacks left in a state of inflation. Next the central
part of the sternum must be got out of the way. Cut
it longitudinally on either side of the keel from the
hinder almost to the anterior end. After that remove
the viscera very carefully, when the extremely delicate
membrane which forms the sacks may be seen, and
also the scarlet sponge of' the lungs at the bottom
of them (the bird lying on its back), and the openings
of the bronchi into the sacks. There are nine sacks
in all, four on each side, and another pair which have
run into one. The hindmost or abdominal pair
are very large, and, when the bird is placed upon its
back, lie over the kidneys and under the intestines,
extending far back behind the lungs. In front
of them are the posterior thoracic, and next to
them the anterior thoracic, sacks. Then comes the
interclavicular sack formed of two which have coa-
lesced. The middle part of this can easily be seen
in the angle between the clavicles or wishbones,
but it also runs out on either side to the shoulder
bones (Fig. 23). The cervical sacks are very small
and lie at the base of the neck.

As yet I have only described the minimum of air
sacks common to all birds : in many species there are
air cavities in the bones, sometimes extending even to
the very extremities of the limbs : in some they are
found under the skin also, and even in some of the
feathers and between the muscles. In a young bird

G

82 THE STRUCTURE AND LIFE OF BIRDS CHAP.

the bones are always filled with marrow, but often as
it grows to maturity the marrow is absorbed, leaving
only a thin dry-looking lining, and the delicate mem-
brane of the air-sacks extends into the cavity. Thus

Fig. 24.— Section of (a) femur of Ostrich ; (l>) skull of Carinate Bird. E, external
opening to ear. The bronchial membrane lines all the small cavities in the bones.

whenever a bone is hollow (if we except certain
parts of the skull), the cavity connects with the lungs
and is lined with the bronchial membrane. When the
cavity in the bone is large, thin plates separate from

vi FORM AND FUNCTION g 3

the inner coat and act as buttresses. Sometimes these
buttresses are bound together and a strong network is
formed. It is a network like this which supports the
beak. In the skull the plates take the form of arches.
In all birds without exception, I believe, some of the
bones of the skull are aerated, the air being derived
mainly from the nostrils and ears. But the beak and
some of the bones connected with it are aerated from
the lungs. Thither runs from each cervical air-sack a
small tube of membrane which lies in an incomplete
bony canal under the vertebrae by the side of the
vertebral artery. On its way to the beak it throws off
branches to the vertebra? of the neck. Every aerated
bone has a foramen or aperture through which the bag
of membrane finds its way. In the humerus it is easy
to find at the end near the body on what is properly the
upper side of the bone, but which in the bird's wing,
when it is folded, looks backward. The interclavicular
sack opens into it. Of all the long bones the humerus
is most commonly pneumatic. An easy and interest-
ing experiment is to tie up the windpipe of a dead bird,
then break the humerus and blow down it through a
blowing tube, when the sacks will at once inflate.
Indeed wounded birds when their windpipes have been
choked with blood have been known to breathe through
a broken humerus that has pierced the skin. Other
bones that are frequently aerated are the breast-
bones, the coracoid, the vertebrae, less frequently the
thighbone, shoulder-blade and merrythought. But a
good many birds are, as I have said, pneumatic to the
very extremities, the Hornbills and the Screamers to
the ends of the fingers and toes. The Gannet has

G 2

84 THE STRUCTURE AND LIFE OF BIRDS chap.

large air chambers under the skin, and when these are
filled it floats like a bladder on the surface.

The problems connected with the lungs and their
extensions are many and difficult, and I shall devote
to them a separate division of this chapter (see p. 105).

The Process of Breathing.

It will be well first to say something about the
mechanism of breathing in man, and then show how
different it is in birds. A man creates the vacuum
within him which the air rushes in to fill partly by
means of the diaphragm, partly by means of the ribs.
The diaphragm is a partition which separates the
cavity which contains the heart and lungs from that
which contains the intestines. Muscles descend from
it to the ribs and stronger ones to the spinal column.
When these muscles contract, the lung chamber is
enlarged, a vacuum is created, the air rushes in and dis-
tends the lungs. Diaphragmal breathing is impossible
to a bird since it has no fully developed diaphragm.
Indeed the oblique septum, to which the name of
diaphragm is often given is apparently so different in
its nature and situation that it has been doubted
whether we can regard it as the same organ as the
diaphragm of mammals. Its arrangement is very
complicated. One part lies on the under surface of
the lungs and under the cervical air-sacks which, thus,
are in a chamber by themselves. The other, the
entirely membranous and oblique part, at its anterior
end connects with the former along the line of the
backbone ; further back it springs from the pelvis.

VI

FORM AND FUNCTION

85

Fig. 25. — Diaphragm of Duck (diagram after Strasser). The sternum has
been removed.
Abs, abdominal. sack ; it runs back under the viscera and opens into the lung ;
sometimes penetrates the pelvis and femur : as, anterior thoracic sack ; axs, axillary
sack, a pouch of the interclavicular ; cs, cervical sack ; D, diaphragm ; g, gizzard ;
Gi», great pectoral muscle turned back ; h, heart ; 1, intestine ; ics, interclavicular
sack, often pn-trates keel ; Lg, lung ; Lg', line to which the lungs reach ; l1 and Lr,
left and right lobes of liver ; ps, posterior thoracic sack ; t, trachea.

86 THE STRUCTURE AND LIFE OF BIRDS chap.

It forms on cither side a sheet which slopes outwards
and fastens to the sternum near to its junction with
the ribs. Cross-partitions divide the chambers formed
between it and the body wall : into these compartments
the air-sacks enter, their walls being so thin that it is
difficult to separate them from those of the chambers
which they line. The diaphragm does not, as in mam-
mals, separate the heart from the intestines. The dia-
gram on page 85 will help to make this description clear.
Obviously a membranous partition like this cannot
do the work of the diaphragm of mammals, but
that it is homologous to it, i.e., the same in origin,
may well be maintained. The difference of position
does not disprove this, for it is well known that a
muscle may shift its point of attachment so that upon
such a question as the nearness of the relationship of
reptiles to birds the evidence of muscles docs not
count for much. Nor is the fact that the diaphragm is
muscular in mammals and membranous in birds in any
way conclusive. In the apteryx it is strong and fibrous.
In a puffin Mr. Beddard found it muscular, and I
myself found it very highly so in another bird of the
same species. Tendon in fact often replaces muscle.
It is certainly possible, on the whole it seems probable,
that the diaphragm in mammals and birds may be the
same in origin though different in function. 1 It is in-
teresting to find that crocodiles that come near to
birds in so many points, are like them also in having
an oblique septum.

1 See Huxley on " Breathing Apparatus of Apteryx," Proc.
Zool. Soc, 1882: article on the "Diaphragm" in Newton's
Dictionary of Birds.

VI

FORM AND FUNCTION

87

man

To return to the subject of breathing,
breathes not only by means of the diaphragm, but by
raising the ribs. This is effected by means of muscles
called the external intcrcostals, which pass downwards
and outwards from each rib to the one below it. The
contraction of these muscles will raise both ribs, as
may be shown by an easy experiment. Take two
thin rods of deal and screw them on to a third piece,
nail a fourth piece to their other ends to keep them
parallel/ Join them by an india rubber band, sloping

Fig. 26. . , . ,. ,,

R K represent ribs ; B the backbone ; S the breastbone ; E is the india-rubber
band representing the external intercostal muscle.

downwards and outwards and too short to reach
without stretching. This will raise the two ribs.
There are muscles fastened to the inside of the ribs
and from that called the Internal Intercostals which
slope downwards and inwards towards the backbone,
and therefore act just in the opposite way— i.e., they
lower the ribs. When a bird is standing or walking, the
breast rises and falls in breathing much in the same
way as it does in man, though the great weight of the

88 THE STRUCTURE AND LIFE OF BIRDS chap.

breast muscles must make it move less easily. Owing
to the thicker coating of feathers it is difficult to see the
movements clearly. When a large bird, a goose or
a crane, utters a loud cry, is the best opportunity.
Then, if he is standing, his breast may easily be seen
to move forward and upward. When the muscles
relax, the breast will sink and the air will be expelled,
but the latter process will be greatly assisted by the
contraction of other muscles — viz., those that lie over
the abdomen and connect the pelvis and the breast.
The action of these will be to drive the air out of the
great hinder air-sacks. The chest is loosely hinged
on to the back by muscles near the shoulder joint, so
that very little exertion on the part of the abdominal
muscles will be required. Take a dead bird and see
how easily the hinder end of the breast works up and
down. Thus the abdominal muscles in a bird play a
most important part in breathing, in a man they play
a very small one.

But most birds breathe most actively during flight,
and then a different system must be adopted. John
Hunter, the celebrated anatomist, held that birds did
not inhale and exhale during flight, but merely used
the air which they had stored in their air-sacks. This
view appears absurd in the face of the fact that they
will sometimes fly hundreds of miles without alighting.
But he was led to adopt it by what is a very real
difficulty — namely, that the movement of the breast in
breathing would seriously derange the machinery of
flight. The socket in which the wing works is formed
mainly by the coracoid, which is buttressed by the
clavicle. Both bones are almost rigidly fixed to the

vi FORM AND FUNCTION 89

breastbone, from which spring the great muscles which
lower and raise the wing. I f the breast were perpetually
moving up and down, a strong stroke, such as flight
requires, would be impossible. M. Edmond Alix gives
what I believe is the right explanation — viz., that a bird
breathes during flight, by moving not its breast, but
its backbone. But he does not explain how this is
effected. After some investigation I have come to the
conclusion that the muscular movements necessary to
flight themselves lend material assistance in the process
of breathing. I have here, therefore, to anticipate
some important points which properly belong to the
chapter on flight. There is a very broad sheet of
muscle, called the Latissimus Dorsi, which arises
from the vertebrae just behind the neck and also
from vertebrae further back, sometimes even from the
pelvis. It attaches to the shoulder bone. When the
wing is lowered in flight, this muscle contracts and
hauls upon the wings, which resist its action since
stronger muscles are pulling them in a different
direction. Since the wings will not give, the body is
lifted towards them, attaining nearly to a horizontal
position. Were it not for this muscle, it would hang
nearly straight down, as you may see by taking a
dead bird and holding it by its outstretched wings.
The Latissimus Dorsi not only keeps the body nearly
horizontal, but expands the air-sacks. For when the
back is raised, the weight of the breast muscles and
the intestines hanging on the ribs will straighten them
out, and so the sacks which lie close under the back
will be distended. This may easily be seen if you
grip the backbone of a dead bird with a pair of pincers,

90 THE STRUCTURE AND LIFE OF BIRDS chap.

when the dead weight hanging on the ribs produces the
result described. And the movement of the shoulder
blades during flight will help to produce the same
effect. In the down stroke the rotating or twisting of
the wing, by which its front margin is lowered and its
under surface made to look backward, causes a very
slight lowering of the anterior end of the shoulder
blade, and this necessitates a considerable upward
swing of the hinder end, a point that can be best
made out by moving the wing of a dead bird as it
moves in flight. This raising of the shoulder blade
acts upon the ribs since they are connected with it by
muscle. They straighten out, chiefly at the joint
which each of them has in its lower half, and so tend
to raise the backbone. And not only this, but, being
pliable, they bend outwards, and so broaden the roof of
the cage which they form. 1 Thus in birds, as in the
lobster and crayfish, progression itself aids greatly
the process of breathing. The external intercostals,
which we saw moved the breastbone forward when
the bird was standing or walking, now play a different
role. During flight the bird's breast is practically
immovable. The entire weight of the body is hang-
ing on the wings and the wings are pressing inwards
upon the coracoid bones and clavicles which are
firmly fixed to the breastbone. The dead weight and
the pressure inwards do not allow the breast to move.
Let us see then what will happen when the external
intercostals contract. It must be remembered that the
action of a muscle is to shorten the distance between
its two ends, and that of the two bones which it
1 See Fig. 2 on p. 8.

VI

FORM AND FUNCTION

91

connects that one will move which yields most easily.
Let the accompanying diagram represent two ribs,
the left-hand one being the
anterior of the two. The
contraction of the muscle
will raise the hinder one,
because that will yield the
more easily, the muscular
hinges at the shoulder
joints allowing the hinder
part of the back to rise.
Moreover the backbone of
most birds that I have ex-
amined bends downwards
easily, and through a con-
siderable arc just in front
of the pelvis. The raising
of the hindmost ribs which
unite with the backbone
behind the point where the
bend takes place, will aid the vertebral muscles in
straightening the back. 1 Wishing to test these conclu-
sions by experiment, I suspended a freshly-killed pigeon
by its wings, and inflated the air-sacks by means of a
blowing-tube. The backbone a little in front of the

1 Other muscles assist. The levatores costarum, which I have
found highly developed in the domestic pigeon, arising from the
vertebrae, then passing backwards and attaching to the ribs some
way down, tend to make the upper part of the rib horizontal,
thus broadening the chamber beneath. The triangularis sterni,
which arises from the inside of the sternum, from its anterior
lateral end, attaches to the sternal rib-pieces, and tends to make
them perpendicular.

27.

-B = backbone
E.I., ext. intercostal;
piece ; ST, sternum.

T), dorsal rib ;
S, sternal rib-

92 THE STRUCTURE AND LIFE OF BIRDS chap.

thigh-joint moved rather more than half an inch, the
movement of the sternum being almost too slight to
measure. I do not wish to represent this experiment
as one of much value. However, the conditions of
flight were so far reproduced that the weight of the
body was hanging upon the wings and so hindering
the movement of the breast while leaving the back
free ; it is true, there was none of the pressure — which
during flight must be very great — of the wings upon
the coracoids and clavicles. But would not the only
effect of this pressure be to render the breastbone and
the bones united with it still less ready to move ?
When a bird flies with his body sloping upward, as
he always does when he wishes to rise, I believe the
process of breathing will be the same, with the
difference that the Latissimus Dorsi will not contract
sufficiently to raise the back nearly to the horizontal.
There is yet another posture which birds commonly
adopt, and in which the problem of breathing does not
seem altogether a simple one. A chicken sleeps with its
breast resting on its perch. And gulls, geese, ducks,
and other birds will often lie with their breasts on the
ground. In this case the movement of the breast is
out of the question, and sometimes ocular evidence
may be obtained that it is the back that moves. I
have spent a considerable time in watching a Chinese
goose at the Zoological Gardens, while it lay on its
breast and uttered loud and uncouth noises. The
hinder part of the back rose visibly. And this was
not surprising, for there was no weight upon the legs,
and it is this of course which makes back-breathing
impossible when the bird is in a standing posture.

vi FORM AND FUNCTION 93

Hut what muscles arc brought into play ? The
muscle that connects the wing with the back-
bone will not help us now. We have to depend on
the external intercostals, the action of which I have
already explained (p. 91) for raising the back. Besides
there are fairly strong muscles running along the
vertebral column, and these will straighten the back
at the point just in front of the pelvis, where, as I
have said, there is a joint which allows a considerable
rise or fall.

The machinery of breathing, then, in birds is very
different from ours. Let us compare the results,
remembering that the object of breathing is to oxidise
the blood. In all lungs the blood is separated from
the air by a very thin membrane which allows the
passage-out of carbonic acid gas and the passage-in
of oxygen. An experiment will illustrate this. If
black venous blood is placed in a bladder and the
bladder placed in oxygen gas, the oxygen will find
its way into the bladder, and the blood will be
arterialised. In fact gases mechanically held in a
fluid tend to diffuse into any atmosphere to which
they are exposed — e.g. the carbonic acid gas in soda
water — and gases separated by a dry porous partition
diffuse into one another. But the oxidation of the
blood is a very complicated process. 1 Here it is
enough to say that when air as it comes from the
lungs is compared with fresh air, it is found to have
gained about 5 per cent, of carbonic acid and to have
lost about 5 per cent, of oxygen. This is proof
positive of the interchange of gases. Moreover it is
1 See Huxley's Elementary Physiology, Lesson IV.

94 THE STRUCTURE AND LIFE OF BIRDS chap.

beyond dispute that the more rapid and complete
the change of air in the lungs, the more rapid and
thorough will be the oxidation of the blood. It is
easy to increase the warmth of the body by taking
long and rapid breaths for a minute or two, and the
increase in temperature is due to the fact that the
blood, carrying more oxygen, burns the tissues more
rapidly.

The lungs proper are never penetrated by fresh
air. There is an amount of air in them which by no
effort can be exhaled. This is called residual air, and
in man averages from 75 to 100 cubic inches. There
is about an equal quantity which may be driven out
with effort, but which remains after an ordinary
expiration. This is called supplemental air. Only ■
the tidal air (20 — 30 cubic inches) passes in and out
in ordinary breathing. Thus, taking the largest
estimates — residual air 100, supplemental 100, tidal
30 — rather less than -1- of the air is changed when an
ordinary breath is taken, and the fresh air penetrates only
the trachea and bronchi, and not the minute air-cells,
which open from them. The stationary air, residual
and supplemental, carries on respiration. It receives
the carbonic acid from the blood and gives it up to
the tidal air, since it is a law of nature that gases,
when in contact, diffuse into one another. Increase
the volume of the tidal air, and the more rapid will be
the interchange of oxygen and carbonic acid gas be-
tween it and the stationary air. If the inspired air is
very poor in oxygen or meagre in amount the process
will be much slower and the whole vitality will be
lowered. In fact the refreshing effect of exercise is due

vi FORM AND FUNCTION 95

mainly to the more rapid breathing caused, and the
consequent more rapid oxidation of the blood. Now,
we have seen that in birds the air rushes in through
the lungs into the air-sacks behind, and that the latter
have a capacity many times as great as that of the
lungs. Not only, therefore, does the fresh air pene-
trate all the bronchial passages on its way to the air-
sacks, but expiration also will bring to the lungs a
supply of air only slightly vitiated, since it will drive
into them the as yet unused air in the sacks. This
fact must be viewed in connection with the known
rapidity of a bird's breathing. According to M.
Milne Edwards big birds, when inactive, breathe
20 — 30 times a minute, small birds 30 — 60 times. x
The thick coating of feathers makes it difficult to
count a bird's respirations. In ducks, which I have
watched closely, they have been from 18 to 22. Even
this lower estimate makes a bird breathe more rapidly
than we do ourselves, for an adult man, when sitting
still, averages only 13 — 15 respirations per minute.
In the case of a young horse, according to M. Milne
Edwards, the average is 10 — 12 per minute, of an
adult horse 9 — 10. In comparing a bird's rate of
breathing with that of other animals, we must bear in
mind the fact, that exhalation brings air that is
practically fresh to the lungs, so that a duck's 18
breaths per minute, taking the lowest estimate, ought
to be counted as nearly 36.

It is possible to obtain more accurate evidence of an
animal's respiratory activity by measuring the amount
of carbonic acid gas given off, for this is, roughly
1 Physiologic ct Anatomic comparcc, vol. ii., p. 487.

96 THE STRUCTURE AND LIFE OF BIRDS CHAR

speaking, equal to the amount of oxygen absorbed.
M. Milne Edwards records some experiments of this
nature, the results of which are very striking. 1 The
amount of carbonic acid gas exhaled by various
animals during a given time was exactly measured,
and then equated to one standard, so that the different
sizes of the subjects of the experiments might cause
no confusion. Thus, the figures that follow enable us
to compare animals of the most widely separated
classes, in respect of the amount of carbonic acid which
they breathe out ; and since, as I have said, this roughly
corresponds to the amount of oxygen absorbed, it is a
measure of the excellence, or the reverse, of their
breathing apparatus.

The slug ...
The snail ...
The toad

2

... 4 or 5
5 or 6

The frog

The guinea-pig ...
The pigeon

7 or 8
... 14 or 15
20

The pigeon is a good deal ahead of the guinea-pig,
the only other warm-blooded animal in the list. The
cold-blooded creatures are far behind the guinea-pig.

To sum up, then, a bird's respiratory system is, as
far as we know, much more active than that of any
mammal. As evidence of this we have — (1) the greater
amount of carbonic acid given off; (2) the more rapid
breathing, the effect of which is much increased by the
air-sacks ; (3) high temperature, which could not exist
without thorough oxidation of the blood.

1 Physiologie et Anatomie comparee, vol. ii., p. 534.

vi FORM AND FUNCTION 97

Lungs of Lower Vertebrate Animals.

In comparing reptiles and birds in a previous chapter,
I said nothing about the lungs, because I thought it
would be more intelligible after some account of the
machinery of breathing and its working had been
given.

If birds really had reptilian ancestors, it would be
very odd if existing reptiles had no trace of any
development similar to the air-sacks that in birds are
so characteristic a feature. There is one reptile that
has unmistakable air-sacks — the Chameleon. They
are small, it is true, but in their nature the same as
the bird's. The snake's one fully developed lung (the
other has shrunk to insignificance) is suggestive of
a bird's ; it is a bag the walls of the front part of
which arc full of blood vessels. The hinder part is
simply a reservoir of air. The same is the case with
the lizard's lungs. In crocodiles, they arc more com-
plicated, not at all like mere bags as they are in
snakes and lizards. In this point, too, crocodiles come
nearer to birds than other reptiles. It is curious that
the swim-bladder of fishes, like lungs, an outgrowth
from the alimentary canal, but, unlike lungs, an out-
growth from its dorsal (or hinder) wall, often has its
anterior half covered with blood vessels, while the
hinder part is simply a membranous bag. The lepido-
sirensare fish, which, if left in the mud when their river
dries up, become air-breathers ; they have true lungs,
pouches opening from the ventral (or front) wall of
the gullet, and these arc furnished with extensions
which have no blood vessels.

H

98 THE STRUCTURE AND LIFE OF BIRDS chap.

Thus, air-sacks arc not peculiar to birds, though they
have turned them to account in a way that is quite
unique. How important a part they play in
respiration I have already described. But they are
far larger than is necessary for this, and we shall now
have to consider what other purposes they serve.

Regulation of Temperature.

Our investigations have already made it clear why
a bird is warm-blooded. Thorough oxidation of the
blood and a rapid circulation bring about the burning
of the tissues which is the cause of animal warmth.
It must not be imagined that high temperature can
be due to a thick coating of feathers. The)- no doubt
help to retain heat, but they cannot produce it. Wrap
a lizard up in blankets, and he will still remain cold-
blooded. He has not* the digestion, the heart, or the
lungs that mark the warm-blooded vigorous animal.

We must now try to understand by what means
warm-blooded creatures in general, and birds in par-
ticular, regulate their temperature. This power is one
of the most wonderful things in the constitution of the
higher animals. In Parry's Polar Expedition, a wolf
was shot, and its temperature was found to be 104 F.,
while the thermometer was nearly 33 below zero.
The greater the cold to which the body is exposed,
the more rapid the combustion that is always going
on within it, so that its temperature does not rise and
fall with the thermometer. But it must always be
remembered that extremes, whether of cold or heat,
must reduce the vigour of the body by obliging it to

vi FORM AND FUNCTION 99

make an effort to resist them. In man the ordinary
temperature is 98 and a fraction ; a slight rise above
this indicates fever, and a slight decline below it shows
a failing of the bodily powers. When in health, the
bod\- can be exposed to enormous heat without itself
growing appreciably warmer. The sensation of heat
comes when great effort is required to keep the normal
temperature. In the hottest room in Turkish baths
the thermometer sometimes rises to 230° F., and some
bathers remain there as long as 20 minutes. But this
is far below the record. Doctors Fordyce and Blayden
were able to remain some time in a chamber heated to
260 F. I have been told that a man who earned his
living by feats of this kind, found himself compelled to
rush precipitately from a heated oven because some one,
who was more scientific than kind, had placed a can of hot
water in one corner. Every one knows how oppressive
the heat of a hothouse is. The heat of the vapour
baths in Russia is said sometimes to rise to 11 6° F.,
but between this and 260 there is a great gulf. We
have in this a hint as to one method of keeping down
temperature — viz., by evaporation. Perspiration, or
rather the evaporation, to which it gives rise, lowers
the temperature of the bod)'. When the air around is
so damp that evaporation is slow, even moderate heat
is oppressive. When through long exposure to a burn-
ing sun, all the available moisture in the body has been
exhausted, there results a feverish heat and an uncon-
trollable thirst. Under these circumstances, a private
soldier will not stop to use his pocket-filter, if he
happens to have been supplied with one, but will gulp
down the most poisonous filth, though he knows it to

H 2

ioo THE STRUCTURE AND LIFE OF BIRDS chap.

be poison. And yet a bird does not perspire at all,
and is perfectly at case when flying under a hot sun.
A man, if he is to endure heat, must resemble one of
the porous earthenware pots used in India for cooling
water. Put them in wind which, however hot, is dry,
and, the evaporation increasing, the water cools all the
more rapidly, and the sahib's bath will be ready all
the sooner. A man's skin is highly porous, being
covered with sweat glands, little tubes leading into the
skin and communicating with the capillary blood
vessels, from which the moisture permeates to the tube
and so to the surface. Even when perspiration is
imperceptible, a great deal is given off in the course of
the day. A bird, on the contrary, has no sweat glands;
it must therefore, have some other means of keeping
down its temperature. True, some evaporation will go
on though there are no pores, for if a bladder full of
water be hung up in the air, the water will ooze
through. Still, the process is a very slow one, and such
evaporation cannot be of much service to a bird.

It is now time to investigate more exactly the various
means by which the body rids itself of superfluous
heat. The processes at work are evaporation, con-
duction, radiation. The chilling effect of evapora-
tion, every one is familar with. We are conscious of
loss of heat by conduction when we touch cold
iron. But the same cause is always at work. By con-
duction, the air that is next to the body is warmed
— i.e., the body gives off heat to the air. Clothes,
according to the material of which they are made,
vary very much in their power to lessen conduction ;
they can never arrest it altogether. Radiation takes

vi FORM AND FUNCTION 101

place when heat passes out into the surrounding
atmosphere, not into the particular molecules in
contact with the body, just as the earth radiates out
its heat on a clear starlight night. In man the skin
does most of this work, a much less but still a con-
siderable amount being done by the lungs. Estimates
by different authorities vary considerably, some credit-
ing the lungs with 20 per cent., others with as little as 9. 1
In birds, the skin undertakes comparatively little of
the work, the lungs and air-sacks far the greater share.
Conduction is much checked by the feathers, though
the bare tracts, called apteria, make the coating much
less impervious than might be supposed. It must also
be borne in mind that owing to the high temperature
of the bird's body, the air will be colder to him than to
ourselves, and, so far, the conditions for loss of heat
by conduction are more favourable. Making all
allowances, the heat given off in this way cannot be
very great ; and, as I have said, owing to the absence of
sweat glands, there is no appreciable amount of evap-
oration. The work is, therefore, of necessity, thrown
upon the lungs and air-sacks. It is these organs that
by means of evaporation and conduction regulate
the temperature of the bird's body. It must be re-
membered that, however hot or however cold the air
inhalcd, by the time it emerges from the lungs it has

1 Dr. Michael Foster {Textbook of Physiology, p. 464, 1SS3
ed.) writes : " It has been calculated that the relative amounts
of the losses by these several channels are as follows : in wann-
ing the urine and faeces about 3, or according to others, 6 per
cent. ; by respiration about 20, or according to others, about 9
only per cent., leaving yy, or, alternatively, 85 per cent., for con-
duction and radiation and evaporation from the skin."

io- THE STRUCTURE AND LIFE OF BIRDS chap.

very nearly the same temperature as the body, and
all the heat communicated to the air is withdrawn
from the bird. The rapid breathing, therefore, that is
natural in flight, will of itself counteract the heating
effect of violent exercise. In the same way, since
they perspire only through the tongue and feet, dogs
maintain an equable temperature when running fast,
by means of quickened respiration. It is, probably,
as regulators of temperature that the air-sacks have
been developed till their cubic capacity surpasses that
of the lungs many times : how many, it is difficult to
estimate; probably ten times at least. They cannot, as
some writers have supposed, do the work of lungs,
since the blood vessels in them are so minute as to
be of little use, whereas, by exposing their very
large surfaces constantly to fresh indraughts of air,
they cause a large withdrawal of heat from the body,
and for this no other effectual machinery exists. It
would be very interesting to discover exactly what
amount of aqueous vapour is given off by a bird in
breathing, so that we might know whether, in pro-
portion to the size of body, it is more than it is in man.
Among other reasons for regarding this as probable, is
the fact that a bird's kidneys secrete little or no
water, so that of the three organs which get rid of the
waste products of the body — the skin, the lungs, and
the kidneys — the lungs alone are available for dis-
posing of any great amount of what is fluid.
Unfortunately, it is impossible to give any exact
figures. As far as I am aware, no evidence on this
point has been obtained by experiment.

Books on comparative anatomy are common, and

Vr KOKM AND FUNCTION 103

the smallest points have been investigated and re-
investigated. But comparative physiology is a less
common study. The physiologists, except when wish-
ing to throw light upon human life, have, as a rule,
neglected the life of other animals. In default of
such experiments we must point to the enormous size
of the air-sacks, far greater than is needed for mere
breathing, and also to the rate of respiration, which,
as I have said above, is much greater in a bird, even
when at rest, than in a man.

We have not yet done with the machinery by
which temperature is regulated. There are nerves
which can cause increased warmth in any organ
or part of an organ which requires it, and which
also exercise a general control. If a small artery
be watched, it will be seen to vary in width without
any apparent change taking place in the heart's
beat. To sec this, cut a hole in a thin piece of
deal, put a frog's foot over it, and tie the toes
so that it cannot move them. The frog will suffer
some discomfort but no actual pain. If now the foot
be examined under the microscope, the blood will be
seen circulating, as the skin is quite transparent, and
the widening and narrowing of the small arteries may
be made out. The same thing may be seen in a
small artery in the rabbit's ear. When little blood is
wanted in a particular part, the artery which supplies it
is constricted or tightened. When much is wanted, it is
dilated. This is effected by the vaso-motor nerves— i.e.
the nerves which act upon the blood vessels — and the
centre from which they act is believed to be the part
of the brain which is called the medulla oblongata

104 THE STRUCTURE AND LIFE OF BIRDS chap.

But they do not all centre in this, and the exact part
of the brain from which some of them come has yet to
be discovered. The vaso-motor nerves not only have
local power, but by combined action can affect the
temperature of the body generally. They lower it
by sending a large flow of blood to the surface ;
more heat will then be lost through radiation and con-
duction, and in man by evaporation. Thus exercise at
once raises and keeps down temperature — raises it by
muscular activity which always generates heat, keeps
it down by bringing blood to the surface. In exposure
to cold, the blood withdraws from the surface, and
protects the vital organs from chill.

It is supposed by high authorities that there are yet
other nerves or nerve-fibres, which have more complete
power over temperature in that they control directly
the amount of oxidation. When a warm-blooded
animal is dosed with the drug urari, it behaves like a
cold-blooded creature. If the nerves that arise from
the medulla oblongata are severed, the results are
the same in kind, though not so striking. This
cannot be due to the vaso-motor nerves, which
only regulate the amount of blood sent along the
arteries.

The subject will be more intelligible, when I have
made clear what is meant by " behaving like a cold-
blooded animal." For purposes of distilling, a chemist
puts various substances in a retort and exposes them
to heat, and the greater the heat applied the faster
the process goes on. A cold-blooded animal has
been well compared to such a mixture of dead
substances in a chemist's retort. Heat increases and

vi FORM AND FUNCTION 105

cold diminishes its activity ; when the thermometer
goes down a few degrees below freezing, it torpifies.

The warm-blooded animal generates heat within him-
self, and, in a certain measure, is superior to external
conditions. The cold-blooded animal is their slave.
It might be thought that fish live through great
cold in hard winters. But since water is densest and
heaviest when it is at a temperature of 39 F., ponds
and pools in rivers are not so cold some way below
the ice as might be thought. It is true that when
fish die during a frost, it is usually from want of
oxygen, the ice not having been broken to allow
oxidation of the water at the surface. There is no
reason, however, to suppose that fish can stand very
great cold any more than other cold-blooded animals
It is true of them as a class that they are at the mercy
of their surroundings.

It is impossible here to spend more space on so ab-
struse a subject. I would refer the reader to Dr.
Michael Foster's Textbook of Physiology, where the
subject is admirably handled. He is not there speak-
ing of birds ; but, in this respect, what is true of one
warm-blooded animal is probably true, roughly speak-
ing, of all.

Problems connected with the Holloiu Bones of Birds.

Not long ago the problems connected with this
subject were settled in a very offhand way. The
heated air in the air-sacks and bones being lighter than
the surrounding atmosphere made the bird a balloon,
and so flight was easy. This theory has withered

106 THE STRUCTURE AND LIFE OF BIRDS chap.

beneath the cruel light of fact. A bird can carry only
a very small amount of air in its sacks and bones,
and the difference in weight between a few cubic
inches of heated or cold air is too infinitesimal
to be worth considering. The fact that an eagle
may sometimes be seen carrying off a lamb
ought to convince any one that the saving of the
tiniest fraction of an ounce of weight would make
practically no difference. True, air within the
bird, whether heated or not, will expand its volume,
and lessen its specific gravity, 1 but it could not
help it to rise, and this is the real difficulty. Moreover,
many birds which fly to perfection, for instance the
swallow, have all their large bones solid. If by any
means a bird attained the lightness of aballoon,he could
not fly. A balloon drifts with every gust ; steering is
impossible ; the wind chooses its course. A machine
which is light as air can have no strength to gain a
velocity other than that of the air-current in which
it moves. The bird-balloon, as light as the wind and
as strong as iron, is a figment of the imagination.

What, then, is the true explanation of the aeration
or pneumaticity of birds' bones ? It is impossible that
it can be of use in the regulation of temperature, since
the air cannot be expelled from them at will. But the

1 It may be well to explain what is meant by specific gravity.
The weight of water is taken as the unit. When it is said that
the specific gravity of gold is 19, it is meant that a cubic foot of
gold weighs 19 times as much as a cubic foot of water. Thus,
when a bird inflates his sacks with air, his weight increases by
the weight of the air breathed in, but his specific gravity is
lessened. An average cubic inch of him does not now weigh so
much ; his weight in proportion to his bulk has gone down.

vi FORM AND FUNCTION

1 07

capillaries in the great expanse of bronchial membrane
must help a little to aerate the blood. With a view to
discovering the main purpose of pneumaticity, I will
briefly set down the chief facts.

(1) Many small birds that are first-rate flyers have
either marrow in all their larger bones, or else in
all except the upper-arm bone ; the Swift in all with
this exception, the Swallow in all.

(2) Most of the big strong-flying birds have a great
deal of aeration.

(3) The Hornbills, which according to good ob-
servers are very poor flyers, are as pneumatic as any
birds or, perhaps, more so than any.

(4) The Apteryx, the wingless bird of New Zealand,
has only part of its skull, and no other bones, aerated.
On the other hand the Ostrich, Emeu, Rhea, and
Cassowary have great hollows in the thigh bones, the
vertebrae, the ribs, the breast bone, and the coracoids.

(5) Birds which dive have solid bones, or only the
shoulder bone aerated.

(6) Birds which spend much of their time in the
water without diving have, at least in all the species
of which I have been able to obtain specimens, nearly
all the bones solid. The Gulls are the most striking
example of this, even the humerus in. the Black-
headed Gull being solid.

(7) There are great differences between nearly
related species — e.g. the Gannet has an extraordinary
amount of aeration, while its near ally, the Cormorant,
has only the humerus pneumatic. The Hornbill is
not very distantly related to the Swift, which has
singularly little aeration.

io8 THE STRUCTURE AND LIFE OF BIRDS chap.

(8) The bones of birds that are highly pneumatic
are, relatively to their length, larger in girth than those
of birds in which the aeration is but slight.

(9) All young birds have solid bones. As they grow
to maturity, if pneumaticity is characteristic of the
species, the marrow dries up and the bronchial mem-
brane extends into the hollow.

These facts look tangled and perplexing, but I
believe it is possible to some extent to unravel them.
I will begin by considering the case of the diving birds.
Much aeration of the bones would be an inconvenience
to them ; as it is, they can regulate the amount of the
body that appears above the water, sometimes sinking
till no more than the head is visible. Very often it is
impossible to see a Red-throated Diver swimming in a
mountain tarn. Only his neck stands out above the
water, and you cannot distinguish it among the reeds.
The Cormorant uses the same device, but he is not
equal to the Red-throated Diver in making himself
heavy or cork-like at pleasure. This power to vary
their specific gravity resides, no doubt, in the air-sacks,
which they can at will empty or innate. Sometimes
they help diving birds, it is thought, in another way :
those which lie under the skin about the neck
and breast of the Gannet may serve as air-cushions to
break his fall when he dashes into the sea from a
height of over 100 feet. 1 Aerated bones, on the other
hand, would be a hindrance and not a help to a diver,
for they would make it harder for him to swim under
water. Probably, too, the marrow in the bones serves

1 See on this point a paper by Mr. F. A. Lucas in Natural
Science, January, 1894.

vi FORM AND FUNCTION 109

a very important physiological purpose. 1 Divers are
frequently exposed to great cold when in the water.
They are protected against this by a peculiarly thick
coat of feathers, and by a deep layer of fat beneath
the skin ; and I cannot help thinking that the marrow
also helps to maintain their warmth. In other animals
it is held to be the birthplace of a large proportion of
the red blood-corpuscles, and unless they are very thick
in the blood, a high temperature cannot be maintained.
But if the marrow is a factory of red corpuscles, what
substitute for this have birds whose chief bones have
only a thin lining of marrow from which the output
must be small ? Though, as a rule, exposed to less
cold than diving-birds, they show in severe weather a
very great power of generating heat. Birds as a class
have more red corpuscles than any other animal. Is
the spleen, which in emergencies (e.g. when much
blood is lost) is a great red corpuscle factory, more
developed in birds which have little or no marrow ?
The vital organs are sometimes strangely versatile.
When an animal's spleen is removed, its work is done
somewhere else in the body and no ill effects are felt.
Putting physiology out of sight, I am going now
to consider why it is that among birds of powerful
flight we find differences so great in the amount of
aeration, and why such a poor flyer as the Hornbill
is, in respect of bones, so well equipped for aerial navi-
gation. To put physiology aside, is to assume that if

1 Physiology is the science which aims at explaining the work
done by the different organs of the body. It deals with all the
processes which maintain life.

no THE STRUCTURE AND LIFE OF BIRDS chap.

hollow bones are advantageous to a bird, natural
selection can bring it about that they become hollow
and that somehow the bird is able to dispense with
the marrow. This would be a bold assumption, did
we not know it to be an accomplished fact. The
bones are hollow, and there is no want of life in the
birds.

We shall find that pneumaticity in a bone implies
Greater girth in proportion to its length, and conse-
quently greater strength ; and that a decrease of
weight has accompani:d the increase of strength,
mainly through the drying up of the marrow, but
partly through a reduction, if we allow for the increased
size of the bones, in the thickness of the hard osseous
shell. I shall give first a few measurements to show
that in the case of birds whose skeletons have little
or no aeration, the girth of the bones is, relatively to
the bulk and weight of the body, considerably less.

Bones highly pneumatic.

Girth of humerus. ins.

Screamer i|

Rhinoceros Hornbill . . . i^

Golden Eagle 1 1

Vulture Monachus 2\

Marabou Stork 2}|

Bones very little or not at all aerated.
Girth of humerus.

Logger-headed Duck . .

Scoter Duck

Nestor Parrot

Red- throated Diver .
Spur-winged Goose . .

ins.
irV

l 3

TS

a

4

I

These measurements speak for themselves, even
without any exact statement of the weights of the
birds ; but the following illustration will do more to
explain the problem of hollow bones. The shoulder
bones of a Skua Gull, which has scarcely any aeration,
of a vociferous Sea Eagle, and a Hornbill, both of which
are highly pneumatic, are placed side by side. The

VI

FORM AND FUNCTION

greater girth of the hollow bones in proportion to their
length is at once obvious. But to bring this out still
more clearly, I have taken the wing bones of the Skua

Humerus of Pomatorhine Skua (a) ; Rhinoceros Hornbill (b) ; and Vociferous Sea
Eagle (c). Drawn to scale.

as the standard, and calculated what would have been
the length of the same bones and of the whole wing in
the Sea Eagle and the Hornbill, if they had been built
upon the same lines ;

ri2 THE STRUCTURE AND LIFE OF BIRDS chap.

Skua ......

Sea Eagle . . .
Rhinoceros Hornbill

Humerus. ' Aggregate Length of
\\ ing Bones.

Girth of
Humerus.

Actual
Length.

It u
Length pro- , , Length pro-
portionate T Actua u 1 Portionate
to Girth. Length. to Girth of
Humerus.

£f inches 4! inches

f 5 Aa

if „ 4s »

4§ inches 13* 13I
7^0- ,, 2o T v 22^

7^ „ i5l 23H

!

Thus, if in the Sea Eagle's humerus length were
proportioned to girth, the bone would be more than
half an inch longer ; on the same principle the aggre-
gate length of the wing bones would be greater by more
than one and a half inch ; the Hornbill's wing would
be lengthened by more than eight inches, its humerus
by more than three ! If now we take a fine saw and
cut the humeri of the Skua and the Sea Eagle from
end to end, we shall find that the walls of the latter
arc not thicker in proportion to the greater girth of
the bone. The larger bone, compared with the small
one, has a girth two thirds as great again, a thickness of
wall only one third as great again. 1 We can now see
why small birds have so little aeration. In their case,
there would be no great reduction of weight since the
exterior shell of the bones forms a great part of their
bulk. In the case of a larger bird, with bones many
times multiplied in size, but the thickness of the walls
increased comparatively little, the removal of the

1 The girths are in the ratio of 25 : 42 ; while 3 14 represents
the ratio of the thickness of the walls, the measurements being
i§q and j4 s of an inch.

VI

FORM AND FUNCTION

1 1

marrow will be a great advantage. This will W clear
if we take two cubes, a side of one of which is twice
the length of a side of the other.

Then the face is four times as large and the cubic
contents eight times as large. This will be true of other
figures besides cubes, so that if the average girth of one
bone be double that of another, and if the length also be
double, its cubic contents will be approximately eight

Fig. 2q. — Cubes.

times as great ; and as the walls do not thicken in pro-
portion to the increased girth, nearly all the enlarged
interior can be filled with air. Clearly, then, a large
bird has much more to gain by dispensing with marrow
than a small one.

Thus the Eagle has gained in point of lightness. It
must also have gained in point of strength, for in-
creased length of wing means an altogether dispro-
portionate increase of work. The longer the wing,

I

ii4 THE STRUCTURE AND LIFE OF BIRDS chap.

the greater the pace at which its extremity will move ;
if the velocity is doubled, it is well known that the re-
sistance of the air is far more than doubled, so that an
increase of strength is required that is altogether out
of proportion to the increase of length. This will be
made clearer, when we come to the subject of flight
(see p. 175).

The Hornbills are a puzzle. The extreme short-
ness of the hand bones, a ridiculous anticlimax follow-
ing upon so grand an ulna and so portentous a
humerus, might suggest that they were once better
flyers, and that the wing is slowly undergoing reduc-
tion. But the mountainous beak seems to show that
colossal bones are an ancient heritage of the family,
that even feeble flight might have been difficult had
they not become hollow, and that existing Hornbills
fly quite as well as their ancestors. In cither case
they have been great gainers by aeration.

The Ostrich and its allies present another difficulty.
But here too it may be said that by means of pneu-
maticity great strength has been combined with
lightness in a way that with solid bones would have
been impossible.

Any one who wishes to realise the relation, in birds'
bones, of slimness to solidity, and of large girth to
aeration, should inspect collections such as those at
the Royal College of Surgeons, or at the Natural
History Museum at South Kensington, where a large
number representing different families may be seen side
by side. It is easy then to see that big long-winged
birds have wing bones thicker in proportion to their
length in order to bear the far greater strain upon them,

VI FORM AND FUNCTION 115

while the aeration of the bones has obviated the natural
increase of weight which would have been a serious
hindrance. But there remains the perplexing physio-
logical problem : what organ of the body does the
work that, in mammals, and, presumably, in birds that
have solid bones, is done by the marrow ? :

The Kidneys.

In man, as remarked above, three organs — the skin,
the lungs, and the kidneys — divide between them the
work of eliminating waste products from the body.
The skin disposes of a great deal of water and a little
carbonic acid ; the lungs of carbonic acid and water,
but water to a much less extent than the skin ; the
kidneys of urea, uric acid (much nitrogen in both, the
debris of the tissues) and water. As these three are
allied organs, doing work that is similar, to some extent
actually the same, it might be expected that in birds,
since their skin is not an excretory organ, the other
two would be unusually active. With the lungs we
have seen that this is the case. And the kidneys are
very large ; they will be found lying behind the lungs
against the pelvis — long dark bodies. Yet they do not
undertake all the work that they do in mammals.
They are very active in excreting urea and uric acid ;
but, as is the case with snakes, it is in a nearly solid
form, the product of their activity being easily dis-

1 On " Aeration of Bones " see Fiirbringer, Morphologic und
Systematic der Vogel, especially pp. 47 and 133 ; Strasser,
Morphologisches Jahrbuch (Leipzig, 1877) ; Dr. Crisp, Proc.
Zoo I. Society ; 1S57.

I 2

n6 THE STRUCTURE AND LIFE OF BIRDS chap.

tinguishable by its whiteness. Upon the lungs alone
must fall the duty of getting rid of superfluous water
in any large quantity.

The Nerves.

About the nerves it is unnecessary to say much>
since they do not differ very materially from those of
mammals. The spinal cord is the great trunk nerve
which sends out branches on either side between the
vertebra?. It broadens out and forms part of the back
of the brain. There is also another system of nerves
called the Sympathetic, which lies in front of the verte-
bral column, and which acts mainly on the intestines
and blood vessels, not on the voluntary muscles. It is
connected with the spinal cord and so with the brain.
Nerves are called afferent and efferent. When any part
of the body comes into contact with anything that
necessitates prompt action — for instance, red-hot iron —
the afferent nerve carries the news to the spinal cord,
and so to the brain. The efferent nerve causes the re-
quisite muscular movements. In every warm-blooded
animal the nerves are highly developed. Otherwise
a highly-organised brain would be of little use. The
keenness of sight and hearing for which birds are
remarkable shows the perfection of their nervous
system. Great strength may co-exist with sluggish
nerves, as in a crocodile. But when a Swallow catches
sight of a gnat, and in less than a second has
taken all the necessary steps — eye communicating
with brain, brain directing the proper adjustment

VI FORM AND FUNCTION 117

of the muscles of wings, tail, neck, and beak — for an
unerring dart and snap at the victim, he has proved
that he possesses nerves of the first order.'

The Brain.

The subject is a very difficult one. It is impossible
as yet to impart interest to it by allotting to each part
of the brain its special function. Some progress is
being made in this by methods of study that arc
scientific and dependable, but, at the same time, slow
and laborious. It is hardly necessary to say that
phrenology which mapped out the skull into pro-
vinces, like an old and well-known country, not like
a half-explored continent, has gone to the limbo where
all systems founded on mere guesswork must go.

If a bird's fragile skull be removed carefully, so
as to leave the brain uninjured, the posterior part,
the cerebellum (cb, Fig. 30), will be easily dis-
tinguished ; in contact with it at their hinder ends are
two large bodies that make up nearly the whole of the
top of the brain. These are the cerebral hemispheres,
the larger development of which makes a bird's brain so
different from a reptile's (c/i). In them all the higher
faculties reside. If they are severely- injured or re-
moved, there is no more intelligence, memory, or
voluntary movement. There is only what is called
reflex action such as is called forth in a hydra or a
coral animal when food touches its tentacles ; they close
upon it without consciousness or intention on the

1 See Coues' Field and General Ornithology, p. 257 and
onward.

nS THE STRUCTURE AND LIFE OF BIRDS chap

animal's part. In the same way when the eyes wink
at a sudden flash of light, we call the action reflex.
The Frog, whose cerebral hemispheres are no longer
in their place, will move its foot when it is irritated :
of thus much, lower parts of the brain are capable.

Fig. 30. — Pra'n of Pigeon (after Parker); A from above ; P, front below; C from
left side(x2); cb cerebellum; c.h. cerebral hemispheres; m.o. Medulla Oblongata;
n roots of cerebral nerves ; o 1. optic lobes ; olf olfactory lobes ; pn pineal body.

If the whole brain is removed and only the spinal
cord is left, even breathing will not continue. When
there is much intellectual power, as in man, the
hemispheres arc highly convoluted — i.e., they are a
mass of folds and wrinkles. When the bird's skull is

vi FORM AND FUNCTION 119

removed, one is struck with the smoothness of the
brain.

The ol factor)- lobes (olf.), in which lies the sense of
smell, arc small cone-shaped objects which project
from underneath the front end of the hemispheres,
their smallness suggesting that birds depend little on
this sense. Formerly it was thought that vultures
" scented the carrion from afar," but Darwin showed
by experiment that this was not the case. He wrapped
some meat in paper, and put it near some condors
that were tethered in a garden. When it was only a
yard off him, an old cock bird " looked at it for a
moment with attention, but then regarded it no more." 1
It was pushed closer and closer till at last it touched
his beak, when the paper was " torn off with fury."

The optic lobes (o.l.) are many times larger — two
rather egg-shaped bodies at. the sides of the brain,
partly below the hemispheres. Their size suggests,
what is really the case, that the vulture finds his food
by sight. His eyes sweep the whole country round
as he flies, and when he swoops down upon a carcass
he is seen by numbers of others who quickly follow.

Towards the back of the brain between and under the
hemispheres lies a small oval object called the pineal
gland or body (pn.). What may now be its function,
if it has any, is unknown. Formerly it is believed to
'have been a central eye. In the bird's skull, in which
the fusion of bones is so marked a characteristic, we
should not expect to find an)- external evidence of
this rudimentary organ. But in lizards a hole in the

1 Darwin's Journal of Researches, chap. ix. (p. 133, Minerva
ed.).

i2o THE STRUCTURE AND LIFE OF BIRDS chap.

front ccntr.il part of the skull bears witness to its
existence. In the Hatteria, the now rare New Zealand
lizard, this hole is very large. As long ago as 1829
it was noticed that in the Sand Lizard (Lacerta agilis)
one of the scales at this point was quite unlike the
rest. In 1884 it was first suggested that the pineal
body was a rudimentary eye — i.e., an eye that had
become functionless. It has now been examined'
in various reptiles ; and partly in one, partly in
another, the lens, the retina, and the nerves, all the
chief characters of an eye, have been identified. But
in one important point, which I shall explain when
I come to what are commonly known as eyes, it is
the eye of an invertebrate, not of a vertebrate animal.
We must go to insects or to crustaceans to find its
fellow. In birds it has lost all resemblance to an eye,
and it has been covered by the hemispheres which
extend over and in front of it. In man it is also
present, and Descartes suggested that this mysterious
object, about the size of a hazel-nut, might be the
seat of the soul.

If the question be asked what any animal wants
with two different kinds of eyes, it is not easy to
answer positively. We can say that many insects
have compound eyes with hundreds of facets as well
as simple eyes (ocelli), the latter having, probably,
very defective sight, extending only to the very
nearest objects. The lower crustaceans have eyes
and a central ocellus ; but in the higher members of
the class, such as the crayfish, the ocellus has been
lost. Possibly in vertebrates, before the two eyes as
we know them had attained to their present perfection

VI FORM AND FUNCTION 121

in focussing, a central eye with a very near range may
have saved its owner occasional hard knocks against
objects close at hand when its superior organs of vision
were gazing upon some more distant scene. 1

Of the lower parts of the brain, I do not intend
to say much. The medulla oblongata, however (m.o.
in Fig. 30), must not be passed over. It forms the
lowest part of the brain, being really a continuation
of the spinal cord. We have already seen that in it
mainly centre the vaso-motor nerves, which govern the
arteries and so regulate the flow of blood. And
through it pass all of the twelve pairs of nerves
which proceed from the brain, except two, the optic
and olfactory ; and these two arc not, strictly
speaking, nerves, but prolongations of the brain.
The muscles that move the eyes, the muscles of
the face, the tongue, the larynx, the lungs, the liver,
and stomach work at the bidding of nerves that arise
from the medulla oblongata.

The Eye.

In most essentials the bird's eye is formed on the
same plan as our own. It is a camera at the back of
which is a nerve which expands into what is called
the retina ; the retina is sensitive to light, and the
image formed upon it is conveyed by the nerve to the
brain, where the impulse given to the nerve becomes
sensation — where, that is, sight actual ly takes place.

Before describing the eye more particularly, I wish

1 For a description of the pineal body see Lubbock's Se?ises
pf Animah

THE STRUCTURE AND LIFE OF BIRDS chap.

vi FORM AND FUNCTION

^3

to distinguish sight from mere sensitiveness to light:
Thus much even an earthworm possesses, for when at
night the light of a lantern is thrown upon him he
hurries into his hole. This is quite different from
seeing a definite image of things. With our eyes
shut, we can tell whether we arc in a bright light or
in the dark, and the earthworm has no power beyond
this. An insect's compound eye, again, is formed on
quite a different principle from the eyes of vertebrate
animals. It has a number of tiny facets beneath
which arc sensitive cells, so that a mosaic picture is
formed. There can be no doubt that eyes of this
description arc very inferior to our own. Among
their great defects is this, that they have no power
of adjusting themselves to different distances. To
return to the vertebrate eye. It is a camera with a
biconvex lens in front — i.e., a lens rounded outwards on
both sides. If a lens of this kind (a common magni-
fying glass will do well) be taken and a candle be
held in front of it, an inverted image of the flame
will be thrown upon the wall. The room must be
darkened except for the candle, and you must be
careful to get the right focus — i.e., to hold the lens at
such a distance from the wall, and the candle at such
a distance from the lens, that the image is clear.
The less convex the lens, the further away you
must hold the candle, and vice versa. Here, then
we have one means of focussing objects at different
distances ; we use lenses of various degrees of
convex it) T .

If the lens is to form a really clear image the light
must fall upon it only from in front ; rays from the

i2 4 THE STRUCTURE AND LIFE OF BIRDS chap.

sides must be carefully screened off, and of course
the box of the camera must be impervious to light.
The remaining essential is a sensitive plate at the
back of the camera on which the image is formed.
All these parts are represented in the eye. The
eyeball is the box of the camera ; it is tough and un-
transparent, and is called the Sclerotic (SC, a/cXyipb? =
hard), only in front it becomes transparent, and is
known as the Cornea (C). Side rays are shut out by a
circular curtain, the Iris (I), with a hole in the middle,
the Pupil, which can be seen opening and contracting
to regulate the amount of light admitted. There is a
crystalline lens (L) of great elasticity, which by the
action of the muscles which suspend it is made more
or less convex so as to focus for objects at different
distances. In front, between the cornea and the lens,
is a fluid called the aqueous humour (AH), and behind
the lens is the less fluid vitreous humour (VH). The
rays of light that fall upon the eyes are refracted or
bent by the curved surface of the cornea, then by the
anterior surface of the lens, and again when they pass
from the lens into the vitreous humour. The cornea,
the aqueous humour, the crystalline lens, and the
vitreous humour may be, therefore, looked upon as
making one compound lens. But of the component
parts the crystalline lens is far the most important,
since it alone has the power of accommodation — i.e.,
of adjusting itself to different distances.

The sensitive plate at the back of this living camera
is called, as I have said, the retina (R). Though thinner
than tissue paper, it is made up of nine distinct layers,
and it is the hindmost of these on which is formed

V]

FORM AND FUNCTION

125

the picture of the world without. It is made up of
very delicate rods and cones — -30,000,000 of the
former and 3,000,000 of the latter in the human eye,
at the lowest estimate. On these millions of sensitive
points the image is formed. They extend over
the back of the eyeball, but there is a central mark,
called the Macula Lutea (ML) or yellow spot, which

The entrance of

is the region of clearest vision
the nerve is rather towards
the nose (ON), and at this
point the eye is blind, as
a simple experiment will
show (BL).

Hold the book at arm's
length, close the left eye, and
fix the right upon the cross
mark, the image of which
will fall upon the Macula
Lutea. The dot will be also
visible. Now move the book
slowly towards you, and the
image of the dot must at
length fall upon the point
where the nerve enters the
eye. At this moment the

+
dot will disappear, then again at a nearer distance
come again into view, as the image of it gets once
more clear of the blind spot. The rods and cones
cover the whole of the back of the eye, except this one
point. Another experiment shows that it is the hind-
most layer of the retina that is sensitive. Let a candle

Fig. 32. — Section of Retina of duck
(after Cajal) ; C, Cone; R, Rod;
S, cells of supporting tissues.

126 THE STRUCTURE AND LIFE OF BIRDS chap.

be the only light in the room. Get some one to hold
it by the side of your eye, and with the help
of a lens to focus the rays upon it. Look at the wall,
which must be uniformly coloured. The shadows of
the blood vessels which ramify in the retina in front of
the rods and cones will be distinctly visible. Not only
is it the hindmost layer on which light makes itself
felt, but the rods and cones look backward. In the
invertebrate eye the retina looks forward, and its front
surface is the sensitive one. In this important point
the pineal body is the eye of an invertebrate. 1 Behind
the retina is a deep layer of dark pigment, called the
Choroid (CH) ; in this the rays after passing the sensitive
cells are absorbed. Were they reflected from one part
of the retina to another, any clearness of vision would
be impossible. But it would be rash to say that the
pigment exists for the sole purpose of preventing
reflection. This coloured layer is continued in front, and
forms the round curtain called the Iris, and, besides this,
where the sclerotic or white of the eye passes into the
transparent cornea, it sends out a number of muscular
frills, which lie behind the iris and which are separate
from it except that they spring from the same point ;
for the Iris, like these frills or, as they are called, ciliary
processes (CP), arises, as I have said, from the choroid
and is attached to the sclerotic at its margin close to
the cornea. It is these ciliary processes, consisting of
striated or voluntary muscle, which enable the eye to

1 See Grenadier's Sehorgan der Arthropoden. Sir John
Lubbock, by an oversight, has stated in his Senses of Animals
that the pigment lies in front of the sensitive cells (retina) in
the eyes of vertebrates. This of course cannot be so.

vi FORM AND FUNCTION 127

focus. When they contract the choroid is drawn
forward, the strain upon the lens is reduced, and, con-
sequently, its surface becomes more rounded. This is
the process that takes place when the sight is adjusted
for near objects. At the same time the Iris contracts
and lessens the amount of light admitted. This
wonderful curtain adapts itself to all circumstances :
involuntarily, by a reflex action we reduce the size of
the pupil when a strong light falls upon the eye ;
voluntarily, though habit makes the action unconscious,
and by calling into play a different set of nerves,
we contract it, when we cast our eyes upon a near
object.

It is now time to mention some of the peculiarities
of the bird's eye. The eyeball is not so globular as in
man ; in front it is much contracted, behind it opens
out like a decanter ; the cornea is highly curved. In
birds of prey, which see great distances, the front
surface of the lens is nearly flat ; in owls, on the con-
trary, it is much rounded, and at the same time the
pupil is very large to admit as much moonlight as
possible.

At the back of the eye, springing from the entrance
of the nerve, is a peculiar fanlike object, the Pecten,
which projects into the eyeball (P). It is full of
blood vessels, and is deeply pigmented, like the
choroid to which it is akin in structure. It is
thought to nourish the vitreous humour ; certainly
it does not push the lens forward for focussing-
purposes as some writers have maintained. Any
one who examines it, not in a diagram, but in the
eye itself, will find that it is far too limp to produce

128 THE STRUCTURE AND LIFE OF BIRDS CHAP.

any such effect. It is wanting, so far as is known, in
only one bird — the New Zealand Apteryx. It is found
in some reptiles, but always less developed than in
birds. It is odd that it does not interfere seriously
with the access of light to the retina. Besides the
central " yellow spot," which however is not absolutely
central, birds have a second similar spot more towards
the outer side of the eye. It has been thought that,
of the four spots thus possessed by the two eyes, two
are used together for binocular, and two separately for
monocular vision. The retina of a bird or a reptile
contrasts with that of a man in another point : the
cones exceed the rods in number. 1 The nictitating
membrane most people have heard of ; but it is often
imagined that it is the privilege of the eagle alone to
possess it, and that its object is to enable him to gaze
at the sun. As a fact, it is found in all birds and
reptiles. Watch the eye of any bird, and before long
you will see a film pass over it and in a moment vanish.
This is the nictitating membrane, which lies in the
front angle of the eye, and can be found without much
difficulty when the bird is dead. Some birds seem to
have great power of moving the Iris, a movement that
in most human beings is always involuntary, though
sometimes it is caused by nerves which, except for the
force of habit, are believed to be subject to our will. If
a Parrot's eye be watched, the pupil may be seen to
contract till it is quite small, though the light remains
as it was, and though the bird, apparently, continues to

1 Sec Fiirbringer's Morphologic mid Systematik dcr Vogel, p.
1069.

vr FORM AND FUNCTION 129

look at the same object. Moreover it is maintained '
that the muscles of the Iris in the Falcon may be seen
to contract without any alteration in the size of the
pupil ensuing, the outer ring seeming to work separately
from the inner ; it is suggested that the work of this
outer ring is to aid in focussing the eye. I have
watched the eyes of Falcons, Eagles, and other birds of
prey long and carefully, and I do not feel certain that I
have seen this. But an eagle in a cage has very little
need of sudden change of focus. It is far different
when he swoops from a great height upon his prey,
and, no doubt, keeps him clearly in view as he falls
like a thunderbolt upon him. It is certain that the
Iris in birds is highly muscular ; and, moreover, both in
birds and in reptiles the muscle is striated, not smooth
as in mammals. This is evidence that its action is
voluntary, and, perhaps, that it is more powerful. A
natural result of the tightening of the belt of muscle
round the lens would be to round it outwards — i.e., focus
the eye for near objects. On the whole it seems prob-
able that the Iris in birds is not only a curtain to
regulate the amount of light admitted, but that it aids
the ciliary muscles in the work of focussing.

The size of the eye varies very much in different
species, and, as a rule, the power of sight seems to vary
in proportion. Here are some figures which bring
this out clearly. 2 In the Owl, the two eyes cleared of
muscle weigh -£% of the whole body, in the Falcon
J.-, in the Woodpecker T V, in the Peacock y^, in the
Goose - 5 } T . In the Apteryx, a night feeder like the Owl,

1 See Bronn's Thier-Reich, vol. " Aves," p. 434.

2 Ibid, p. 425.

K

130 THE STRUCTURE AND LIFE OF BIRDS chap.

the eyes are very small, but the Apteryx is the one
bird in which the nostrils open at the end of the beak :
it trusts more to scent and touch than to sight.

Birds' keenness of sight is most remarkable. Vul-
tures, as I have already mentioned, descry their prey
from enormous distances. A Gannet, flying ioo feet or
more above the sea, will distinguish a fish near the
surface from the surrounding water which it so nearly
resembles, and pounce upon it. It is a common
amusement on a steamer to feed the gulls that follow
the boat with small pieces of biscuit, which, when
thrown, float, often invisible to the human eye, in the
wilderness of foam which covers all the wake of
the ship. The gulls, flying some thirty or forty feet
above the water, will swoop down upon them with un-
erring aim. Often, when you think they have missed
a small fragment, they will at last find it far in the rear
of the vessel.

The colours of birds' eyes are very various. In
the Shag the Iris is emerald green ; in the green-billed
Toucan, light green of the same shade as the beak ;
in the Ariel Toucan, like the tip of the beak, pale blue ;
in the Black Stork deep red ; in the Eagle Owl red-
orange ; in the Javan Fish Owl light yellow ; in the
Indian Kite nearly white.

The following examples would seem to show that
dark plumage implies a dark shade of colour in the
Iris, and vice versa.

Plumage. Iris.

Angolan Vulture Wing coverts white Pale almost to

whiteness.

A Cockatoo Dark blue Dark brown.

Ditto Light blue Nearly white.

V!

FORM AND FUNCTION 131

Plumage. Iris.

Red-backed Buzzard. ..To a great extent light brown ..Light brown.

Cinereous Vulture .. ..Dusky Dark brown.

Foster's Mil vagtj Mainly black Very dark.

Shag Dark with green gloss Emerald green.

Indian Kite A good many whitish feathers.. Nearly white.

Indian Owl Much of it black Dark brown.

Flamingo Light pink Light yellow.

Javan Fish Owl Some light brown on nearly all

its feathers Bright light yellow.

But the eye is not always light or dark according to
the shade of the plumage. The Crowned Pigeon,
whose plumage is a light blue-gray, has eyes of a rich
scarlet, just the colour of holly-berries. For the first
few months of his life, the Gannet's eyes are almost
black, but they soon turn to a pale, almost white, hue,
long before he has exchanged the dusky-gray attire of
his youth for the snow-white of his maturity. As a rule
the Iris is brown in young birds. The brighter tints
come with adult years, and in some species they are
limited to the male. 1

The Ear.

I shall first briefly describe the main features of the
human ear, then point out the chief differences between
it and the same organ in birds. The essential part
is in the sidewall of the skull ; and here there is a bony
" labyrinth " consisting of three winding tubes of bone,
which are filled with fluid (L, in fig. 33). There is

1 Dr. Gadow (Newton's Diet, of Birds, vol. i., p. 230) refers to
a paper on this subject by Th. A. Bruhin in Zool. Garten, 1870,
pp. 290-295, which I have not read.

K 2

32 THE STRUCTURE AND LIFE OF BIRDS chap.

Fig. 33. {a) Human ear— diagrammatic ; (?>) ear of Owl, after Gadovv ; (c) of

Thrush, after Retzius.

c, Columella ; cch, Cochlea ; e, Eustachian tube ; ex, outer opening of Ear ;

L, Labyrinth; Lg, Lagena ; m, Membrane, closing the drum ; n, entrance of auditory

nerve; nn, Nerve endings; 01, 2, 3, the three Ossicles, Stapes, Incus, Malleus;

ft, Pterygoid bone.

vi FORM AND FUNCTION 133

also a great extension, called, because it is shaped like
a spiral shell, the cochlea, and into this too the
fluid extends. A membranous bag, also filled with
fluid, extends throughout the ramifications of the
cochlea and the labyrinth. On the inside of the
membranous bag within the labyrinth, at certain
points where it is attached to the bony wall, are hairs
which are believed to communicate with the nerve of
hearing. So far, 1 have been describing the ear
proper. The rest of the machinery has for its object
the communication of the vibrations of sound to the
fluid in the bony labyrinth, from which they pass to
that in the membranous bag which lies in it, from that
to the hairs which connect with the nerve. The
apparatus for conveying sound vibrations to the laby-
rinth is rather complicated. There is, to begin with,
a membrane which stretches across the external
aperture of the ear. When a sound sets the air moving
in waves, which we speak of as vibrations, they strike
against the membrane. To this membrane and the
chamber behind is given the name of the drum of the
ear, and on the further side of the drum is the
labyrinth described above. Three bones united
together have their one end resting against the outer
membrane just mentioned, the other against another
membrane that at one point takes the place of bone in
the bony labyrinth. Thus, the vibrations of the outer
membrane are transmitted by the united three bones
to the window of membrane in the bony wall of the
labyrinth, from there to the fluid in the labyrinth,
next to the fluid within the membranous bag, and lastly
to the hairs within the bag that connect with the nerve.

134 THE STRUCTURE AND LIFE OF BIRDS chap.

A few more points must be mentioned. The bony
labyrinth has a second window of membrane, and this,
yielding, allows greater vibrations to be imparted to
the fluid. In the cochlea, are very peculiar cells,
called the rods or pillars of Corti, forming two rows all
along the spiral, in all from four to six thousand of
them. They lie upon the inside of the membranous bag,
following the line along which it comes into contact with
the wall of bone. They stand leaning on one another,
and rather remind one of the keys of a piano. There are
delicate hairs at their ends. It is possible that each
of these rods vibrates to a certain note and no other.
If you put on a table several tuning forks which have
different pitches, and if you set vibrating another,
then if one of those on the table is of the same pitch,
it also will vibrate. The rest will be motionless and
silent. So these rods of Corti have been thought to
respond each to a certain note. In the labyrinth
there are no similar cells, and it has been suggested
that the membrane there is sensitive only to noise as
distinguished from music.

It is not known exactly to what part of the brain
the nerve of hearing leads — i.e. where we have con-
sciousness of sound.

The ear has two openings, the external one with
which every one is familiar, and another through what is
called the Eustachian tube to the mouth (E). The
two tubes from either ear unite and open into the
roof of the mouth just behind the two openings from
the nasal passages.

I must now describe the main differences between
the human ear and the bird's.

VI FORM AND FUNCTION 135

(i) No species of bird has what can properly be
called an external car. The Owl has a Hap of skin,
forming a kind of valve, by which it is said that it can
close the car at pleasure. Certainly it possesses
muscles for this purpose. Often the car valve is larger
on one side than the other, the whole skull being at the
same time lopsided. During the breeding season, the
cock Capercailzie has moments of complete deafness,
owing to a fold of skin which becomes swollen with
blood and closes the opening of the ear. In other
species the flap of skin is very little developed.

(2) The three small bones which in the human cat-
convey the vibrations of sound from the membrane
which forms the outer wall of the drum of the ear to
the inner membrane that forms a window in the bony
labyrinth are represented in birds by one bone, the
columella (C). But it is almost certain that this is
formed by a fusion of three, corresponding to those
which we find in mammals. It was usual, till recently,
to see in the quadrate bone, to which the lower jaw of
birds and reptiles is hinged, one of the three bones of the
mammalian ear. If these three are combined in the
columella, where are we to look for the quadrate in
man and other mammals ? The best authorities arc
of opinion that it is represented only by an insig-
nificant ring of bone, called the annulus, which forms
a frame for the membrane of the drum of the ear.

(3) In place of the spiral cochlea birds have a
slightly curved bone to which the name of the lagena
has been given (Lg). It is similar in reptiles.

(4) The absence of the organ of Corti in the bird's
ear is a remarkable fact. It is true there is a very

136 THE STRUCTURE AND LIFE OF BIRDS chap.

delicate membrane, no doubt sensitive to sound, in
the corresponding place. But the distinctive pillars
or rods, leaning upon one another and forming arches,
are not there. It has been held, as I have said above,
that in those cells lies the power of distinguishing
nice differences of tone ; in fact, that when we say of
some one that he has " an ear for music," we speak
of what is supposed to depend on a high development
of the organ of Corti. And yet we cannot imagine
that birds can be such good singers without having
" good ears." Power of appreciation must accompany
power of song. The fact is that the ear, whether in
mammals or in birds, is an extremely complicated organ
about which there is much to learn, and the absence
of the pillars of Corti in birds is unexplained.

There can be no two opinions about the acuteness
of birds' sense of hearing. It is fine to see an old
Heron, put on the alert, at the slightest sound of a
human foot, by his wary ears, turn in the direction
whence the sound comes his equally wary eye. The
Curlew is all ears. The Thrush hears the worm moving
beneath the ground and waits for his appearance above
the surface.

The Organ of Voice.

As I have said above, a bird's upper larynx at the
top of the trachea or windpipe has no vocal chords,
and is, therefore, incapable of producing sound
though tone may be raised or lowered by it. There
is a lower larynx, to which the name of syrinx is
commonly given, the mechanism of which is, in all

VI

FORM AND FUNCTION

37

important respects, the same as that of the human
larynx. There are two membranes corresponding to
those called in man the vocal chords, which can
be stretched tight, and made parallel to one another.
When thus stretched, they are set vibrating by the
passage of the air between them, and a note is pro-
duced. The syrinx is in principle a reed instrument,

Mu

Fig. 34.— Syrinx.

Raven, a with bronchi ; a, b, c, half-rings ; where the two bronchi face each other
there is nothing but membrane, b, side view ; the outer part of the lower end of the
trachea and of the nearer bronchus being cut away; M, Membranous inner wall ot
bronchus; m.s, Membrana semilunaris; p, Pessulus ; c, Muscles of Syrinx; mu,
Muscles (after Owen).

though, in the relative position of the vibrating mem-
branes thus set edge to edge, it is, as far as I know,
unlike every instrument commonly used. There are
three varieties of syrinx, distinguished by their different
positions in the trachea or bronchi, but I shall describe
only the one which is by far the most common.

Near to the point where the windpipe divides to form

I3§ THE STRUCTURE AND LIFE OF BIRDS chap.

the two bronchi leading to either lung, a bony enlarge-
ment ■ is found, formed partly from the lower rings
of the windpipe, partly from the upper ones of
the bronchi. The latter on the inner side are of
membrane only. A bar of bone, the pessulus (P in
fig. 34 B), formed where the sides of the two bronchi
meet, passes across the syrinx from front to back.
From this bar a membrane, scalloped like a half-moon
on its outer edge, the membrana semilunaris (M.S. in
fig. 34 B), extends some way across the mouth cf the
bronchus. Opposite to it from the outer wall of the
syrinx projects another membrane. On the other
side of the pessulus is a similar crescent-shaped mem-
brane with another facing it. Thus, there are two
pairs of membranes, and there are muscles which can
tighten each pair and make the edges parallel. Many
birds have only two pairs of muscles for this purpose,
one pair passing to the trachea from the clavicles, the
other from the breastbone. But the majority of them
have at least one additional pair of syrinx muscles,
some as many as seven pairs, all having both points
of attachment on the trachea. Long vocal chords I
make a low voice, short ones a high voice. Hence
treble notes are characteristic of most birds and other I
small creatures. By tightening the chords the tone is
raised, by relaxing them it is lowered. The fact that
birds have so little range of voice seems to show that
the tension does not vary very much. The harsh, gruff
note of the Nightingale, and the abortive attempts of
the Cuckoo, when their vocal time is past, may be due
to the relaxation of the chords.

The chief difference between the syrinx of a songster

vi FORM AND FUNCTION 139

and that of an unmusical bird is that the muscles
of the former are, in most cases, more numerous and
stronger. The syrinx of the Skylark is almost a ball
of muscle, whereas the Pigeon's has but very little
to show. But it is very remarkable how muscular a
syrinx some few non-singers have. Among these arc
the Crow and the Raven. Perhaps a more striking
instance is that of the Bullfinch who sings very feebly
in the wild state. The hen-bird also, who, I believe,
is almost voiceless, has highly developed voice muscles.
In the cock-bird they have clearly not lost their power,
for in captivity he becomes a splendid vocalist.
However first-rate the syrinx and its muscles may be,
it is wonderful that so small a creature as, for instance,
a Nightingale, can produce such an amount of voice.
Even the Wren sends out a flood of powerful notes.
The Thrush's song is wonderful as a tour de force.
If the bird were nothing but a musical instrument, the
volume of sound sent forth would be astonishing ; and
when we consider the variety of functions which its
small body has to perform its musical powers supply
far greater reason for wonder. The air-sacks, no doubt,
are a great assistance. Those great reservoirs of air
must make it easier for the bird to avoid the awkward
crises that come to the untrained human vocalist
when he finds, at the moment his grandest notes are
expected of him, that his voice is becoming thin and
feeble for want of breath. The trachea sometimes
takes strange forms which might be thought to influ-
ence the voice. In the Drake, just in front of the syrinx,
it has a big box-like appendage, which looks as if it
might be intended to give the voice greater resonance.

140 THE STRUCTURE AND LIFE OF BIRDS CHAP.

This great air-chamber is entirely wanting in the Duck.
Yet the quack of the Duck is loud and sonorous, that
of the Drake is thin and without any body in it. In
some species of Crane, the trachea winds round
about within the keel of the breastbone, which is

tr
b

cl

Fig. 35, showing convolutions of Trachea of Mantchurian Crane.

b, Network of bones ; br, Trachea dividing into two bronchi; cl, Clavicle; co, Cora-

coid ; sc, Scapula ; tr, Trachea at entrance into keel.

formed of two thin sheets with, in places, a light bony
network in between : after all these windings it at
length divides and enters the lungs. Cranes have a
loud and striking crow, but it is not nearly so striking
as the crow of the barndoor Cock, whose windpipe
takes the shortest course to the lungs. The whistling

V1 FORM AND FUNCTION 141

Swan shows convolutions of the trachea very similar
to those of the Crane. Whatever other purpose it may
serve, the long coiled windpipe ensures the thorough
warming of the air before it reaches the lungs.

Muscles and Tendons.

To muscles all movement in the body is clue.
When acted on by the motor nerves they contract
and become shorter, with the result that the bone or
other organ connected with them is moved. The
nerve, in reality, gives a series of small shocks which
owing to the elasticity of the muscle, result in one
movement. The diminution in length of the con-
tracting muscle is balanced by an increase in breadth
and thickness. Great as its force is, it is not a perfect
machine. Like a steam-engine it only converts a
fraction of its total energy into work, the rest taking
the form of heat. In a steam-engine the work done
is rarely more than one-tenth of the total energy. In
a muscle, as far as we know at present, it varies from
one fourth to one twenty-fourth.

There are two kinds of muscles: (1) striated or
striped ; (2) unstriated or smooth. All muscles which
we move voluntarily are striated, and it is these which
move most quickly. The unstriated muscles, on the
other hand, which aid in carrying on the processes of
life in the body, move slowly and are subject to the
sympathetic system of nerves which are not under
the control of the will. The muscle of the Iris in man is
altogether exceptional ; it is unstriated ; its action is in
some cases voluntary, in others involuntary, according

142 THE STRUCTURE AND LIFE OF BIRDS chap.

as its activity is due to one or another set of nerves ;
it moves with great rapidity, instantaneously enlarg-
ing or reducing the size of the pupil. 1 The muscles of
the heart, too, are peculiar. Though involuntary, they
are striated, and yet unlike other striated muscles.

The amount of contraction possible to a muscle
varies with its length ; its strength depends upon its

£///'.;I;7/

mm 4j
iliiiiiiii

B

* fiiii i
iS

Fig. 36 (after Huxley).
A, Striated muscle of frog ; B, of mammal, "teased out " ; c, non-striated muscle.

thickness. Thus a short thick muscle will have strength,
but no great range ; a long thin muscle, great range,
but little strength. It has been found that a muscle
cannot contract more than one- third of its length. It
will be important to bear in mind this principle, when
we come to consider the varying lengths of the
breastbone and, consequently, of the muscles arising
from it, in birds with different methods of flight.

See p. 127.

vi FORM AND FUNCTION 143

Muscle is related to another kind of tissue which yet
in its function is very different. Tendons have no
power of contraction. They are . merely cords by
which, in many cases, the ends of muscles are
fastened to the bones. In youth there is compara-
tively little tendon in the body, nearly all is muscle,
and to this is due the springiness of the limbs. In
age one of two things happens : either the muscle
undergoes a kind of degeneration, fat making its way
in among the tissue, as we often see it, in small streaks
and flecks, in beef; or else the tendon by which the
muscle is attached grows longer, while the muscle
grows shorter, an increasing stiffness being the inevit-
able result. Long tendons, for quite different reasons,
to be explained soon (see p. 208), are characteristic of
birds. When, as they move, they have to rub against
hard surfaces of bone, tendons are protected by little
bags filled with moisture ; sometimes they are com-
pletely sheathed at these points. Sometimes their
working makes grooves in the bones. This can be
well seen at the ankle-joint of birds or where the toes
spring from the metatarsals. Tendons themselves,
in some cases, change their nature, and become
sesamoid bones as they are called. Such bones are,
for instance, the knee-cap, the pisiform bone, a small
bone that can be felt on the outer side of the wrist,
and the marsupial bones of the kangaroo.

Bones.

Much has been said on this subject in the opening
chapters (see especially Chapter II.), and in the

144 THE STRUCTURE AND LIFE OF BIRDS chap.

present one under the head of " Hollow Bones."
For what remains the reader is referred to the re-
marks on " Passive Machinery " in the next chapter.

Ligaments.

Ligaments are like tendons in having no power of
contraction, but, unlike tendons, they are not con-
nected with muscles. Their usual function is to
fasten two bones together at the joint, and to limit
the amount of freedom with which one turns upon
the other. When a skeleton is obtained by macera-
tion — i.e., by leaving the carcase in water till the flesh
is easily removable, many of the ligaments still re-
main and keep the bones in their proper connection.
There are some which answer very different purposes.
The horse's head is supported by a strong elastic liga-
ment attached to the upright spines of the vertebrae.
The bird, as I shall show in the article on " Passive
Machinery" in the next chapter, has several which
are remarkable for their elasticity, some, if not all, of
these having been originally tendons.

Feathers — Structure and Development.

A feather is a very elaborate appendage. When
we are told that a Peacock's or an Ostrich's plume, or
the wing-feather of an Albatross is an " epidermic
growth," part, that is, of the horny outer skin, we
seem to hear words that explain nothing. There is
another " epidermic growth," the nature of which it is
perhaps hardly less difficult to realise — the horn of a

VI

FORM AND FUNCTION

45

rhinoceros. But even with feathers, if we begin with
the simplest instead of the most elaborate, the diffi-
culty will appear much less, though it may not entirely
vanish. A still better plan will be to begin with the
scale of a reptile, and show how it corresponds to a
bird's feather. The scale proper is formed from the

Fig. 37.

(n) Feather of Duck, carrying nestling down feather ; (/>) Nestling down of Thrush;

(c) of Pigeon ; (d) Thread feather of Goose ; (7>), (c), and (it) after Gadow ; f, Feather

proper ; N, Nestling down.

skin, its horny coating from the epidermis. Where a
feather is to grow, there is a little skin papilla or
pimple, which corresponds to the scale proper ; the
actual feather is formed from the epidermis that
covers the papilla, and corresponds to the horny
covering of the scale. On the wings of the Penguin,
or on the legs of birds of the Ostrich kind — e.g., the

L

146 THE STRUCTURE AND LIFE OF BIRDS CHAP.

Rhea— may be found primitive feathers that are not
very different from the scales on the bird's own legs
or on a lizard. Birds in general have down feathers
among the large ones, and these down feathers are
often merely a little fluff at the top of a quill, though
sometimes they are almost perfect miniatures of a
typical feather. Besides these there are thread-
feathers, filoplumes, always growing close to the
base of one of the large feathers. Sometimes, like
hair, the thread-feathers are perfectly simple and un-
branched ; the branches are never more than a very few.
The Nightjar has, bordering the mouth, a number of
bristles that look like filoplumes, but are really ordinary
feathers of which only the shaft remains. There are also
found on some birds, notably on some Parrots and on
the Heron, powder-down feathers, so called because
they shed a fine powder. They continue to grow, and
the ends of their branches give off a whitish dust which
is at once greasy and dry. What purpose they may
serve is quite uncertain.

By the help of these simpler specimens we must
try to realise that the most elaborate feather is only
a much-divided scale. Such a feather I must now
describe, and then try to show how it has grown from
a skin papilla. Take a large one from the wing or
tail of any common bird. The semi-transparent base
is the quill (0, fig. 38) ; it has two small apertures,
one at the bottom, the other at the top, where the
branches begin, on the under-surface (U 1 and 2).
At the lower one the papilla entered to give the need-
ful nourishment, and if a young feather be taken, the
quill will be found full of blood (P).

VI

FORM AND FUNCTION

'47

When the quill is dry and hollow, the feather is
in most ways a dead thing, but the fact that in
some birds there is a change of colour without a
moult, and without the loss of any part of the feather,
shows that it has not entirely lost life. The stiff rod
above the quill is the rachis or shaft (S). It is grooved

-v$^§

^D

Fig 38. — Contour Feathers of Heron.

(a) plume-like feather with little or no interlocking.

(b) pennaceous or perfect flight feather.

B, Barb ; d, Downy ends of the lower barbs ; p, dried remains of Pulp ; Q, Quill ;

s, Shaft or rachis ; s ?, After-shaft ; v, Vane formed of the two webs on either side ;

u 1, Inferior umbilicus ; u 2, Superior umbilicus.

down the under-surface. The branches on either side
are called barbs (B), and the barbs to right and left
together form the vane of the feather. The barbs
give rise to barbules — i.e., little barbs on either side.
The barbules end in barbicels— i.e., still more diminu-
tive barbs. The barbicels belonging to the barbules
on the side of the barb that is nearer to the quill

L 2

148 THE STRUCTURE AND LIFE OF BIRDS chap-

are smooth and hairlike, with only an occasional im-
perfect hook near the edge of the vane. Those on
the further barbules end, many of them, in perfect
little hooklets. By means of these the barbules of
two neighbouring barbs are locked together.

Fig. 39 (after Pycraft).
To show method of interlocking.
A and B, Barhicels running in grooves ; c, Barhule on near side of barb :
on further side.

Barbule

If you look at a feather under the microscope, using
a not very high power, you will see that the barbules
on the distant side of the barb are not only hooked
but waved, and the smooth hairlike endings of the
opposite set that meet them are neatly tucked under-
neath into the hollow of the wave. It is not the case,

vi FORM AND FUNCTION 149

as is sometimes stated, that the barbules themselves
interlock. It is the hooklets that fasten one barbule
to another, and this they do in such a way that, while
keeping a firm grip, they increase the elasticity
natural to the material of which the feather is made
(Fig. 39A). The edges of the barbules, that have to
be laid hold of by the hooklets of those opposite to
them, are folded over. Below this folded edge is a
channel between the two adjacent barbules that lie
parallel to one another. The hooklet keeps hold of
the edge, and at the same time is able to move up and
down in the channel. Hence the wonderful play of
the vane of a wing or tail feather when pressure is
applied to it. In the softer, partly plume-like feathers
the mechanism is not so perfect ; in some cases the
hooklets do not exist. Such feathers are not imper-
vious to air, and they are much less strong and much
less elastic. All feathers, it will be noticed, are con-
cave underneath, a form that adapts them for resisting
pressure from below and not from above.

In nearly every case there is a small after-shaft (S 2,
fig. 38) arising just below the small pit at the top of
the quill. Generally it is insignificant and escapes
notice, unless attention is specially called to it. In the
Pigeon it is minute ; in the Cassowary, on the contrary,
it is as large as the main shaft. It is curious that in the
embryo feathers of this bird there is no after-shaft at
all. In no bird except the Emeu does it appear till
the feather proper grows, and this has been thought
to show that in primitive birds after-shafts were not
found.

The development of the feather now demands our

150 THE STRUCTURE AND LIFE OF BIRDS chap.

attention. Most birds, before they
are fledged, have upon them em-
bryo or nestling feathers, which are
very similar to the " downs " of
mature birds described above. Be-
ginning the history of one of these
from its earliest days, we find first
a papilla upon the skin. From the
epidermic covering of this springs
the nestling feather with a number
of thread-like branches, all starting
from the same point, so that it is,
in fact, a feather with the rachis
left out. We may look upon it as
a quill split uniformly the whole
way round into numbers of narrow
pieces ; it does not, like the later
feather, face one particular way.
Preparatory to this splitting, the
epidermic cells over the papilla
group themselves round the centre,
and their little elevations are in-
dications of the barbs that are soon
to appear. When the time comes,
the feather proper drives out the
nestling feather, and carries it on
its tip. The two are not really
distinct, but parts of one and the
same growth, the real feather with
the nestling on the top having been
formed even in the egg. The
quill does not dry up, so that the

vi FORM AND FUNCTION 151

pulp, as it is called, is the same in the nestling down
and the more lasting and stronger formation that
follows it. The change, therefore, bears no resem-
blance to the shedding of milk teeth and their re-
placement by permanent ones. The early tooth is
driven out by the later one ; the two are not in any
way connected.

After the first moult, the feathers develop without
any nestling " downs " as precursors. Otherwise the
process is not, in essential points, different. The cells of

,M.S

^-SH

AS

Fig. 41. — (After Gadow). Showing development of feather.

AS, cells forming after shaft ; b, cells forming barbs ; MS, cells forming main shaft ;

sh, horny sheath surrounding whole feather.

the papilla, or rather of the epidermis over it, arrange
themselves starwise. Two of the columns of cells
which cause this starlike formation grow broader and
longer than the rest, and go to make the rachis of the
feather. Two on the opposite side form a secondary
shaft, of which, as I have said, most feathers retain
some trace. At the same time there is a growth
inwards, so that in some cases the bone is reached.
On the ulna the marks of the great wing feathers are
easily discernible.

The cap found on the top of young feathers is
formed from the outermost cells of the epidermis, the

152 THE STRUCTURE AND LIFE OF BIRDS chap.

horny tube that overlies the papilla. Of this tube one
side only, as a rule, is much developed, the other, that
forms the after-shaft, being stunted. The little pit at
the top of the quill is the remains of the aperture
through which the pulp once forced its way, extending
even to the top of the rachis (U2, fig. 38).

The pulp retires when the feather is complete,
leaving only a few white flakes in the quill to mark
its former presence (P). When the feather is to be
moulted the papilla revives.

Varieties of Contour Feathers.

Contour feathers is a general name for the feathers
which are visible on the surface and which shape the
bird, to distinguish them from " downs," " thread-
feathers," and " powder-downs."

The name plumes is generally reserved for feathers
which are merely ornamental or a protection against
cold. They have not that perfect system of inter-
locking that makes the wing and tail feathers air-
proof. To the great wing feathers, the name remiges —
i.e. rowers — is given. Some spring from the hand and
are called primaries. The Pigeon has eleven such
feathers, six of them attached to the second meta-
carpal bone, the rest to the bones of second and third
digits. If, as often happens (in the Starling, for in-
stance), the outermost is very short, it is called a bastard
primary. The " thumb " carries no primary feathers. 1
The rest of the remiges, called secondaries and inner-

1 Sec p. 42 on the question whether this is really the thumb.

VI

FORM AND FUNCTION 1 53

most secondaries, spring from the ulna or the humerus,
The name tertiaries for the latter has now been disused.
The total number of the remiges varies very much, the
Humming-bird having only sixteen, and the Albatross
up to fifty, the variation being found in the secondaries
much more than in the primaries. Covering the bases
of the remiges are the wing-coverts. The great flight-
feathers are not originally the hindmost ; by their
enormous development, they push the two rearmost
rows to the lower face of the wing, where, to show
their origin, they still carry the after-shaft undermost.
The large tail feathers are called rectrices or steerers.
They always make an even number, but may be as
few as eight, or, it is said, as many as twenty-four. Some-
where about twelve is the normal. Sometimes they are
useful in distinguishing species. Thus the Common
Cormorant has fourteen, the Shag only twelve. Shielding
the bases of the tail feathers are the tail-coverts. In
the same way we speak of neck-coverts and ear-
coverts.

Though feathers are to a great extent dead things,
they are in connection with the living parts of the
body and, so, are frequently moved. Pelicans maybe
seen raising their feathers to dry them after a swim.
An old Hen with chickens raises them in anger. The
Long-eared Owl lifts his great "ears" to inspire
terror ; the Cockatoo raises his top-knot to add to his
dignity; the Peacock in pride of heart spreads his plumes
or rattles his quills. The behaviour of the Turkey-
cock is easy to interpret.

To make these movements, there are distributed
generally muscles which move the skin and with it

154 THE STRUCTURE AND LIFE OF BIRDS chap.

the feathers. By far the most remarkable of such
movements are connected with flight.

It is interesting to put side by side some of the most
wonderful forms of feathers, bearing in mind the like
origin of all : for instance, an Ostrich's plume, a
Penguin's tiny scale-like wing-feather, one of an
Albatross's mighty remiges, a Cassowary's plume with
its equal shafts, a hackle from the neck of a Barndoor
cock, a plume from a Bird of Paradise, a Lyre-bird's
tail feather, a spur from a Cassowary's wing (a great
wing feather that has lost its barbs so that the shaft
alone is left), one of the Motmot's two extraordinary
tail feathers, one of the grandest from a Peacock's
train, and, to complete the collection, one of the
stumpy business-like set with which a Woodpecker
props himself as he climbs.

Feather Tracts.

Except in the Penguin the feathers do not cover
the whole of the body, but only certain feather tracts.
The bare regions are called Apteria, and are some-
times devoid even of down — for instance, in the Wood-
pecker and the Sparrow-hawk.

Our common birds have most of them a bare tract
down the breast, which is very convenient when you
wish to skin them. In most sea-birds you have to
work through a thick, almost impervious, mass of
feathers before you can begin operations. Feather
tracts, especially down the neck and back, have been
found very useful for purposes of classification.

vi FORM AND FUNCTION 155

Moulting.

Moulting, as I have already said, is a reptilian
characteristic, and corresponds to the shedding of
the horny covering of the scales. It is due to the
papilla which once more extends into the quill
and causes the feather to fall off. In the Cassowary
and Emeu the tip of the new feather extends into
the base of the old one, which it carries for a time,
but the two are only connected mechanically.
They do not, like the nestling down and the feather
that follows it, make up one organ. Most birds
moult completely in the summer or autumn.
Many have a partial moult, at which only small
feathers are shed, in spring. It is sometimes stated
that migratory birds have two complete moults, 'one
before each migration, but it is probable that none of
them shed their great quill feathers more than once.
The Cuckoo's main moult is believed to be in spring :
sometimes he has arrived in England before its comple-
tion. In autumn, after he has left us, the smaller feathers
are once more changed. Swifts also, it is supposed,
moult in early spring long before they come to us,
a slighter moult taking place after their departure.
Swallows and Martins moult only once — in very
early spring or even in winter — being distinguished
by this from the Swifts, which moult twice. With
most migrants, as with other birds, autumn is the
great season for donning new feathers. Spring is the
time when the wedding plumage is put on, and the
Ruffs, the Golden Hovers, the Dunlins, the Linnets,

156 THE STRUCTURE AND LIFE OF BIRDS chap.

and a host of others come out like different birds.
Ducks and their allies are quite unlike most species
in this respect. The Mallard, the male of the Wild
Duck, to take an example, gets his fine feathers
in autumn and is in full splendour by October. He
is still wearing these same plumes when the pairing
time comes round. While the eggs are being laid and
sat upon, his plumage is fading, and before long a
moult begins. In June or about that time his faded
finery is shed and replaced by a dull garb very similar
to that of the Duck. Even earlier, towards the end
of May, he drops his big wing feathers, and, since they
all go nearly at the same time, he is incapable of
flight. Till then, he has been a most dutiful partner,
watching over his mate upon the nest, and warning
her if there happens to be danger when she is leaving
it to refresh herself with a bath and food. When his
moulting begins she is left to herself, arid often has,
unaided, to take her young ones to the water and
educate them. By the end of July the Mallard is
again possessed of fully grown remiges, but his dull
plumes are still upon him, and it is not till October
that he sheds these, and once more looks his best.
The Duck does not moult till her young ones are off
her hands. Geese, like Ducks, shed their quill feathers
all at once, and, standing, tumbled and helpless, pre-
sent a truly pitiful sight. It is only some water-birds
who moult in such haste. When the time comes for
it, they are very careful to be on or near the water so
that in case of danger, they may make use of what is
now their only means of escape, their power of
swimming. Mr. Seebohm describes a great procession

vi FORM ANT) FUNCTION 157

of Bean Gccsc that he saw in the north of Russia,
making for the water when their moult was imminent. 1
By the time the young can fly, the old birds have
renewed their quills, and they start for the south
together. Any land bird with such a system of
moulting would be reduced to a sad plight. He would
be worse off than the Crayfish, who has cast his shell
and, cowering in a hole, waits for the new one to form
and harden. As far as is known, all birds who are
not at home upon the water, shed their large feathers
at intervals, a pair at a time, one feather from each
side. In Hawks and other birds of prey, the intervals
are very long, and the process continues nearly the
whole year. In Homing Pigeons — the breed now in
use for " carrying " — and, I believe, in other pigeons
also, the moult lasts nearly half the year. About
May the tenth of the eleven primaries counting from
the outermost is lost. A month later the ninth eoes.
By that time the tenth has grown nearly to its full size ;
when the ninth is about half its proper length, the
eighth falls ; the others follow at intervals of from
eight to fifteen days. In the tail, which has twelve
feathers, the two which are fifth from the sides fall
first. When the new ones are grown to three quarters
of their full length, the two central ones are shed ; the
remainder fall in this order : the fourth, the third, the
outermost, the second. The Pigeon suffers much as
the moult approaches its conclusion. He flics with
difficulty, and is liable to arthritis, commonly known
among Belgian fanciers as La Maladic des ailes.
Badly fed birds have a defective moult. If Pigeons
1 See his Siberia in Europe, p. 287.

158 THE STRUCTURE AND LIFE OF BIRDS chap.

after being fed daily are left to pick up their own food,
the moult is arrested. Bird fanciers hasten the moult
by putting their victims in a dark and rather cold
place. Pigeons, which do not pair, put off their moult,
and so are in splendid condition for flying. By some
process at present not understood in Europe, the
Japanese check the shedding of the tail feathers of
Cock Chickens, and so produce the enormous growths
(ten or twelve feet long) with which we are familiar.
The Ptarmigan moults no less than three times in
the year. After the nesting season he sheds many of
his smaller feathers and becomes gray ; in autumn he
moults again, and in winter is arrayed in white, with
feathers on his legs supposed to be intended to
prevent him from sinking into the snow. A partial
moult in spring arrays him in his breeding plumage of
black and gray-brown and white. The big wing-
feathers are white at all seasons. The claws are
shed in July and August, and have grown to their full
length again before the bird puts on his winter dress.

The moulting of birds in their first year presents
great varieties. In most songsters it begins thirty or
forty days after they have left the nest. Hawks and
their allies keep their first plumage till next summer.
Young Ducks first appear in the same dress as their
parents in late autumn. Geese have only down feathers
till six weeks old ; after that appear feathers proper,
which they shed between September and December.
In their second autumn, like mature geese, they moult
completely in the space of four weeks. A young bird
of aquatic habits can afford to be content with a
covering of down for a long time after his birth. A

vi FORM AND FUNCTION 159

young Partridge has quill feathers big enough to
enable him to fly, to some extent, very soon after
leaving the egg. These are shed and replaced several
times during his first summer and autumn, thus keep-
ing pace with his rapid growth. Thrushes, Blackbirds,
and Fieldfares have one complete moult in their first
autumn.

Change of Colour without Moulting.

In spring the cock Gray Linnet becomes the " Red
Linnet," and appears with a crown and breast of
crimson in place of the dull gray of winter, and
yet it is certain that no feathers are shed. In cap-
tivity he gradually loses his crimson splendours, which
fade to ochre-yellow. After the first moult he assumes
and retains the dull plumage of the hen. The fore-
head of the Redpoll becomes blood-red and his throat
and breast carmine, equally without the shedding of
a feather. The nape and back of the Brambling turn
from reddish-brown striped with black to pure glossy
blue-black without any moult ; and, to take one more
instance, the Blackheaded Gull, in the course of a
fortnight, dyes the white plumage of his head black, or,
more strictly speaking, a very dark brown. In some
cases the explanation is perfectly simple ; the crown
and breast feathers of the Linnet have wide gray
borders which in spring break off and let the crimson
that was before covered up become visible. The same
is the case with the Redpoll, Brambling, the Snow
Bunting, whose back plumage becomes black in spring,
and the Blue Throat. In some of these cases Gatke

160 THE STRUCTURE AND LIFE OF BIRDS chap.

attributes the change not to a breaking off of the
edges, but to a peeling of the barbules. However this
may be, he must surely be right when he maintains
that in spring there is a rounding off of the ragged
edges of feathers. The Linnet's nuptial plumage
would be but a sorry garb if the dropping away of the
edges left what remained all ragged. A far more
remarkable cause of change of colour is the entrance
of fresh colouring matter into the feather, which can-
not therefore be an entirely dead thing. This is what
takes place when the Blackheaded Gull puts on his
spring head-dress, the colour, according to Gatke,
appearing first at the edges of the feathers and
gradually extending till the whole is dyed. In winter
the breast of the Dunlin is almost white, in spring it
becomes black, the pigment working its way to every
part of the feathers through channels as yet un-
discovered. By a similar process the head of the
Little Gull changes in spring from white with a dash
of ashen-gray to black. As in the Linnet in captivity,
so in the Herring Gull there takes place a withdrawal
of pigment, for the head having been gray in winter
becomes snow-white in spring. In these cases no
indication of moulting, such as half-grown feathers, is
ever found. The plumage of the Wood-sandpiper is
an interesting study, since it supplies an example of
the influx of fresh colour into the feathers and also of
the rounding off of ragged edges. Birds in captivity
sometimes show these changes well. This year the
Knots at the Zoological Gardens appeared with the
chestnut-coloured breasts proper to them in spring,
but whether the change in their case is due to the dull

vi FORM AND FUNCTION 161

margins of feathers being shed or to the influx of
fresh pigment, I do not know. 1

Spurs.

Spurs are outgrowths of bone covered by a horny
sheath formed from the epidermis, and, thus, they re-
semble the horns of oxen and antelopes. The cock's
spur is familiar to every one. Some birds, for instance
the Double Spurred Peacock, have two on each foot.
Spurs are also found upon the wings, for example in
the Crested Screamer of South America which is now
to be seen at the Zoological Gardens. The Cassowary's
spurs, which are really feathers, I have already de-
scribed. These are found in both sexes, as ordinary
wing-spurs sometimes are. All spurs are used in
fighting, and well-developed leg-spurs are the privilege
of cock birds. We should expect, therefore, to see
them, as we do, mainly in those species which are
polygamous, and which consequently have an excess
of males among whom there is constant war in spring-
time. It is with his spur that the game-cock slays
his rival.

The Beak.

In the beak the horny covering which overlies the
bone is a growth of the epidermis just as spurs are.

1 On moulting- see especially (i) Bronn's Thier-Reich r vol.
" Aves," pp. 538-542 ; (2) Seebohm's Brit. Birds, passim ; (3)
Gatke, Die Vogclwarte Helgoland, pp. 156-166; (4) Le Pigeon
Voyageur, by F. Chapuis, pp. 103-111. I am indebted to Mr.
C. M. Adamson's book Some more Scraps about Birds, printed
for private circulation

M

i62 THE STRUCTURE AND LIFE OF BIRDS char

In all birds the upper beak moves slightly, in parrots
freely. It is always growing, but constant friction
against hard substances and of the upper against the
lower beak prevents this from being apparent. The
duck's beak acts as a strainer : the whale, in the so-
called whalebone, has a similar instrument which lets
the water pass away while retaining the food. The

Fig. 42.
(1), Beak of Falcon showing toothed edge ; (2), of Duck showing strainer.

beaks of Humming-birds are bent or otherwise shaped
so as to suit the forms of particular flowers down the
corollas of which they dive for the honey. Falcons and
other birds of prey have their upper beaks cut into
teeth, an assistance in tearing their food. And in con-
nection with this, it must be remembered that flesh-
eating birds have nothing worthy of the name of a
gizzard. Hence some tearing of the food is desirable.

vi FORM AND, FUNCTION 163

Mr. Beddard mentions that the Great Spotted Wood-
pecker ate the caterpillar of the Buff-tip moths partially
after much pecking. 1 Was this because the conspicuous
colours frightened him or because the skin was over-
tough ? A Magpie rubbed the hairs off a caterpillar
before eating it. On the other hand a Gannet swallows
a mackerel whole. A Cormorant is only troubled by a
whole fish if he happens to swallow him headforemost
and so get the fins the wrong way. He has been
known to swallow a Starling with beak and feet and
everything appertaining to him, and to attempt to
swallow a young kitten. 2 The parrot gnaws his food
carefully like a dyspeptic. The great freedom with
which his upper beak works enables him to put its long
curved point to the front margin of the lower beak
when occasion requires. With this long point he
scoops out a Brazil nut when he has cracked the shell
like a piece of shortbread.

The beak aided by the long and supple neck takes
the place of a hand. When the forelimbs became wings
and the former reptile, now a bird, stood comparatively
erect on two legs, some form of hand was clearly
necessary. The parrot uses his feet to lift food to his
mouth, but most birds know no hand but their beak.
It is also a weapon of offence, many birds being able
to give a powerful stroke not unlike that of a snake,
and far more promptly administered. When some
members of the Challenger expedition visited Penguin
kl rookeries " they found they must wear thick gaiters

1 Animal Coloration , by F. E. Beddard, p. 155.

2 See The Home of a Naturalist, by the Rev. B. Edmondston,
P- 77.

M 2

i6 4 THE STRUCTURE AND LIFE OF BIRDS chap.

to protect their legs from the formidable beaks among
which they had to run the gauntlet. The Woodpecker
pecks a hole in a tree in which to make his nest. His
beak is the hammer with which the Nuthatch, swing-
ing at the hips, cracks his nut. A Thrush may be
seen picking up a snail and dashing it on a stone to
break the shell. The beak is also used to preen the
feathers, even a short-necked bird being able to bring
it to bear on almost any part of his plumage. When
there is an oil-gland at the root of the tail, the bird with
his bill presses the oil from it and distributes it over
his feathers. The Tailor Bird uses it as a needle, and
partly to its skill are due the beautiful nests of many
of our small birds.

The Foot.

It will be enough to mention a few types to show
how the anatomy has adapted itself to different modes
of life. The normal number of toes is four, the fifth
or " little toe " having been lost. The first, as a rule,
points backward. The Emeu, the Rhea, and the
Cassowary have only three, having lost the first as well.
The Ostrich has only two, the third and fourth, and the
latter of these two is small and bears no nail. As in
the horse, it is the middle toe which carries all the
weight. Among English birds the most striking
difference is between the webbed feet of the swimmers
and the separate-toed feet of the perchers, climbers,
waders, and runners. The Gannet, the Cormorant, and
their allies have all four digits connected by the web ;
in most swimming birds the first is free. There are

VI

FORM AND FUNCTION

165

various intermediate stages before we arrive at separate-
toed feet. In the Dabchick and the other Grebes, the
toes are not connected, but there is on cither side of
each a broad expansion of skin. In the Kingfisher
the second, third, and fourth toes arc fastened together

Fig. 43.
Foot of (i), Woodpecker ; (2), Grebe.

for most of their extent. The Woodpeckers, Cuckoos,
and Toucans have a most curious form of foot called
zygodactyle or yoke-toed, the first and fourth toes
pointing backward, the other two forward — a foot
specially adapted for climbing. In the Swift all the

1 66 THE STRUCTURE AND LIFE OF BIRDS chap.

toes turn forwards. The number of Phalanges or
segments in each varies very much in different species.
Usually the first toe has 2 ; the second, 3 ; the third,
4 ; the fourth, 5. The Swift has in the respective
toes only 2, 3, 3, 3. This and the extreme shortness
of his legs must account for his inability (if the inability,
as is popularly supposed, exists) to rise from the ground.
Mr. Howard Saunders denies the correctness of the
popular belief, but I am not sure that the bird is
not in difficulties when he finds himself among grass
of any length.

Perching.

Most of our common birds would soon fall victims
to some nocturnal beast of prey, if they had not the
power of maintaining themselves on a bough during
sleep. To see the machinery by which this is effected,
take a bird of some size and cut through the skin at the
back of the ankle-joint. We find there, first, two tendons
belonging to muscles which have nothing to do with
the toes, one of which attaches a little above the foot,
the other just below the ankle-joint. As they pass this
joint, these tendons spread out and form a sheath in
which run several of the tendons that bend the toes, and
which are bound together by connective tissue but easily
dissected apart. Cutting down deeper we come to other
tendons passing to the toes, making the number in all
up to seven. Of these the Hallux or first digit (our
great toe) has 1 ; the second and third, each, 2 ; the
fourth, 1 : while another tendon divides into 3, the
branches going to toes 2, 3, 4 respectively. This

\ I

FORM AND FUNCTION

167

branching tendon is the one to which most interest
attaches, and it is easily distinguished from the others :
it lies the most deeply imbedded of all at the ankle-
joint, in a cartilaginous or bony tunnel. In a great
many birds it connects with the tendon that bends the
Hallux, and the absence of connection or the form of

Fig. 44.

Flexor tendons of toes 'm(a) Fowl; (l>)— after Gadow — Passerine Bird. (1) deep,
divided tendon ; (2) the tendon that bends the hallux or first toe. In Passerine Birds
they remain unconnected.

connection have been found very useful in classifica-
tion. 1 Tracing the tendons upwards we shall find
them passing into muscles that arise partly from the

1 Some of the chief varieties are well shown in specimens at
the British Museum, S. Kensington, but it is much the best
plan to dissect them out.

1 68 THE STRUCTURE AND LIFE OF BIRDS ch\p.

top of the Tibiotarsus (drumstick) partly from the
lower end of the Femur or thigh-bone. When the
leg bends at the ankle, there is a pull upon the
tendons, the muscles are stretched, the toes are
bent and grasp the perch on which the bird sits.
Thus he is maintained in position by his own weight,
which bends the leg and so causes the toes to grip.
The strain on the muscles is, probably, not great.
Chickens and, I believe, other birds rest their breast-
bone upon the perch, and so get support nearly in the
vertical line in which lies the centre of gravity. The grip
of the toes, therefore, is wanted only to steady them.
This bending of the toes, as a necessary consequence of
bending the leg at the ankle-joint, is not altogether
peculiar to birds. A squirrel's toes will open or close ac-
cording as his leg is straightened or bent. In birds what
was once, probably, a trifling or useless feature has been
developed in order to supply a vital need. Birds, such
as Gulls, which do not sleep upon a perch and are rather
ill at ease upon one, have this toe-grip only in a most
rudimentary form. I have seen the Black-headed Gull
alight on railings, and at the Zoological Gardens, when
in a small aviary where they have not much ground to
wander over, Gulls will remain perched for some time,
though apparently uncomfortable, on the thin bar
allotted them. This is no proof, however, that they
could sleep upon a perch. Others, which are not
ordinarily perchers, are quite capable of adopting
arboreal habits. The annual flooding of great tracts
of country in Siberia has brought this about in the
case of the Snipe. 1

1 Vide Seebohm's Siberia in Europe, p. 147

VI

FORM AND FUNCTION

169

There is connected with this subject another strange
phenomenon. In many birds a thin muscle, called
Ambicns, arises from the Pelvis just under the thigh-
joint and passes forward on the inner side of the leg
to the knee, before reaching which it becomes a tendon :
it curves round the knee in a little tendinous tunnel
occupied by itself alone, then doubles back on the
outside of the leg and passes into one of the muscles
which bend the toes as described above. It is very
characteristic of birds that a muscle should, by means

A

Fig. 45. — Leg of chicken, the side next the hody.
a, ambiens muscle ; k, knee-joint.

of a long tendon, do its work at such a distance : but,
curiously, it is not found in by any means all the
perching birds. And, besides, this the same muscle is
to be found in crocodiles. 1 This must not, how-
ever, be taken to prove any very close relationship with
birds. The fact that it is found in two families of birds
may help to prove that they are closely allied, but such
evidence is less dependable when we are dealing, not

1 Refer to Appa?-eil Locomoteur des Oiseaux (M. Edmond
Mix), p. 443.

i;o THE STRUCTURE AND LIFE OF BIRDS CHAP.

with the relationship between families within the same
class, but with the relationship between two classes.
In the crocodile the muscle in question appears either
not to connect with the toe-flexor muscles or else to
be altogether functionlcss, tor when I have bent the
ankle-joint o( a young American alligator, most pro-
bably resembling a crocodile in this point of anatomy,
no effect at all has been produced upon the toes.

The habit o( standing on one leg is common to
many birds. The Heron is well known for it.

'*Ni v L;h upon that hour
When the lone hern forgets his melancholy,
Lets down his other leg ami, stretching, dreams
Of goodly supper in the distant pool."

Flamingoes, Storks, and Cranes can frequently be
seen in this posture at the Zoological Gardens.

It is said to be a restful one, and it must have
merits or they would not adopt it. Hut if the leg be
watched it will be seen to be perpetually swaying to
and fro. In fact the balance is only maintained by
the help o( perpetual small muscular adjustments, of
which, no doubt, the bird is capable while asleep,
some lower part oi~ the brain working when the cere-
bral hemispheres, the seat of conscious life, arc at
rest.

Su imming.

People maintain that they have seen from a boat a
Shag " flying under water," swimming, that is, by
means o{ his wings. Among the diving birds at the
Zoological Gardens there is frequently a Shag, and as

vi FORM AND FUNCTION 171

he chases the fish in the tank, he holds his wings
motionless, just slightly lifted from the body. It is
the same with the Indian Darter — he strikes with his
legs only. The Penguin, on the other hand, swims
almost entirely with his wings. For him swimming
is flight under water, the only flight possible to him ;
his legs arc used only for steering, or for an occasional
upward kick to force him downward. The Rough-
faced Shag strikes with both feet simultaneously, the
Indian Darter's is an alternate stroke, and the same is
the case with the Gull. The Swan and the Duck take
almost simultaneous strokes with both feet, yet one
is always just a little behind the other.

I have already mentioned how various diving birds
by driving most of the air out of their air-sacks cause
only a small part of their body, or nothing but the
neck, to appear above water. Some birds dive to
great depths. The Shag begins by jumping up in
the water and taking a header, then he strikes hard
upward. One was caught once in a crab-pot twenty
fathoms below the surface. There is one kind of
Penguin which is said to swallow stones for ballast
and vomit them up again at the mouth of his burrow-
on returning. 1 It would be worth while watching
long to prove this true or untrue. Certainly a diving-
bird is in a dilemma if he wishes to descend to a
great depth and stop there long. He must take in
an abundant supply of air, but this will make him
over-buoyant.

lairds which sleep floating upon ponds or tarns

1 Sphouscus Magellanicus ; see Report of" Challenger" Ex-
pedition) vol. ii., p. 127.

172 THE STRUCTURE AND LIFE OF BIRDS CH. VI

are in danger of drifting to the bank and falling
victims to any beast of prey. To prevent this ducks
and others have the habit of sleeping with one
leg tucked under the wing, while with the other they
keep gently paddling so that they revolve in a circle.
In summer time, when they have had a long day,
they will begin this early when there is still some
light, and then is the time to watch them. This
remarkable habit is a kind of sleep-walking turned to
good account, and is, no doubt, perfectly compatible
with complete unconsciousness. 1

Some of the best Books on the Subject.

(i) Bronn's Thier- Reich, vol. "Aves."

(2) Milne-Edwards' Physiologic ct Anatomie cojnparce.

(3) Max Furbringer's Morphologie wid Systematik der Vogel.

(4) Various articles by Dr. Gadovv in Newton's Dictionary of
Birds.

(5) Michael Foster's Text-book of Physiology.

(6) Huxley's Elementary Physiology.

(7) Coues' Field and General Ornithology.

[See references given in footnotes.]

1 My attention was first called to this interesting point by
Mr. Thompson, head keeper at the Zoological Gardens.

CHAPTER VII

FLIGHT
The Wings as Levers, the Air as Fulcrum

ARCHIMEDES was prepared to move the world if
he could find a fulcrum for his lever. The problem of
flight seems almost equally difficult : the body must
be lifted by levers, and the fixed points on which they
are to work must be found in the air. But before I
show how the bird surmounts this great difficulty, a
word about levers is necessary. Levers arc rigid
rods resting on a fulcrum or fixed point ; at another
point in the rod is the weight to be moved, and at a
third point the power is applied. There arc three
kinds of levers, the difference lying in the relative
position of the three points mentioned. In the first the
fulcrum is in the middle, in the second the weight, in
the third the power. Of the first we have an example
when a poker, rested on the bar of the grate, raises
the coal. An oar is an instance of the second ; the
boat is the weight, the fulcrum is the water upon
which the oar works. And this makes clear an im-
portant fact, viz. that the fulcrum is not always an

174 THE STRUCTURE AND LIFE OF BIRDS chap.

absolutely fixed point, though, of course, the lever
would be improved if it could be made so. The third
kind of lever in which the power is applied at a point
between the weight and the fulcrum is not often used,
because it does not economise labour. We have
an instance of it in the treadle of a sewing-machine,
where the force required is so slight, that economy
is unimportant. Wasteful as it seems to be, this third
kind of lever is the common one in the bodies of
animals. All three classes are represented, but ex-
amples of the first and second are comparatively rare.
Consequently there have been people who have main-
tained that the human body is a clumsy machine made
on antiquated and unscientific principles. Such an
idea shows the danger of a little knowledge. When we
use a lever, we wish to move a weight with compara-
tively little effort, however much we may lose in the
speed and amount of the movement. In the levers of
the body rapidity is a great object. The arm is a series
of levers of the third order, and by their help it can be
drawn in quickly, then shot out again to deal a sudden
blow. If we try to hold out a weight at arm's length,
we then find the weak point of levers of this order.
To economise effort with them you must apply the
power near to the weight. In the case of the arm,
we should require a biceps, springing, as now, from
near the shoulder but attaching near the wrist, and
this, besides other inconveniences, would entail great
slowness of movement. In a bird's wing the leverage
which aims at moving a weight with great rapidity is
to be seen in its greatest perfection. Very powerful
muscles arc required, but the muscles are there.

vii FLIGHT 175

The wing-lever must find its fulcrum in the air.
A strong breeze, or, better, a hurricane may give us
a notion how this is done. The air, which, when
still, seemed to ofTcr no opposition to our progress,
becomes, when moving at a great pace, an obstacle
through which we must shove our way with effort.
It makes no difference whether it is. an actual wind
that opposes us, or whether it is one existing only
relatively to ourselves, being produced by our own
rapid travelling. A bicyclist if he rides fast on a
calm day is retarded by a breeze due to his own
velocity. If there is a light breeze ahead, this may
be doubled by the pace at which he rides, so that
what to a pedestrian is hardly a breath of air is
magnified by the bicyclist into a wind. The resistance
of the air, then, increases if we move through it more
quickly. But this is only a very vague statement,
that gives but little idea of the facts. Some experi-
ments were made by Newton showing that in many
cases the resistance of the air increases as the square
of the velocity. These experiments depend on the
fact that a body, when let fall, gains in velocity for
some time, after which it maintains a uniform pace.
Nothing but the resistance of the air can check a
progressive increase in velocity, and when the pace
becomes uniform it is clear that the resistance of the
air is exactly equal to the weight of the body falling.
Newton took glass globes of equal size, but unequal
weights, corresponding to the figures 1, 4, 9, 16.
These he let fall from a height, and measured their
velocities when each had settled down to its uniform
pace. The velocities were in proportion to the

176 THE STRUCTURE AND LIFE OF BIRDS chap.

numbers I, 2, 3, 4, but the resistance of the air was,
as we have seen, equal to the weights of the globes
which were as 1, 4, 9, 16, the squares of the numbers
which represent the velocities. Experiments at once
more elaborate and more accurate have been made
since. Professor Marey concludes from many made
by himself, that the resistance increases in a less
proportion for velocities between o and 10 metres per
second ; when 10 metres per second is exceeded, then
the rule of the square of the velocity under-represents
the rate of increase of the air's resistance. When
the speed attained is very great, in the case of a
bullet for instance, Newton's law does not hold at all :
the rate of increase of resistance altogether outpaces
the square of the velocity. The rate of movement of
a wing is comparatively moderate, so that here it
might seem that we should be safe in applying
Newton's law. There is liability to error, however
from another cause. A wing is very different from
the glass globes with which he experimented — it
presents a concave and irregular surface with rough
edges. Such an object passing through the air, which
is not a perfect fluid, but viscous, must, like an oar
forced through the water, produce eddies, and this
complicates the problem so much that our greatest
authorities confess that we know very little of the
resistance to a surface like that of a wing. It is
necessary to say this, since the rule " resistance of air
increases as the square of the velocity" is often
quoted as if it held true of all surfaces and all
velocities. Nevertheless it comes near enough to the
facts to be of great value, and probably when we

VII

FLIGHT

177

apply it to a bird's wing we understate the rate of
increase of the air's resistance.

It will now be well to take a particular instance.
Let W A and W B represent the same wing in
different postures, a and b the same point in it. Let
a b be one inch in length, and A B three times as long.
When the wing descends, a passes through one inch
of air, A through three inches. But the resistance of
the air will be, at the lowest estimate, as the squares of
! anc | 3_that is, at A it will be nine times as great as
it is at a. It is by rapid movement of its wings,

Fig. 46.

then, that the bird obtains a fulcrum on which they
can work as levers. However large an expanse a
wing might offer it would be useless, unless it were
driven through the air at a great speed ; it could not
possibly obtain the comparatively fixed point that
every lever must have. Though there is still much
to be said about the shape of the wing and the way
in which the air acts upon it, we have advanced far
enough to understand the system of leverage. The
weight to be raised is the body of the bird, the
power lies in the breast muscles and is applied not far

N

178 THE STRUCTURE AND LIFE OF BIRDS CHAP.

out upon the wing, the fulcrum is at a point not far
from the tip. As a fact, of course, the fulcrum is dis-
tributed over the whole wing, but since, owing to its
more rapid motion, the end meets with far more
resistance than the base, we may consider it to be
not far from the tip. Here it will be well to mention
something that often makes living machinery puzzling.
The different parts are not distinct. For instance,
when a man breathes, his chest is a suction pump.
But there is no separate piston. The walls of the
chest, that is, the walls of the pump itself, expand and
so cause a vacuum. In the same way we have been
speaking of the bird's body as the weight to be raised,
of the wings as levers, and of the power as residing in
certain muscles. But the muscles in question form
part of the body, and they and also the wings go to
make up the weight. Nor have we yet done with the
complications in which we get involved when we
study the wing as a lever. When it is being moved
rapidly through the air in order to gain a fulcrum, by
the help of which to move the body, the weight is, at
first, at the extremity in the shape of the resistance
of the air that has to be overcome, while the fulcrum
is at the shoulder-joint. When the fixed point has been
gained, then the end of the wing becomes the fulcrum,
and the body is the weight. But it is only in imagina-
tion that we can divide the down-stroke into two such
periods. During the whole of it we have at the near
end both a weight and a fulcrum, during the whole of
it both a weight and a fulcrum at the further end.
The body is always suspended from the wings, the
ends of the wings never cease to move as they strive,

vii FLIGHT 179

each, for their fixed point. The fact is that the dis-
tinction between the fulcrum and the weight is an
artificial one. The power applied acts on both ends
at once, and if only one moves, or if one moves more
than the other, we speak of the weight as being at
that end. A weight at the other end which does not
give, or gives less, we call the fulcrum. In a bird's
wing both ends move, but since the object is to obtain
for the extremity as fixed a point as possible and to
raise the body, the term fulcrum is reserved for the
air. The bald statement, " The air is the fulcrum," is not
incorrect, but it leaves out of sight a most interesting
process. It is the rapid motion of the wing that wins
for it a comparatively fixed point, and throughout the
process the air is being moved by a lever that has for
its fulcrum the shoulder-joint. The oar, though a lever
of the second order, presents the same difficulty, but in
a less puzzling form. People who have never thought
of the subject are apt if asked what is the fulcrum on
which an oar works, to reply " the rowlock." This is
as much as to say that it is the aim of the oarsman
to displace as much water as possible. It is only,
however, by making the displacement of water a
preliminary object, that he gains a fulcrum by which
to move his boat.

We must now consider the working of both wings
at once. In order to understand this we may imagine
a boat rowed by oars employed as levers of the same
order as a bird's wings. The rowlocks would be in
the middle of the boat, and the oarsman would sit on
either side holding the oars between rowlock and
blade. They would have to face the bows, and this,

N 2

180 THE STRUCTURE AND LIFE OF BIRDS chap.

perhaps, would be the only advantage. But it is quite
possible to propel a boat in this way, and such a
system of rowing would illustrate what takes place in
flight. True, the blades of a bird's "oars" face
differently, so that, while they propel him, they at
the same time raise or maintain him in the air. But
the system of leverage is the same. This diagram is
a further illustration.

Fig. 47.

A* and Z are rigid rods representing the bird's wings hinged at I 'to V V the bird's
body ; a b and c b are the muscles which lower the wings. The shortening of a b
and c b will cause Y 3" to rise, since the air resists the descent of X and Z. After
Alix, Appareil Locoiiioteur des Oiscaicx.

Horizontal Flight.

Why does a bird advance horizontally when it
works its wings up and down ? The common metaphor
which makes them oars is picturesque, but may be, as
I have shown, misleading. Vergil, who, in describing
Daedalus' wings, uses the expression " Remigium
alarum," doubtless never intended to commit himself
to any theory of flight. If in a scientific work, wings

VII

FLIGHT

arc spoken of as oars, it must be borne in mind that
they arc oars of a peculiar kind. The French arc
fond of the terms "vol rame " and "vol a voiles,"
which have the merit of neatly distinguishing ordinary
from sailing flight.

Wings work by movement up and down. If they
faced as the blades of an oar face, they would be
useless for flight at the rate of fifty miles an hour, since
the stroke would be over before any force could be
put into it. If a boat is moving rapidly— at the rate,
say, of a mile in five minutes— its pace alone brings the
oar in quick to the oarsman's chest, even if he puts

Fig. 48.

little force into it. Hence the importance of " getting
on at the beginning," to use the language of rowing
"coaches," otherwise the happy moment is missed.
Rowing, then, is out of the question for a bird which
is to move at a speed far greater than that of any

boat.

How, then, is horizontal movement gained ? To
understand this a knowledge of the parallelogram of
forces is necessary. The proof of the principle involved
will be found in any book on elementary mechanics.
I shall merely try to make clear what is meant. If
two forces act upon a body at one point, they combine
to make one force. If B A and C A are two forces

i82 THE STRUCTURE AND LIFE OF BIRDS chap.

acting at A, the comparative length of the lines
representing the comparative magnitude of the two
forces, then there results a force which is exactly
represented both in magnitude and direction by D A,
the diagonal of the parallelogram. If, now, we start
with a force represented by DA, we can do in imagi-
nation, what often happens in reality, break it up
into two forces BA and CA, or other forces acting
along two other lines, which must, however, con-

Fig. 49.

verge on A from opposite sides of DA. This fact
is made use of a great deal in sailing. A sail SL is
slung obliquely across the boat, and the wind falling
upon it tends to move the boat in the direction XY,
at right angles to the sail. In the same way when a
stone, thrown slantingwise at a window by some one
standing some way off, but near the wall of the house,
breaks a pane, the fragments of glass are knocked into
the room in a direction at right angles to the window.
To return to the boat, the force XY can be broken up

FLIGHT

183

into two forces, one represented by XM, the other by
XR. But the force XR will have little effect since the
water offers great resistance to the movement of a
boat sideways through it ; at any rate if it has a keel or
a centreboard. The force XM will cause the boat to
move in the direction she is meant to go. This is an
excellent illustration of what happens in the flight of
a bird. When the wing is descending its front
margin is lower than its hind margin ; it is turned so
that the long feathers slope upwards. For simplicity

x Y

Fig 50.
section of wing ; X, front margin ; Y, hind margin ; W = wind caused by

the down stroke ; BA, resultant force, which is resolved into BC, BD.

we must imagine that the air is quite still. The wing
descends with great velocity through it, " making its
own wind " like a bicyclist. The resistance of the
air will be equal to a wind blowing vertically upward
(W in Fig. 50). When it strikes the inclined surface
of the wing its force will act in a direction at right
angles to it (BA). The force BA may be divided
into two forces, BD, which supports the bird, BC,
which drives him onward.

Whether Professor Fettigrew's description of the
wing's movement agrees with the account I have

1 84 THE STRUCTURE AND LIFE OF BIRDS chap.

given, is difficult to say. He speaks of it as working
like the screw of a steamer. If this is taken to mean
that the surface of the wing during the down stroke
has much the same incline as the blade of a screw, the
description is no doubt correct : the opposite wing will
correspond to a blade of a screw revolving in the
opposite direction : both tend to drive the bird
forward and, at the same time, lift it, as the descending
blades of the screw of a steamer propel and lift the
ship. But it is impossible to go beyond this and
compare such fractional rotations as the wing is
capable of to the complete revolutions of a screw :
during the up stroke the wing is certainly not a screw-
blade propelling the bird in a forward direction.

In horizontal flight the hind part of the body is
raised duri-ng each down stroke by muscular effort. 1

The Bird in Motion — Support Given by the A ir.

If a bird flies at a great pace he derives far more
support from the air than if he flies slowly. He is
perpetually coming to fresh columns of air, each
series of which is able to sustain his weight for a
moment. If a gull be tied to a string, it is said that
he cannot support himself when he comes to the end
of his tether, and though some other birds are not
under these circumstances reduced to complete help-
lessness, the downfall of the gull is a significant fact :
since he has no onward movement, the air gives his
body little or no support, and, besides this, the air
which his wings are beating has rolled in to fill the
1 See p. 257.

vil FLIGHT 185

vacuum made by the last stroke and is already in
motion downwards. The predicament in which his
wings find themselves may be illustrated by the screw
of a steamer ; till the vessel begins to move on, it
churns the same water round and round and gets very
little grip. Another illustration will help to explain
why the large body surface proves so poor a parachute.
Thin ice will often bear a skater who moves rapidly
over it, when it would break if he stopped for an
instant. A particular square yard has not time to
break before he has transferred his weight to another.
Or we may put it thus, that he is, as it were, supported
by long skates that spread the pressure over a great
area. This fact is turned to account by all birds.
Though the principle is always at work except when
they ascend almost vertically, we see it most clearly
when they get up pace and then glide onwards with-
out moving their wings which are fully or partly
extended, the body being sloped at a slight angle
upwards ; when this position is adopted, the resistance
offered by the air is very little. The bird cuts edge-
ways through it, and of the little resistance there is,
the greater part acts in an upward direction, or, in
other words, supports the bird's weight. Sir George
Cayley made some very interesting calculations with
regard to this. 1 Experiments had shown that if a
flat surface one foot square, a piece of board for
instance, be moved forward horizontally at the rate
of 2 3 6 feet a second, the resistance is one pound ;
whereas if it be held at an angle of 6° to the

1 " On Aerial Navigation," Journal of Natural Philosophy,
Chemistry, and the Arts (Nicholson's), xxiv., p. 164 (1809).

1 86 THE STRUCTURE AND LIFE OF BIRDS chap.

horizon and moved forward as before, the resistance
will be only f lb. in a direction perpendicular to its
surface. If the velocity is then increased to 37*3 feet
per second, the resistance will be equal to 1 lb., since,
as we have seen, it increases, speaking roughly, as the
square of the velocity. 1 Sir George Cayley estimated
that the weight and wing area of a Rook were in
the proportion of 1 lb. to the square foot. The Rook-
would therefore maintain his level, if having a velocity
of $7'$ feet per second he placed himself at an angle
of 6° to the horizon. This calculation has great value
since it emphasizes and connects three important
facts : (1) if a bird inclines his body at a small angle to
the horizon the resistance of the air is much less ; (2)
the resistance being at right angles to the plane of his
body and wings will tend to support his weight ; (3)
the support given increases enormously with increase
of velocity. But it is easy to show that it is inaccurate.
To begin with, a Rook does not present a plane
surface like a piece of board. It presents irregular
concavities which cause the air to offer a far greater
resistance ; but how much greater, has not been dis-
covered either by calculation or by experiment.
Besides this, the whole f lb. resistance offered by
the air will not go to the bird's support. A small
fraction of it will oppose his onward progress, the pro-
portion which is thus a hindrance and not a help

1 § lb. x 37 '3 * 37'3 = f x I (nearly) = lib. A mistake in
23-6 x 23-6 5 " 3i

the figures made by Sir George Cayley has been corrected by

Professor Roy, See Newton's Dictionary of Birds, vol. i., p.

263.

VII

FLIGHT 187

diminishing as the bird's position approaches nearer
and nearer to the horizontal. This reduction of
waste need not, however, mean an absolute gain.
He will have to bear in mind that if he reduces the
upward inclination of his body to the vanishing point,
his course will be inevitably downwards. When the
angle becomes extremely acute, nearly all the resist-
ance comes in the form of support, but this fact will
avail him nothing if the total of resistance be too
small. Thus the Rook has to make a calculation : if
his velocity be so many feet per second, at what angle
must he set himself in order not to lose elevation ?
This will vary very much with the pace of the bird.
The Swift and the Rook are to one another as an
express and a parliamentary train : consequently the
Swift can venture upon a much more acute angle than
the Rook : consequently he will lose pace less rapidly
and will be able to glide further. 1

Mathematical problems are often simplified as
Sir George Cayley has simplified this. The com-
plications in Nature are so great that, in many
cases, it is necessary to eliminate some of them,

1 It is estimated that the horizontal is to the vertical resist-
ance as the sine of the angle made by the bird's body with the
horizon is to the cosine.

Thus if BA represents the bird's body the relative lengths of
AC and CB represent the ratio of the horizontal resistance of
the air to the vertical resistance.

Fig. 51

iSS THE STRUCTURE AND LIFE OF BIRDS chap.

if the problem is to be tackled at all. Every one is
familiar with the pipes of different dimensions in
arithmetic books which are opened at various times,
some of them filling, some of them emptying a cistern,
the problem being to discover at what precise time
the cistern will be full or empty. Such problems give
healthy exercise to the brain, but we must not suppose
that the behaviour of water passing through pipes is
a thing that can be absolutely predicted. The present
calculation shows us general principles. But, since it
does not take into account the resistance of the air to
such an irregular surface as that of a bird's wing, it
does not enable us, in the case of a particular bird, to
fix the exact angle at which he must set himself, if he
wishes, having attained a certain velocity, to glide
onward and maintain his level.

Though the resistance offered by the air to surfaces
like those of a wing cannot be accurately measured,
yet it is possible to obtain some notion of its amount.
If you hold an umbrella so that the inside faces a
strong breeze, it feels a great strain, and is likely to
give at every point. If, on the other hand, the out-
side meets the blast, the air passes harmlessly off its
slippery convexity. Herr Lilienthal, the German
engineer, who has sailed through the air a distance of
over 500 yards with only a slight descent, once, as he
was carrying his wings to the place of trial, was
cheered by the fact that the air gathered in their
curved under-surfaces and relieved him of all the weight.
If we take a single big wing feather and wave it
through the air, we feel that the resistance varies
according as we turn the concave or convex surface to

vii FLIGHT 189

the front. The difference is far more apparent when
we take the whole wing. Above, a bird s wing is con-
vex, so that it passes easily through the air ; the
under-surface is concave and lays hold of it. I am
referring mainly to the part of it which is nearest to
the body and which forms a kind of irregular cup, its
hinder side gently sloping away. Experiments have
been made with a view to measuring the resistance of
the air to concave surfaces, but the results do not help
us much. The irregularity of the wings has not been
reproduced, the velocity has been uniform, whereas
that of the wing is very different at different stages of
the stroke, and no account has been taken of the
variation of the curves during flight owing to the
elasticity of the great feathers.

The duty of the near part of the wing is to a great
extent that of a parachute. Between the strokes, the
bird drops, and were it not for these umbrella-like
supports, the drop would be greater than it is. The
work of propelling is, as we have seen, done mainly
by the extremities of the wings, which move with far
greater rapidity. It is possible to find approximately
the centre of the action of the air — to find a point in
the wing so situated that the air shall act with equal
force on the near and far sides of it. If the wing were
a triangle, this point would be in a line drawn from
the middle of the base to the apex, f of its length
from the shoulder. If it were a rectangle, it would be
at a point on the corresponding line f of its length
out. Professor Marey estimates that in the actual
wing it is at a point about | of its length from the
base. It must vary much in wings of different shape.

190 THE STRUCTURE AND LIFE OF BIRDS chap.

But it is not only when we compare base and
extremity that the division of labour is unequal. If
we divide the wing in imagination by a line drawn
down its middle from base to tip, then the front half
will do far more work than the hinder half, when the
bird is gliding at great speed or moving his wings
rapidly through the air. A boat sailing at an angle
to the wind with a sail slung obliquely across it
supplies an illustration of this. If she moves rapidly,
only the forepart of the sail will do much work. The
wind is blowing, say, at right angles to the boat. It
will strike the forepart of the sail, over all the rest of
which there will only be a backward current of air,
which has been turned from its course by the forepart.
The faster the boat sails, and, also, the nearer to the
wind she sails, the truer this will be ; the narrower
will be the margin of sail that really works. This
is called the law of Avanzini. It holds with regard
to the bird's wing, which during the down stroke moves
rapidly forward as well as downward, and, of course,
shares the onward movement of the whole bird. It is
truer of the swiftly moving extremity than of the
slower inner part, and this accounts for the remarkable
way in which long wings, notably those of the Swift
and Gannet, narrow towards their ends. Professor
Pettigrew made some experiments which illustrate
this. He cut away the hinder part of a Bluebottle's
wings and apparently it could fly equally well. In
the same way with Sparrows, the removal of the same
part of the wing seemed to do little damage. Still to
describe the flight after these mutilations as " perfect "
is to go too far. No failing may strike the eye. A

VII

FLIGHT

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192 THE STRUCTURE AND LIFE OF BIRDS chap.

man with his hands tied behind his back may appear
to walk with ease, but in the course of a long tramp,
it would hamper him much. In stopping-, in rising
and till great velocity is attained, a broad expanse of
wing is of use.

For the understanding of gliding flight also, it is
most important to bear in mind the law of Avanzini.
If a bird wishes to descend rapidly, he must partly
flex his wings, so that they may present a less
extended front and, consequently, receive less support.
If he wishes to descend very gradually or maintain
his level or glide upward, he must open his wings to
their full stretch, so as to have the support of as long
a front line as possible. The amount of work done
by the front margin and, consequently, the trajectory
of his flight will of course vary with the pace x (fig. 52).

The principle just explained can be seen at work
in little paper contrivances. Take a piece of paper
shaped thus

Fig. 53-

Fold it along the line AB so that the two sides slope
upwards. Put in a pin along the line with its head
near A. Hold it on a Siant with A at the lower end
and let it drop. It will glide some distance and very
likely show an upward tendency ending in a somer-
1 See Newton's Dictionary of Birds, p. 265.

VII

FLIGHT '93

sault. Then put in the pin with its head near B and
drop it as before, but make B the lower end. It will
glide by a steep descent to the ground. It is worth
while improving the cut and balance of these little toys
till they behave properly, for they admirably illustrate
gliding flight : when A leads, a bird with wide-spread
wings is represented ; when B, a bird with wings
partly flexed.

Before closing this subject I must refer to the old
fallacy that the bird, owing to its hollow bones, is
like a balloon. This has been already dealt with in
the previous chapter (see p. 105).

The General Shape of the Bird.

If a ship were built on absolutely perfect lines, when
she lay at anchor and swung to the tide, the pressure
of the stream upon the bows would be exactly balanced
by the closing of the waters again at the stern, so
that the only force straining at the cable would be due
to friction. The case would be far differrent if the
vessel presented a flat surface to the current at right
angles to it. The lines on which a bird is built are
first-rate : its body is not unlike in shape to that of a
fish, and investigations have shown that fish are con-
structed so as to meet the minimum of resistance in
passing through a fluid.

194 THE STRUCTURE AND LIFE OF BIRDS chap.

Passive Machinery.

The Passive Machinery consists of bones, ligaments,
tendons, membranes and feathers. None of these
have, like muscular tissue, any power of contraction.
Any activity they may show is really due to their
being acted on directly or indirectly by muscles.

It will be well first to make some general remarks on
the different kinds of joints. Passing over what are
called imperfect joints because they allow of very little
motion — those between the vertebrae in man are good
examples — we have (i) ball and socket joints where the
rounded surface of one bone fits into a cuplike hollow
in another ; (2) hinge joints where only motion back-
wards and forwards is possible ; (3) double hinge joints,
e.g. that by which the thumb articulates with the
wrist, or those between the neck vertebrae of birds ; I
compared this joint above to two saddles put one
atop of the other ; it has been well likened to the
articulation between the rider and the saddle ; (4) pivot
joints, e.g. that between the skull and the atlas
vertebra.

In the wing we have examples in full working order
only of the ball and socket, and hinge joints. The
shoulder joint comes under the former head, and yet
it is very different from the types of which we have a
familiar instance in the thigh joint, in which a round
ball fits into a deep cup. At the shoulder the cup is
shallow and imperfect, and the bone which fits into it
is not round. The result is that it works with great
freedom, so much so that it has sometimes been called

vii FLIGHT 195

a universal joint. It is at this point only that the wing
turns and twists, when it is fully extended : chiefly at
this point, when it is partly flexed. The other joints
are hinges : had they anything of the " universal "
joint in them, the wing would not have that stiffness
which is indispensable, if it is to stand the pressure
upon it. When the wing flexes some free play in other
directions is possible. It will be interesting now to
compare the elbow joints of men and birds. In man
as in birds the ulna {i.e. the hinder or postaxial one of
the two armbones) articulates with the humerus by a
simple hinge joint. The difference lies in the articu-
lation of the radius, the praeaxial bone. In man
this moves with considerable freedom, articulating,
as it does, with the humerus by a pivot joint. If
you lay your elbow, forearm, and the back of your hand
upon a table, you can, while still keeping the elbow
immovable, turn the arm so that the palm of the hand
faces downwards. The radius revolves upon its pivot
joint and the hand with it. In the last-mentioned
position it lies across the ulna. It will be useful to
remember that the thumb continues the line of the
radius. When the back of the hand is downwards,
the position is called supination {supinus = lying on
the back) ; when the palm is downwards, pronation
{promts = face downwards). In the bird the differences
are great. When extended for flight, the whole wing
is pronated and the elbow joint is immovably stiff. It
is true that if we straighten our arm and then place the
elbow on a table, the hand will not turn so readily as
it did when the arm was bent at the elbow. Still a
great deal of free play remains. In the bird the whole

O 2

196 THE STRUCTURE AND LIFE OF BIRDS chap.

limb becomes rigid. I think this is due mainly to the
ligaments. When the wing is flexed there is much less
rigidity, for, to say nothing of the movements necessary
to flight, the different parts of the wing must face
different ways in order to fold neatly over the body,
the upper arm looking upwards and inwards, forearm
upwards and outwards, the hand outwards and only
very slightly upwards.

The way in which the wrist joint has been modi-
fied is remarkable : the hand has very little of the up
and down movement that comes to ours so easily, and
when the wing is fully extended, none at all. Its only
free movement is away from the thumb and towards
where the little finger would be. Our wrists are very
stiff, if we try to move them thus. This peculiarity in
the bird's wrist may be traced to the radius. When
the wing is folded the bone slides forward and, extend-
ing beyond the ulna, forces the hand into the position
described. When the wing is straightened the radius
slides back and brings the hand into line with it.

The wing presses with tremendous force upon the
bones that support it. W r hen it descends like a flail,
with its face during the first half of the stroke looking
not only downwards and backwards, but also outwards,
there must be great pressure inwards upon the pivot on
which it turns. I have in an earlier chapter shown
whence this pivot derives its strength. The shoulder
is the meeting-place of three bones (the coracoid, the
scapula, and clavicle), though only the two former
actually help to form the joint. The coracoid and
clavicle slope outwards, and it is this outward slope
that gives them their power to resist the pressure

VII

FLIGHT 197

inwards, the scapula being useful mainly for hingeing
the back and breast together. A bird with a broken
clavicle is said to be incapable of flight. 1 But the
coracoid is much the stronger bone of the two. Its
broad base is fixed in a long groove in the sternum, to
which it is tied by powerful ligaments, and, besides
this, a strong membrane, covering all the space between
it and the clavicle, binds the two together. If skeletons
representing a number of species arc examined, I
believe it will be found that in birds of powerful flight
the coracoids project outwards more than in inferior
flyers. I have no accurate measurements of my own
to give. But I once looked carefully through the
collection of breastbones at South Kensington and
noted clown that in the Albatross, the Adjutant, the
Golden Eagle, and other birds that fly well, the two
coracoids made a large angle with each other and
were, at the same time, strong and short : whereas in the
Crowned Pigeon, Game-fowl, Goose, Parrot, Pheasant,
they formed a small angle and were at the same time
in proportion to the size of the bird longer and weaker.
I ought to mention however, that Furbringer, a very
great authority, holds that the size of the angle made
by the coracoids varies according to the size of the
bird : the greater the bird the greater the angle.' 2
He recognises exceptions to the rule, and these
exceptions will, I think, be accounted for by the
power of flight of the particular species. His rule has,
no doubt, considerable foundation : a big bird takes

1 Yet some Parrots, whose clavicles are rudimentary, fly well.
- SeeMaxFurbringer's Untersuchungen zur Morphologie und
Systcmatik der I "ogel, p. 740.

198 THE STRUCTURE AND LIFE OF BIRDS chap.

slower and stronger strokes, the rate of movement of
the further end of the wing increasing enormously with
increase of length, and the resistance of the air in-
creasing out of all proportion to the increase of
velocity. Thus, in proportion to his weight and bulk,
a big bird will require a firmer pivot for his wing than
a small one. But the comparatively small inter-cora-
coidal angle in the Pheasant (if I have estimated it
correctly) shows that power of flight as well as size
must be taken into consideration.

The working of the passive machinery may be seen
well in a dead bird. Only extend the arm, and the
hand is extended ! Only extend the hand, and all the
large feathers are spread ! The radius, as I have
explained, slides back when the forearm is extended
and pulls the hand with it, bringing it into line with
the arm. Then follows the sudden expansion of the
wing feathers like a fan, effected by means of elastic
ligaments through which the primary and secondary
feathers pass, and which are stretched directly the
anele between the hand and forearm is widened. The
system of ligaments is elaborate. If the skin is removed
it will be seen that each of the great feathers is fastened
to the bone by a stringy tendinous mass. Even in
the dry skeleton they leave their mark. The elastic
ligaments can be made out without any dissection.
One, through which the quill of each feather passes,
can be clearly seen extending from the extremity of
the handbone or metacarpal to the armpit, including,
therefore, all the secondaries, and, with the exception
of those that spring from the fingers, all the primaries.
It is in fact a continuation of a tendon connected with

VII

FLIGHT

199

200 THE STRUCTURE AND LIFE OF BIRDS chap.

a muscle that arises from the ribs. A little nearer the
bases of the quills there is another ligament which,
instead of being pierced by them, runs along the lower
side only. These two ligaments, according to M.
Edmond Alix, get confounded at the hand and elbow,
but in Pigeons I have found them running still separate
beside the hand.

The above description may possibly have given the
impression that the bones and ligaments supplied
some motive power. This can only come from muscles.
The forearm is put into line with the humerus by
muscular effort, and the effort required is greater since
the hand is necessarily extended by the same move-
ment, and the extension of the hand requires more
force since it involves the stretching of the ligaments
in which the feathers are set. Thus, indirectly, the
hand and the great feathers are prepared for flight by
the action of the triceps muscle that extends the fore-
arm. This must not be supposed to mean that there
is no special muscle to extend the hand. There is
one for this purpose arising from the further end of
the humerus. And it must be remembered that the
radius cannot by its sliding movement bring the hand
absolutely into line with the forearm : the finishing
touch must be given by the muscle just mentioned.

It is a marvellous piece of machinery which thus
spreads the wings. But what is perhaps the most
remarkable thing with regard to the secondaries — the
great feathers that spring from the forearm — has yet
to be mentioned. They are shifted in such a way
that they prevent the passage of air during the down
stroke, but let it pass during the up stroke. With this

vn FLIGHT 201

view each is connected by a little triangle of tendon
with a muscle, 1 which arises from the upper-arm bone
at the end further from the body and attaches its
other extremity to one of the wristbones and also to
one of the metacarpals further on. Working un-
opposed, it bends the wrist. When the wing extends,
its resistance tightens up the wrist-joint and helps it
to bear the strain of flight. When the wing is bent, it
lies in a slightly curved form. It is straightened out
when the wing straightens, and this, combined with the
sloping of the feathers outwards from their bases, as
they spread, stretches the little tendons that arise from
it and are fastened to the feathers. These little tendons
slope outwards, away from the shoulder, and passing
under the quills fasten to their further side (fig. 54).
When tightened, they rotate the feathers so that the
near side of the vane is pressed hard against the off
side of the one that overlaps it, thus preventing the
passage of air. During the up stroke the tendons
relax and the feathers are no longer pressed tight
together, so that the air can now pass through them.

It is worth remarking that though the Triceps
controls the wings from the elbow joint to the hand,
yet that the two united fingers and all the feathers
upon them are not under its sway, but depend upon
their own muscles to extend them, lower them and
raise them — small movements the importance of which
it is so difficult to estimate. The bastard wing also is
moved by its own muscles.

The wing-area is greatly increased by two mem-

1 The muscle is called flexor carpi ulnaris by M. Alix ; the
cubital anteneur, see p. 412 in App. Loc. des Qiseaux,

202 THE STRUCTURE AND LIFE OF BIRDS chap.

branes, one of which, called the anterior membrane,
stretches from the head of the clavicle to the hand.
In the Gannet it is of great breadth, and it is so placed
that it makes an almost upright wall along the front
margin of the wing, which thus presents a deep
umbrella-like hollow, such as a parachute requires.
In most birds the membrane is slung more horizontally.
In all it is at once stretched, when the wing opens.
The other membrane lies behind, in the armpit, and
fastens the wing to the side.

The great feathers make some movement, without
assistance except from the air. From below they are
concave and during the down stroke the concavity is
much lessened owing to the pressure upon them. When
the muscles cease to lower the wings, the feathers
regain their full curve owing to their own elasticity,
and their bending thus is equivalent to a slight
prolongation of the stroke. Up till this moment they
have yielded to the air, they now strike down against
it. The result of their action is that the stroke is
longer and less violent, and that the strain upon the
wing is lessened.

The air lends very material assistance in another
movement, to effect which there is, as I have shown,
muscular machinery provided. The outer webs of the
feathers are very narrow compared with the inner ones.
The result is that the air acts much more strongly
upon the latter, forcing them upward, so that each
feather has the inner side of its vane pressed closely
against the one which lies next to it and above it on
the side nearer to the body. 1 Thus the shape of the

1 This interesting fact was first pointed out by Professor Roy.

FLIGHT

203

feathers themselves greatly reduces
the work of the muscles and tendons
in making the adjustments that arc
required in order to render the wing
impervious to air (fig. 55).

The Active Machinery.

The great muscles of flight lie below
the shoulder joint, and yet upon them
falls the task of raising the wing. All
the great mass of muscle lies behind
the joint, and yet the wing must be
lowered without being pulled back-
ward. Besides this the muscles can
incline the wing rightly for upward
flight, for a downward swoop, for a
sudden halt ; they can adjust it to
every varying breeze, to every current
or eddy that can be turned to account.
In addition to the breast muscles,
there are many smaller ones which
help in these niceties of adjustment.
When we consider the number of
these, each with its special office, each
giving the wing a slightly different
turn from any other, it is extraordinary
that Professor Marey should state that
" the muscular apparatus of the bird,
like that of the insect, has nothing FlG - 55— Primary

„ & wing feather of Heron

to do with the course of the wing ; fis? 8 than natura ! size )

53 ' T. he outer web is nar-

devation and depression are almost the row -

2o 4 THE STRUCTURE AND LIFE OF BIRDS chap.

only movements which it can produce." 1 Almost
everything beyond the mere up and down motion he
attributes to the resistance of the air. It is true that
some important movements can be set down wholly
or in part to this. Still much is left for the muscles
to do.

Before their position and their working can be

a. b.

Fig. 56.— Humerus of left wing. («) lower ; (b) upper side.
E, elevator muscle attaches. The point of attachment varies, in distance from the
shoulder and from the pra^axial margin. The action remains much the same. F,
air foramen ; ld, latissimus dorsi attaches ; gp, great pectoral or depressor ; P3, third
pectoial.

understood, some further account must be given of the
humerus. Its position, even when at rest, is abnormal ;
it has received a twist at the joint which has left it
set differently from the humerus of any other animal,
so that you must not use the terms " above " and
"below," "postaxial" and " praeaxial," before care-
fully finding your bearings. In addition to this it
1 Animal Mechanism, p. 214,

vii FLIGHT 205

so rotates in the course of the stroke th.it further
bewilderment is apt to arise. When the wing is
folded, what is really the upper side of the humerus
looks partly upward, but mainly towards the body.
It may be easily recognised by an unmistakable
landmark, the foramen or aperture through which the
bone is aerated (see Fig. 56). In the primitive an-
cestors of birds, with their forelimbs not yet adjusted
for flight, this would have been on the upper surface,
and postaxial. On what is really the under surface,
but in existing birds looks forward instead of down-
ward when the wing is at rest, there is another land-
mark. Where the broad expansion at the rear end
of the bone begins to narrow down, there may be seen
a rather long mark on the bone (GP). This is
where the Great Pectoral muscle attaches — on what is
properly the under side near its prseaxial edge. In
describing what we may call the geography of the
bone, we must always state what would be the case
if it were set as it is in other animals, and as it is in
the bird itself during the down stroke.

At its near end it is broad, and, according as a
muscle attaches at one margin or the other, can be
made to rotate either way. A pull from below on the
prseaxial margin will make it turn its under surface
backward. A pull on the prseaxial margin of the
upper side will make it face forwards. And so forth.

The Great Pectoral muscle springs from the lower
part of the keel (supposing the bird is placed breast
downwards), partly also from the sides of the breast-
bone above the keel and from the ribs where they
join the breastbone. From the breast it passes forward,

206 THE STRUCTURE AND LIFE OF BIRDS chap.

fastens firmly to the clavicle all along its length, and
then by a very short tendon attaches where I have
pointed out the mark of it on the humerus, on
its under side and its further or pmeaxial margin.
I have in several species found another less strong
attachment, also on the under side of the bone,
but near its postaxial margin ; and I cannot help
thinking that the bird can make the muscle act mainly
on the one or the other at his pleasure. In both cases
the wing would be lowered, but it would be rotated
differently ; working through its main tendon attached
to the prseaxial margin of the bone it causes the wing
to face to the rear as it descends.

Every one must have noticed that, when the breast of
a chicken is carved, the greater part of the meat flakes
off clean from a thinner layer that lies below in the
angle between the upright keel and the nearly hori-
zontal sheet of bone below. This upper part (the
bird now lying upon its back) is the great pectoral
that lowers the wing, the part below is the Elevator,
muscle. The muscular fibres in the Great Pectoral may
be easily seen in an uncooked bird running forwards,
or, more strictly, forwards and inwards, converging
from both sides on a line of tendon in the middle.
The set of the fibres shows the direction in which
they pull. But since the muscle does not arise only
from the breastbone, but also from the clavicle, the
result of its action is not to draw the wing backwards
and downwards, but simply downwards. The forward
movement is, as I shall show, due to the action of
the air (see p. 213). To turn to the Elevator muscle.
Unassisted, it could not raise the wing since it

vii FLIGHT 207

lies below it. But the humerus turns as on a pivot
at the joint, and if it be pulled inward it will be
raised to an upright position, if only the pull comes
from a point not below the pivot. A tendon, therefore,
must be made to pass through a pulley fixed at a
point high enough, and must attach to the inner side of
bone some little way out from the joint. By such means
a mast may be raised, if it be prevented from slipping
at its base. A pulley on a level with the shoulder-
joint is available. There are two bones, as explained
above, which meet to form the socket, the coracoid
and the scapula or shoulder blade. On their inner
side the top of the clavicle or merrythought meets
them, and the three together form a tunnel of bone.
Through this passes the tendon and fastens on to the
back of the humerus near its prseaxial margin.

The upstroke is at first in a backward direction,
mainly owing to the action of the wind due to the bird's
own velocity ; such a wind must necessarily drive the
wing backward and at the same time lift it. 1 The move-
ment to rearward, which is often difficult to detect in
photography, is clear in diagrams obtained by Professor
Marey by various methods. 2 Towards the end of the
upstroke the wing is moved forward, till it stands above
the pulley and the pivot, mainly by the action of the
Elevator muscle. The way to see the working of
these muscles is to take a dead bird and cut away the

1 The third pectoral muscle, springing from the outer and
lower part of the coracoid and from the sternum close to the
ribs, assists— its chief office being to draw the wing backward.
It attaches to the upper side of the humerus, just over the air
foramen.

2 See p. 221.

2o8 THE STRUCTURE AND LIFE OF BIRDS char

Great Depressor and the Elevator from the bone while
leaving their tendons attached to the humerus,
then pull them and watch the result. Much that
otherwise must be obscure then becomes perfectly
clear. Not the least interesting point is the way in
which the Elevator, being attached where it is, raises
the front or praeaxial margin of the wing, so that in
attaining its position for a fresh stroke it turns the
edge to the air and meets with little resistance. These
two muscles can also be worked at the same time, the
one antagonising the other ; the Depressor lowering
the wing to the horizontal, while the Elevator holds
the praeaxial margin fast, so that the hinder part
cannot be tilted up. A bird sets his wings thus when
he wishes to halt. A movement that looks perfectly
simple may be due to the combined action of a
number of muscles.

The Spreading of the Wing.

It is a strange thing that in these days when it is
boasted that machines can be made to do most things
that a man can do, that sailors should still have to
run up the rigging, be the weather foul or fair, and
straddle across the yards in order to furl or set
the sails. It would not seem to be beyond human
ingenuity to devise machinery by the aid of which
this work should be managed from the deck. Some
progress towards this has, I believe, been made. In
the bird we find such machinery brought to great
perfection. Instead of men we have muscles, and by
the machinery of tendons and ligaments these muscles,

vii FLIGHT 209

situated on or near the body, arc able to spread the
wings and regulate their utmost extremities. It is
all-important that it should be so. All the weighty
organs must be accumulated in the body. To speak
metaphorically, the wings must be made up of very
little besides masts, sails, and cordage.

The great muscles that move the humerus I
have already described ; they spring from the breast-
bone and neighbouring bones. The muscles that
bend or straighten the arm at the elbow arise from
the top of the coracoid and from the anterior end of
the shoulder-blade respectively. Nearly all their bulk
and weight is near the body. One of them, the
triceps, does a great work ; it straightens the elbow-
joint, whereupon the hand is extended and the great
ligaments get to work and spread the feathers, and
then only small points remain for small muscles to
see to. There arc ten muscles springing from the
further end of the humerus. These, of course,
are not large, and most of them extend only to the
wrist or metacarpal bone. Two of them, however*
by the help of tendons move the fingers. Altogether,
to do this work, there are five long muscles spring-
ing either from the humerus or the ulna. And
one of these attaches to the much-reduced thumb,
called the bastard wing. 1 There are also no less
than eight very small muscles springing from the
wrist and metacarpal bones, and filling up the space
between the latter. All of these are attached to the
fingers or " thumb," which, actually, engrosses no

1 See p. 42, where it is discussed what fingers in our hand
correspond to the three in a bird's wing.

210 THE STRUCTURE AND LIFE OF BIRDS CHAP.

less than four ; the second digit (the first finger as we
call it in our own hands) has three, and the third digit
only one. But it must be remembered that these two
digits are fixed together so that one cannot move
without the other. Counting long and short muscles
together we have thirteen to move the " thumb " and
fingers. Of these the " thumb " has five, one long and
four short.

Active and Passive Machineiy — Summary.

(i) All the muscles of any size at all arise not
further from the body than the far end of the humerus
or the near end ofthe forearm. All the really large
muscles arise from the body. The spreading of
the wing is much simplified by the fact that the radius,
when the elbow-joint is straightened, slides back and
extends the hand. The extension ofthe hand involves
the stretching ofthe elastic ligament, and, consequently,
the spreading of the great feathers. The straightening
of the arm also spreads the anterior and axillary
membranes, thereby greatly increasing the wing area.

(2) The fingers must have a great deal of freedom
of movement, since they have so many muscles
attaching to them. Similar evidence seems to show
that the " thumb " cannot be quite rudimentary.
Later on I shall show that it is sometimes used.

(3) Muscles, which, if worked separately, have con-
trary effects, may work together with good results.
Thus if the muscle which bends the wrist be contracted
at the same time as the one which puts the hand in a

vii FLIGHT 2ii

line with the forearm, the result will be that the
hand will be pulled hard against the wrist. This will
help the ligaments to resist the very great strain put
upon them. The wonder is that, with all its strength,
the wrist joint does not succumb.

(4) The air, to a considerable extent, determines
the movement of the wing. This will be explained
more fully later on.

(5) There are muscles which rotate the great
secondary feathers and hold them in the best position
to make the wing impervious to air. All the great
wing feathers are so shaped that the action of the air
upon them assists the muscular machinery.

Such is the wing, at once strong and light ; pliable or
stiff, according as pliability or stiffness is needed at
different parts, or at different times ; quickly spread
and quickly flexed, capable of the nicest adjustment
to suit every phase of flight ; worked by machinery,
all the weightier part of which is massed upon the
body or close to it, and turning on a pivot which
stands firm under all pressure.

Weight of the Breast Muscles.

To realise how important are the muscles that
lower the wing, it is only necessary to know what
proportion their weight bears to that of the whole
body. I have weighed those of two Wood-pigeons
and two domestic pigeons, and have found in each
case that they accounted for either just under or just
over one fifth of the total weight, In one of the

P 2

212 THE STRUCTURE AND LIFE OF BIRDS chap.

Wood-pigeons 1 the three pairs of breast muscles
represented three thirteenths of the whole — i.e., a little
less than one quarter, a very large proportion, the
average, among birds, being less than a fifth. The
Elevator is very small compared with the Depressor,
often so light that the most delicate scales are re-
quired if trustworthy results are to be obtained, The
differences in different species are very striking. In
the Wood-pigeon I have found the weight of the small
muscle to be a little less than one fifth of the greater,
in the starling just over one ninth.

Movements of the Wing partly due to the Action
of the Air.

The muscle which lowers the wing rotates the
humerus, as I have shown, so that the under surface
of the wing looks backward and downward. And
since the whole expanse of feathers lies to rearward
of the bones, the action of the air will tend to turn
the wing round in the same direction, just as the
wind swings a sign-board. When once an upward

1 These figures do not agree with those given by two German
investigators, Legal and Reichel, in the Jahres-Berichte der
Schlesischen Gesellschaft fur Vaterlandcidtur, 1879. They

give — ( = — = more nearly i than %) to represent the

3'43 V 343 '

relative weights of the three pairs of pectoral muscles and of
the whole body. This seems impossible, nor is it clear why in
the pigeon the breast muscles should weigh so enormously
heavier in proportion to the weight of the body than in any
other bird. In the case of other species their figures are not so
startling, and they may be more trustworthy.

vii FLIGHT 213

incline from the front to the hinder margin has
been brought about, the resistance of the air will
no longer act vertically, but at right angles to the
plane of the wing. And the force thus acting may
be resolved into two, one raising the wing, the other
uniinc it forward. When the end of the descent has
been reached, muscular action and the air acting
mainly on the front margin, 1 cause the wing to change
front and face forward and downward. The wind
due to the bird's own velocity will act on the oblique
surface, and lift it backward and upward. Thus when
the bird is flying rapidly the air relieves the Elevator
muscle of a great part of its work, and this accounts
for its small size. The velocity due to the action of
the great Depressor muscle comes to the assistance
of the small Elevator. Even when there is no great
speed to create a current of air relatively to the bird,
the descent of the body during the upstroke helps to
lift the wing. In the same way during the down-
stroke, the work is lightened and the extremity of
the wing appears to travel a much greater distance
than it really does. For the raising of the body
means a relative lowering of the wings, and helps
them home just as the motion of the boat seems to
help the oar through the water.

The Tail.

The tail feathers have their elastic ligament. At
their bases they arc firmly held by muscles, and arc
arranged like a fan. Some little way out from their
1 See p. 190.

2i 4 THE STRUCTURE AND LIFE OF BIRDS chap.

base they are bound together by a strong elastic band,
the outer ends of which pass into muscles which arise
from the vertebrae of the tail. When these muscles
contract, the fan opens, the distance between the quills
being increased where the elastic band holds them,
their bases meanwhile being held firm. The tail itself
can be moved in any direction, having several pairs of
muscles attached to the vertebrae and to the pelvis at
different levels so that it can be raised or lowered, and
also (since one member of a pair of muscles can con-
tract without the other) moved to right or left. It is
often important that the tail should form a hollow on
its under side. This is ensured by the form of the
feathers which is similar to that of the primaries and
secondaries in the wing. The outer webs are very
narrow in all except the one or two central pairs ; the
broad inner webs offer more resistance to the air and
are forced upward, each pressing hard against the
outer web of the feather next to it on the inner side.
The result is that the tail, besides being almost im-
pervious to air, presents a curved surface with the
concavity underneath. The Woodpecker uses his
strong tail feathers to support him as he clings to a
tree ; and consequently one is not surprised to find
that in him the muscles which lower the tail are very
highly developed.

Rate of Stroke.

There are three ways of estimating this, one of
which has been tried, I believe, only in the case of
insects. These three ways are— -(i) that of unassisted

vii FLIGHT 215

observation — to watch a bird flying and count the
strokes per minute; (2) to determine the note made by
the vibration of the wings, and from that to calculate
the velocity ; (3) to apply machinery by which the
bird or insect registers each stroke. These three
methods may be called, respectively, the method of
observation, the acoustic, and the graphic method.

(1) The first can only be employed when the bird
is flying slowly, and even then it often happens that
two observers do not agree. But it is impossible to
bring any machinery to bear upon a bird in a state
of liberty, so that the third method gives us the wild
wing-beats of some poor wretch, the subject of alarm-
ing experimentation. I have repeatedly counted the
strokes of Gulls making long flights, and find 120 per
minute to be a common rate. Friends whom I have
got to count for me have come to conclusions
not far different. With birds like the Pigeon, whose
stroke is much more rapid, the estimates are far from
dependable. The Puffin's wings move so rapidly that
you only see a shimmer in the air, and you can no
more count the strokes than you can see the individual
spokes of a wheel in rapid motion.

(2) The acoustic method depends on the fact that
a tuning-fork when its vibrations have a certain fre-
quency gives off a certain note, and in the same way
the wings of an insect beating, as they sometimes do,
20,000 times per minute. But the exact tone varies as
the insect flies towards or away from us. An engine
whistle sounds shriller as the train approaches. In
our present investigations there is besides this the in-
superable difficulty that the whirring of a bird's wings

216 THE STRUCTURE AND LIFE OF BIRDS chap.

is often quite inaudible. Even when we can hear the
note it does not tell us much. It depends on the
rapidity with which the extremity of the wing moves,
not on the number of wing-beats in a second, and so
cannot tell us what we want.

(3) We now come to the graphic method, in apply-
ing which Professor Marey has shown wonderful skill.
A sheet of paper blackened by the smoke of a candle
was stretched upon a cylinder, which was made to
revolve at a uniform rate. An insect, the frequency
of whose wing movements was to be studied, was held
by the abdomen in a delicate pair of forceps, and was
placed so that one of its wings, at every up or down
stroke, brushed against the blackened paper. With
birds the method was, necessarily, far more elaborate.
On the extremity of the wing an apparatus was placed
which at each alternate movement broke or closed an
electric circuit. In Professor Marey's book {Animal
Mechanism, p. 230) the bird may be seen carrying his
burden of electric wires and other machinery. As in
the case of the insect, a tracing was made on a
revolving cylinder.

The results were :

Sparrow

Wild Duck

Revolutions of wing
per second.

13

9

Pigeon

Screech Owl

8

5

Buzzard

3

These results seem quite dependable, and it is much
to be regretted that we cannot arrive at conclusions

vii FLIGHT 217

equally certain with regard to less flurried flight.
The slowness of a bird's stroke compared with an
insect's was most remarkable, a common fly attaining
the astounding rapidity of 330 per second, and a bee
190. It must be borne in mind that the extremity of
a long wing may move very rapidly, though the number
of strokes per minute be few, and that a bird's wing
with its long sweep may produce more effect, even
when we allow for the greater weight to be supported
and propelled, than the insect's can by its many rapid
pulsations.

There is the further question of the comparative
velocity of the up- and downstrokes. Photography
has proved what we should not have anticipated — viz.,
that the upstroke is the more rapid of the two. 1

Duck

Duration of Duration of

upstroke. downstroke.

— sec. *p sec.

60 60

^ 4^

Pigeon -^ sec. ^-- sec.

& 60 60

01 jo

Buzzard — 2 sec. ^ sec.

60 60

This is accounted for by two facts — (1) that the
air offers little resistance to the passage of the rounded
upper surface ; (2) that when the pace attained is great
the air itself lifts the wing and relieves the Elevator
muscle of a great part of its work.

1 See Marey's Vol des Oiseaux, p. 101.

THE STRUCTURE AND LIFE OF BIRDS chap.

Phases of the Stroke. l

Instantaneous photography has in many ways
surprised the world. Horses no longer trot or gallop
as they did. Birds get their wings into the most
surprising positions, utterly unlike anything to be seen

Fig. 57 (after Marey).
Gull flying.

in the conventional bird of the artist. Many Gulls
have been photographed on the wing by Messrs.
Wyles, at Southport ; some Storks, the favourite bird
in the Fatherland, by Germans ; the American Eagle
and other birds, by Mr. Muybridge ; and Professor

Fig. 5& (after Marey).

Gull flying— 50 images per second.

Marey has obtained images of flying Gulls, at the rate
of fifty per second, showing the various phases of the
up- and down-strokes. The Southport Gulls are

1 Some of this has been necessarily anticipated. See
"Active Machinery," and "Movements of the Wing partly
due to the Action of the Air."

VII

FLIGHT

19

220 THE STRUCTURE AND LIFE OF BIRDS chap.

winging their way quite unconscious of cameras, and
sensitive plates, and fame. On the other hand, Mr.
Muybridge's American Eagle, with his somewhat
draggled plumes, looks like a scared captive, conscious
that the camera is being aimed at him. However, it
is only under conditions like these last, that photo-
graphs such as those of Professor Marey, showing the
position of the wing at every stage, can be obtained.

When the bird is flying with great energy, he raises
his wings high till in some cases they touch one
another, and this is the cause of the slapping noise
that we hear when a Pigeon rises from the ground.
The wing next moves forward and downward, its
under surface looking backwards and downwards. In
its forward movement it meets with little resistance,
since it cuts edgeways through the air. When it can
strain no further forward and down, it is drawn back-
ward and bent sharply at the wrist-joint, facing during
the process forwards and downwards. During the last
period of the upstroke there is a further turn, and it
moves edgeways forward. 1 This description of the
movements of the wing refers to very vigorous flight,
such as we most commonly see when the bird is
getting up steam. When he has plenty of way on
there is no need for him to take these very long and
exhausting strokes, unless, like the Duck, he is one
of those that seem always to fly with effort. In birds
of long flight, the wing does not rise very high or
descend very low, and it is flexed very little, if at all,
when it is raised. These points can be made out if a
Gull is watched when it is flying steadily. In some of
1 Further details under next heading.

VII FLIGHT 221

Messrs. Wyles's photographs of Gulls, flying horizon-
tally at some height from the sea, there is not a single
one among the large number that has its wings flexed.
(See frontispiece.)

Figures described by different parts of the Wing.

If you take an insect (a Bluebottle, for instance, or a
Drone) and hold it down, the wing tips as they vibrate

Fig. to (after Marey).

Figure described by tip of Crow's wing. 1

The backward movement neary? is largely due to the flexing, the forward movement

near st to the straightening of the wing.

will be seen to describe a figure of 8. If the sunlight
be made to fall upon them, it will be all the clearer.
I have tried painting the tips with body white, but
the rapid vibration soon dislodges the paint. It is
much harder to discover the trajectory of a bird's
wing. Professor Pettigrew has strongly insisted that
it is an 8 as in the case of insects, but the lower circle

1 I have corrected the mistake by which the words " right "
and "left" have been accidentally transposed in Professor
Marey' s explanation of the figure.

222 THE STRUCTURE AND LIFE OF BIRDS chap.

of the 8 either does not exist, or at any rate is reduced
to very small dimensions. Applying machinery to
the bird's wing, Professor Marey succeeded in tracing
the figure described by the outer end of the humerus.
It is an ellipse with the long axis inclining down-
wards. The difficulties were found to be great in
obtaining by the aid of similar machinery a tracing
of the course followed by the wing tip. Professor
Marey at length hit upon the following plan. He
fastened a small piece of white paper to the tip of a
crow's wing, and as the bird flew in front of a perfectly
black screen, he took a photograph of this moving
speck of white, while of course no image of the crow
appeared upon the plate. In the figure the long
forward sweep of the downstroke comes out very
clearly ; at last the line curves backward, the wind
of the bird's velocity making the wing retreat : the
muscles arrest this backward movement, and we have
an apparent forward twitch (due perhaps to the onward
momentum which the wing-tip shares with the whole
body), forming the small loop at the bottom. Soon
the wing hunches up at the wrist and for some distance
the tip moves upward and backward (fl). It will be
noticed that as the bird moves from right to left with
increasing speed, the tracings alter, and at last the
smaller circle disappears altogether. The figure, in
fact, varies considerably even when the same bird is
experimented on, especially when it begins to get up
speed. When the velocity is at all considerable, no
ellipse is actually formed. It only exists in imagina-
tion, like the ellipse which we say the moon forms
in revolving round the earth. The earth itself,

vii FLIGHT

123

meanwhile, is moving rapidly onward, so that the
line of the moon's course never meets or crosses itself.
When the moon moves backward and circles round
us, its backward movement is only relative to the
earth. In reality it is moving onward, only with less
velocity. To apply this to a bird's wing : suppose
that a pigeon is flying at the rate of thirty miles an
hour — i.e. at a fairly easy-going pace if the weather be
good, and with a stroke of 300 per minute. This
gives an advance of 2 -J-t yards per stroke. More than
half of this advance will be made during the down-
stroke. The downward curve, therefore, will be
altogether out of reach when the time comes for
putting in the upward one to complete the ellipse,
and when a velocity of fifty or more miles an hour
is attained, the ellipse becomes still more theoretical.
But though it is desirable to point this out it hardly
diminishes the interest of what Professor Marey has
proved, that the trajectory of the humerus and of the
wing tip, when the figure is not destroyed by the
rapidity with which the bird travels, is an ellipse
with, in the latter case sometimes, a small loop at
the lower end.

The Bird's Trajectory.

Meanwhile the bird's trajectory is an undulating
one. It rises with every downstroke, and sinks with
every upstroke. Though in the course of the latter
there seems to be in some species a slight momentary
rise, caused by the wind of the bird's velocity catch-
ing the wings and his whole surface, yet the main

2-4 THE STRUCTURE AND LIFE OF BIRDS chap.

movement is, as we should expect, downwards. The
broadjumper, as he rises and descends, describes in
the air a curve not unlike the curves of the bird's
trajectory. Many of our small birds — e.g. the House-
sparrow — may be seen as they fly to take a regular
jump upward, and then with outstretched wings glide
onward and slightly downward. The centre of gravity
of course rises and falls, but not so much as a particular
point in the bird — the eye, for instance ; for when the
bird rises his wings are lowered, and this lowers the

Fig. 6i (after Marey).

The space between the two straight lines shows the limits of the rise and fall of the
eye. The cross shows the position, according to calculation, of the centre of gravity
at the moments when the wing is highest or lowest.

centre of gravity ; it is at a higher point in the bird
when he sinks, for at this moment his wings are raised
aloft. Thus the centre of gravity moves along in an
undulating line, but its oscillations are not so great as
those of the eye, since every rise and fall of the bird
is accompanied by a fall and rise of the centre of
gravity.

Long Distance Flight.

Long wings are best suited for long distances.
With a single short stroke they send the bird far on

VII

FLKiHT

22 ^

his way, the further end of the lever moving with
great velocity and force. The short wing must beat
frequently and with a longer swing. Put the wing of
a Swift beside that of a Duck, and compare the two.
The cut of them at once shows the flight of each.
The Swift's is long and very narrow towards the end,
the duck's short for its size, and rounded. The Swift

Fig. 62

Breast-bone of (a) Frigate Bird, (/>) of Duck, ^rds natural size.

cl, clavicle ; co, coracoid ; k, keel ; st, sternum proper.

covers, probably, 60 miles in the hour without great
effort and without a very rapid stroke : the duck keeps
up a good pace, but only by means of strokes
that are at once very rapid and very long. If a
Gannet be watched as he goes leisurely onwards it
will be seen that his wings do not rise much above
or descend much below a horizontal line from the

Q

226 THE STRUCTURE AND LIFE OF BIRDS CHAP.

shoulder. A gull as he travels keeps his wing slightly
bent at the wrist, as photographs show, and probably
also at the elbow, and does not flex them any more
than this for the upstroke : they are merely rotated
{i.e. a turn at the shoulder raises the front margin),
and then lifted for the downstroke again (frontispiece).

The birds which take long strokes have long
pectoral muscles arising from long breastbones : those
which take short strokes have the pectorals corre-
spondingly short. I have above explained the rule
that the amount of contraction possible to a muscle
depends upon its length (see p. 142). If we bear this
in mind it will be very interesting to set side by side
the skeletons of a Frigate Bird and a duck. The breast-
bone of the former is extraordinarily short, but deep,
showing that his typical wingstroke is a very short
but powerful one ; the duck's has a great superiority
in length, but not in depth, suggesting the quick long
strokes of his far more laborious flight.

Some small birds cross wide seas, and we are apt to
think of them as having no special qualification for
such voyages. Golden-crested Wrens come to us
in flocks from Norway, and sometimes, wearied out,
cluster in flocks on the rigging of fishing smacks.
But the wings are, for such tiny creatures, long and
fine ones.

Upward FligJit.

Watch a lark as he is mounting. He holds his
body inclined steeply upwards, often at an angle of
about 60 degrees with the horizon. While he is in
this attitude his wings are set so that their under

vii FLIGHT 227

surfaces face downwards and slightly backwards.
Why, then, if they are adjusted nearly as they arc
for horizontal flight, is his progress almost vertically
upward ? The natural action of his wings is to drive
him onward as well as upward. But not much onward
movement can take place, since the air offers too much re-
sistance to the expanse of his breast and tail. His on-
ward velocity is, in part, therefore, converted into upward
velocity ; and thus he is giving a practical illustration
of the working of the parallelogram of forces. He
always faces the wind, and derives, no doubt, great
help from it ; how, I shall explain later on (see p. 239).
Some birds can ascend much more rapidly, i.e. at a
much steeper incline than others. The reason of this
is, I believe, that they have greater freedom at the
shoulder joint, so that they can turn their wing further
over, giving it a steeper slope from the front to the
hinder margin. If a bird has but little power of
doing this, when he inclines his body upward so as
to form a large angle with the horizon, his wings will
beat backwards and forwards instead of up and down.
Wishing to test this I examined the wings of a good
many birds, some of them alive, some just after they
were shot. 1 found that the lark when its wing was
extended could lower its front margin a great deal
without any strain. The same was true, though not
quite to the same extent, of the jackdaw, jay, crow,
chough, magpie, rook, raven, quail, plover, eagle, all of
which, I believe, are capable of ascending at a fairly
steep incline. On the other hand the gannet, herring
gull, blackheaded gull, pelican, cormorant, had none
of them much power of rotating the wing, the cormorant,

Q 2

228 THE STRUCTURE AND LIFE OF BIRDS chap.

I think, least of all. The pelican's flight I do not
know, but the rest ascend by a comparatively gentle
gradient ; some of them even require the help of a
head wind, if the ascent is to be made with reasonable
ease. The cormorant, when he leaves his fishing,
struggles hard before he gets clear of the water,
advancing quickly but ascending slowly, a striking
contrast in every way to the skylark as he mounts
lightly to the upper air.

Downward Flight.

It is a beautiful thing to see a pigeon, with wings
partly flexed, glide downwards through the air, then,
as he nears the ground, suddenly give his body an
upward instead of a downward slope, spread his wings
to stop himself, and, with all the grace of the " Herald
Mercury," alight.

The suddenness of the change from horizontal or
upward to downward flight is very striking. It is often
maintained that it is due to a shifting of the centre of
gravity by the elongation of the neck or of the legs, or
by some of the other little manoeuvres known to birds,
which have this object. But the pigeon has no length
of neck or legs to extend. If you watch him change
all at once the incline of his body, you will see no
working of either. The very quickness of the move-
ment shows that it must be due to muscular action,
and it is no doubt the work of the muscle called the
Latissimus Dorsi, which in horizontal flight raises the
hinder quarters by hauling upon the wings and which,
as I have shown, probably helps in the process of
breathing (see p. 89).

vii FLIGHT 229

A jerk due to the sudden contraction of this muscle
would have exactly the effect which we see when the
pigeon in a moment gives his body a downward slant
from tail to head. I lawks have, of course, exceptional
power of swooping suddenly downwards, and I have
found in the kestrel an altogether unusual development
of this muscle. It has occurred to me that this power
may be also in part due to the pliability of the waist
in upward and downward directions, which some birds
have in a much greater degree than others. The kestrel,
swallow, sand-martin, all three of them able in a
moment to put their bodies at any incline, have this
qualification to a remarkable extent. Ducks, on the
other hand, are very stiff at the waist, and they are,
comparatively, but poor performers. The gulls and
terns, however, are also remarkable for the same
stiffness, and this makes me doubt whether pliability
of waist is absolutely essential for these sudden up
and down movements. At any rate, much nimbleness
can exist without it.

The wings, as pointed out above, are partly flexed
as the bird descends, since it is the front margin which
mainly gives support when he is gliding, and if he
were to form an extended front, he would travel more
slowly and with a less rapid incline downwards, or in
a horizontal or upward direction (see p. 190).

Horizontal Flight and Gliding Flight.

These have already been described (pp. 180, 190,
223)-

230 THE STRUCTURE AND LIFE OF BIRDS chap.

Flight in Troops.

The formations adopted by birds when flying in
troops are very interesting, but unfortunately not much
is known of the subject. A wedge shape, the point

/ \

Fig. 63 (after D'Esterno partly).

Ducks in formations for troop-flying.

The arrows in 1 and 2 represent the direction of the wind. In 1 the wash passes
between those to leeward ; in 2 the leeward line is too far off to feel it. In 3 (wind
ahead or aft or none), travelling each along his own line, they escape the wash and so
also in V-formation.

leading, is the favourite, certainly among most of the
great water birds such as swans, cranes, and pelicans
(see Fig. 63). Whatever the formation, the object is
not that those in front may break the resistance of the
air for the benefit of those behind them. A bicyclist,
driving as he does a mass of air before him, gives

VII

FLIGHT 231

material assistance to another following close after.
A bird's body, on the contrary, is so shaped that it
does not meet with much resistance and consequently
does not " break " the air to any great extent, and, in
addition to this negative result, the action of his wings
causes a back current, to which, in fact, his progress
onward is due. A bird who finds himself in this
back current is probably at least as badly off as a boat
in the wash of another boat. In whatever form birds
marshal themselves for flight, their aim probably is to
save each from the wash of those in front. Even when
they are apparently flying in Indian file, as ducks
often do, we need not see an exception to the rule.
It is by no means certain that their heads point
exactly along the line : very probably all are following
parallel tracks a short distance apart (see 3, Fig. 63).
It is probable that the formation varies according
to the direction of the wind relatively to the line
of flight. Unfortunately the wind at a great altitude
does not always blow from exactly the same point of
the compass as the wind below. Another difficulty
is that a flock of birds far above us may be flying
some in one horizontal plane, others in another, though
they appear to us to be in the same. It is very
difficult to make trustworthy observations.

Changes en route take place frequently. The
leader drops behind or there is a general rearrange-
ment, apparently quite orderly, each falling without
confusion into his place.

!32 THE STRUCTURE AND LIFE OF BIRDS chap.

Wind and Flight.

Before going on to describe other varieties of flight,
it will be well to make clear, as far as possible, how
the wind may help a bird to rise to a higher elevation.
Many begin early to gain the skill that is necessary
if they are to avail themselves of this assistance.
As soon as he has the use of his wings a young gull
may be seen for a good part of the day busily
practising. And the great proficients in the art —
eagles, vultures, storks, and albatrosses — have acquired
their skill by experimenting on all varieties of currents.
The problem presents far greater difficulties for us.
With labour and complicated contrivances we obtain
some notion of what wind is, of its force, its currents,
its eddies and gusts. By long and painstaking ob-
servation the naturalist discovers what curves the bird
describes in the air, whether he looks down the wind, or
in the teeth of it or across it, and what angle his body
forms with the horizon. Mercilessly the mathematician
applies his laws showing that the theory of soaring
that the naturalist fondly cherishes is built on an
absurd assumption, while he himself, without inventing
conditions which possibly do not exist, may fail to
show how the aerial play of the gyrating vulture is
physically possible.

With all these difficulties before us, let us first lay
a sound mathematical foundation. However tempting
the theory may be, that a Senior Wrangler has one
system of mathematics and a Pelican a totally different
one, we must not be beguiled by it.

vii FLIGHT 233

The great principle to be borne in mind is that a
bird can be helped to rise by a horizontal wind, only
if he have a velocity of his own relatively to it. A
man in a balloon that is sailing swiftly with the wind, is
not conscious of his motion unless he looks down at
the earth and marks his progress by the hills, trees,
and houses that he leaves behind. Instead of con-
sidering that there is a breeze blowing, we may think
of the earth as turning round without carrying its
envelope of air with it. And this would have no
effect on a balloon and its occupants.

A bird is often compared to a kite, and the simile is
a correct or misleading one according to circumstances.
If he has velocity of his own and inclines his body
upward, the wind due to his own passage through the
air will lift him like a kite. His own momentum takes
the place of the string, and the force of the current
that meets him is resolved into two, one acting upward,
and the other horizontally. But supposing that he
has no momentum, no velocity of his own, he is a kite
without a string and is carried with the wind, not
lifted by it.

It is possible that a bird may obtain relative velocity
without any effort of his own by the help of irregularities
in the wind. There may be currents of different
velocity forming strata, or it may come in gusts. It
has long been known that the wind near the earth's
surface is retarded by friction. With a view to obtaining
accurate measurements as to the amount of retardation
and of the height to which the speed of the wind con-
tinues to increase, some experiments were undertaken
by Mr. R. C. Gilson, Mr. J. P. Gilson, and myself.

234 THE STRUCTURE AND LIFE OF BIRDS chap.

These were by no means so complete as we had
hoped they would be, owing to an accident to our ane-
mometer. This small instrument, which presents sails
like those of a windmill to the wind, and, as it turns,
registers the length of the stream of air that has acted
on it in the allotted time, was all that was required
for the lower levels.

The place chosen, New Romney sands on the coast
of Kent, has exceptional advantages for such experi-
ments. There arc no hills or cliffs to tilt the wind
upwards, no trees or other obstacles to make it play
tricks, so that it was a horizontal wind without com-
plications that was experimented on.

The following are averages made out from four
series of experiments : —

Velocity of the wind
Altitude. per minute.

2 inches 515 feet.

1 foot

2 feet

4 feet

7 feet 6 inches ...

736^
770
918
1,021

As the wind comes in gusts, it is necessary, if the
results are to be trustworthy, to make a number of
measurements and strike averages. The figures show
that the rate of increase diminishes as you ascend, the
difference in velocity at two inches from the ground
and one foot being very great. The following are the
averages from only two series : —

Velocity of the wind
Altitude. per minute.

7 feet 6 inches ... ... ... ... 1,375 feet.

9 feet 6 inches ... ... ... ... 1,457^ „

The plan for measuring the velocity of the wind at

VII

FLIGHT 2 35

higheraltitudes was to run the anemometer upthe string
of a kite by means of another string passed through a
pulley ; as soon as it had reached the right altitude a
third string, passed between the vanes and hanging
loose, was to be pulled out, thus allowing the instru-
ment to work. The altitude was to be measured by
means of a sextant. Alternate hurricanes and calms
put obstacles in the way of kite flying, and finally the
anemometer struck work and refused to register. But
results that arc far from valueless were obtained by
attaching the kite to a spring balance which measured
the pull at different heights.

AUitUde ^nr ed ^ Pull of kite.

sexta.ni. ,.

420 feet ••• 2 5lbs.

357 feet ^Ibs.

350 feet... 2 4 lbs -

204 feet l81bs -

126 feet... 15-20 lbs.

60 feet 14-19 lbs.

42 feet 11— 20 lbs.

There is the question whether the greater length of
string carried as the kite mounted higher may not have
to some extent modified the results. Probably, however,
the error clue to this cause was but slight. At low
levels, from 126 feet downwards, the jerking of the kite
made an accurate measurement of the pull very diffi-
cult. But the series suggests, though it does not prove,
that there is a steady though diminishing increase up
to 420 feet. The pull at 357 feet measured during a
temporary lull is the only false note in the scale.

As a supplement to these kite-experiments, I tried
the plan of letting go a number of small balloons,

236 THE STRUCTURE AND LIFE OF BIRDS chap.

inflated with hydrogen gas in different degrees, and
allowing them to race : those which were more dis-
tended, rising to a height of 500 feet or more, quite
outpaced the smaller ones floating below them. But,
as far as I could judge, small differences of elevation
at the higher levels did not much affect the velocity.

Since making these experiments, I have found that
far more elaborate ones, with kite-wire suspended
anemometers, had already been recorded. They
showed that the velocity of the wind increased up to
a height of nearly 1,100 feet above the ground, but
that the rate of increase there was very slight, the
difference being only forty-four feet per minute
between the velocities at altitudes ot ;g$ feet and
1,095, and eighty-five between 549 and 795. l

It seems probable, then, that the wind increases in
velocity up to a certain altitude, but that above that
there is no appreciable increase. This being so, we
must look for some other irregularity of which a bird
soaring over a level plain can make use when he has
passed this limit. It is within even* one's experience
that wind often comes in gusts. But we not unfre-
quently speak of a steady breeze, meaning one which
is almost or entirely free from such fitfulness. Our
experiments with an anemometer at New Romney
went to show that a stead}- wind, in the strict sense
of the term, does not exist. We had to expose the
instrument for not less than a minute at a time, make
a series of experiments, and strike averages, in order
to obtain dependable figures. Professor Langley has
by means of a delicate anemometer made more thorough
1 Xature, April 22, 1886.

vii FLIGHT

investigations, and his results show that the actual
variation is far greater than would ever have been
thought likely. 1 When he measured the velocity at
intervals of seven to seventeen seconds at an altitude of
fifty-three feet, it varied from ten to twenty-five miles
per hour, and the inertia of the anemometer may, as he
says, have reduced the apparent variability. The greater
the velocity of the wind, the greater was the fluctua-
tion. On one occasion he found that a wind blowing
forty miles an hour would almost in an instant drop to
a calm. His anemometer once stopped dead for one
second in a high wind. Another trial, when the
velocity was measured every second, showed that a
wind of twenty-three miles an hour may in ten seconds
rise to thirty-three miles, within ten seconds fall
again to twenty-three, then in another thirty seconds
rise to thirty-six. As to the cause or the nature of
the apparent gusts it is difficult to speak positively.
It is impossible that the onward movement of a large
volume of air in motion can be suddenly checked.
Possibly the irregularity may be due to eddies, the
anemometer during an apparent lull being really within
the centre or the back current of an eddy, whereas
during what seems a sudden gust it is in the eddv's
onward sweep. But, whatever the explanation, the
inequality certainly exists, ready for a bird who has
the skill to make use of.

But is the wind blowing over the level plain hori-
zontal ? No one would imagine that it is otherwise,

1 American Journal of s v. January 1S94: '"Internal

Work of the Wind." by Professor S. P. Langley. [Since
published separately.]

238 THE STRUCTURE AND LIFE OF BIRDS CHAP.

when no exceptional cause intervenes, had not Herr
Lilienthal tried to prove, by means of a vane working
vertically up and down, that its normal direction is
upward, at an incline of 3 — 4 to the horizon. 1 If
this is really the nature of wind when not interfered
with by hills or any irregularities and when there
is no updraught from a sun-heated surface, there will
soon be no air left in which Herr Lilienthal, who
is ambitious to be a modern Daedalus, may try his
wings. If a vane under these conditions were to point
upwards, it would be more reasonable to regard the
fact as an indication of the eccentricity of all vertical
vanes, or of that particular one. While at New
Romney I had one made for me having for its larger
arm a piece of thin deal one foot long by six inches
broad, exactly balanced by a lump of lead attached to
the shorter arm. It is true that so small a deviation
as 3 — 4 would be hard to detect, but this instrument
indicated, as far as I could judge, that a wind blowing
over a level expanse is perfectly horizontal. Ex-
periments on the direction of the wind on, or on
either side of, a small barrier had more interest for
me. While standing on a bank only two feet high,
its tripod lifting it four feet above the bank, the vane
pointed decidedly upwards. Five yards to leeward of
a bank eight feet high it indicated that the wind blew
downwards, making, a large angle with the horizon;
there was but rarely an upward gust. Ten yards from
the bank the direction was still mainly downward, but
with not unfrequent upward movements. At twenty

1 Der Vogelflug als Grundlage der Fliegekunst, by Otto
Lilienthal. (Berlin, 18S9.)

vi! FLIGHT 239

and thirty yards' distance the wind came in wild gusts,
as often upward as downward. Twelve yards to
windward of a bank only rising eight feet above the
level the vane was not quite steady, but on the whole
horizontal. At a distance of six yards there were
occasional upward swings ; at four yards' distance
there was a decided upward tendency, and this though
the bank itself presented only a very gentle incline.
These facts go far to explain the soaring of birds over
hill-tops or cliffs.

Rising with the help of the Wind.

One way to catch Condors is " to place a carcass
on a level piece of ground within an enclosure of sticks
with an opening, and, when the condors are gorged, to
gallop up on horseback to the entrance and thus enclose
them ; for when the bird has not space to run, it cannot
give its body sufficient momentum to rise from the
ground." 1 A Cormorant, wishing to lift himself from
the water, faces the wind, and flaps along the surface
for some distance and at considerable speed, before he
is able to mount upward. The Red-throated Diver is
said to be so powerless to begin his flight without a head
wind to help him that if you sail down upon him with
the wind he must cither fly towards you or remain upon
the water, the third alternative of diving often not
occurring to his mind. I have never seen a Lark rise
facing any way but head to the wind. In fact, all birds
seem to derive great assistance from this.

1 Darwin's Journal of Researches, chap. ix. p. 133 (Minerva
Library).

240 THE STRUCTURE AND LIFE OF BIRDS chap.

I long thought perfectly true what some writers
on flight still maintain, that a uniform horizontal wind
would lift the bird if he faced it, as it lifts a kite. But
a fact that ought to have been obvious has now been
pointed out to me — viz., that the bird, a moment after
he has left the ground, becomes part of the moving
current, so that it will make no difference as far as his
upward progress is concerned whether he fly with it
or against it. There might as well be not breeze
enough to shake an aspen leaf. You may imagine
him flying in a globe filled with air, the globe itself
moving with the current. The fact that the globe is
moving will not affect the bird's flight.

Mr. R. C. Gilson first showed me how it is that a bird
wishing to rise derives advantage from facing the wind.
He is perpetually passing from a slower into a faster
current. Thus at every stage he has his own inertia,
which is equivalent to momentum, to help him. When
he first jumps from the ground, if we divide the wind
in theory into separate layers, he has at his service
the whole velocity of the lowest layer. As he passes
out of this he is helped by the difference in velocity
between layers Nos. I and 2. As he ascends higher,
the rate of increase in the wind's speed diminishing,
he will be helped less, but, as I have shown, he will
not be left entirely to his own unaided efforts, at
any rate until he has passed an altitude of 1,000 feet.

Oinuard FligJit and Air Currents.

When at the seaside it is blowing with a violence
that must startle an anemometer familiar only with
sheltered places inland, it is a beautiful sight to see a

VII

FLKiIIT 241

Gull flying at right angles to it without moving his
wings. A diagram will make clear the method.

He descends swiftly at a gentle incline, making an
angle with the wind that, but for leeway, would be a right
angle, then suddenly turns and faces the wind, rising
by a short steep ascent to his former level, after which
he begins the process over again. He descends from
the more rapid current into the slower, and so has the
advantage of his relative velocity, after which he again
ascends, and profits by his passage from the slower to
the swifter. The upper current lends him pace, the

J^-

Fig. 64.
W shows the direction of the wind ; G the line of the Gull's flight.

lower we may look upon as a floor off which he rebounds
like a ball. As he ascends into the more quickly moving
air, he has, belonging as he does now to the slower
current below, an inertia which makes the wind act
upon him as upon a kite. A glance at the line of his
course will show how he keeps making good his leeway
as he goes. The Albatross, those who have been lucky
enough to see him say, has brought this mode of pro-
gression to perfection, working onward by the hour
without a motion of his wings. A Gull is often reduced
to putting in a stroke or two, though I have seen them
cover a considerable distance without once resorting to
this. It is said that some birds make head directly

R

242 THE STRUCTURE AND LIFE OF BIRDS chap.

against the wind in something the same way, half
flexing the wings for a rapid descent down a gentle
incline, then spreading them for a steep ascent. Their
course is a perfectly straight one head to the breeze,
only rising and falling. 1

During the terrific blizzard that fell upon our east
coast in January 1894, I saw gulls making their way
against the wind by flying as low as possible along the
sand by the seaside, appreciating the fact that even a
blizzard is comparatively mild near to the earth's
surface.

Soaring.

When Eagles, Falcons, and Buzzards were common
in our islands, the phenomenon of soaring was much
more familiar than it is now. The popularity of
hawking brought it to everybody's notice. Sportsmen
or their gamekeepers are now making everything in
the shape of a bird of prey, beyond a Kestrel or a
Sparrow Hawk, a rare sight. But, happily, an oppor-
tunity still occasionally comes to us to see a bird
soaring in grand style, a Buzzard among the hills of
Westmoreland or over Cornish cliffs, or a Raven over
some rocky headland on our coast. But the grandest
soarers of all are not natives of Britain. They may
be seen in many lands, but nowhere better than over
a great plain in Upper Assam. There they have been
watched through a telescope by Mr. S. E. Peal, who
has well described their circling flight in Nature?

1 See Marey, Vol des Oiseaux, p. 18.

2 See Nature for November 4, 1880; September 26, 1889;
May 21, 1S91,

vii FLIGHT 243

The skilled performers are Pelicans, Vultures, Storks,
and, perhaps the finest of all, the Adjutant Birds.
Under different circumstances the last-named may be
seen at the Zoological Gardens, looking lethargic and
far from athletic, their length of leg more apparent than
their mighty spread of wing. They rise the first 100 —
200 feet by their wings, and then, apparently without
the faintest suspicion of a wing-beat, sweep round in
spirals, or what is more properly called a helix, gaining
ten to twenty feet, it is estimated, with each gyration,
the wing and tail being rigidly extended and the
primary feathers separated. During the first part of
each turn of the helix they are flying with the wind,
their direction being slightly downward ; at the
end of the descent they sweep round and face the
wind, which carries them upward. When the curve
described is towards the left, the right wing points
upward and the left downward, but the two wings
always form one rigid rod, and only move with the
body (see p. 250).

In this way they attain an elevation of one or two
miles, and so restful does this upward circling appear
that Mr. Peal is of opinion that they go aloft to doze.
There is always a wind at the time, generally from
the N.E. or S.W., blowing steadily at a rate of five to
ten miles an hour, and since the plain presents a dead
level of 200 miles by 60, the direction of the current
can hardly be upward. When there is no wind there
is no soaring. It is remarkable that there is always
a considerable loss of leeway. This description agrees
in all essential points with that given by other
observers.

K 2

244 THE STRUCTURE AND LIFE OF BIRDS chap.

Birds, like all matter living or dead, are subject
to physical laws. We must, therefore, not relegate
soaring to the realm of miracle, but try to find some
solution of the problem.

There are three explanations possible, all of which
may be true, each of them being applicable to par-
ticular circumstances.

(i) There may be upward currents. We have
already condemned as fantastic Lilienthal's notion
that the general tendency of the wind is upward.
But, as I have shown, in particular places it un-
doubtedly is so. A Raven or a Vulture soaring over
a hilltop is probably making use of alternating
upward and downward currents, or else, if the streams
of air in that particular spot flow steadily, sweeping
from a downward into an upward one. This means
of soaring is, it cannot be doubted, put at his disposal
through the diversion of the wind when it comes in
contact with a high ridge. If the results I have
described could be produced by a bank only a few
feet high, what great irregularities, what tremendous
up and down blasts, may be caused by such a barrier
as a hill of only moderate height ! Gulls are very
quick in finding out such currents, and enjoy playing
in them. When the wind is blowing hard against a
cliff and is diverted upward, they will fly again and
again to its base and let themselves be carried to the
top. Or they will float and circle high above a cliff,
profiting no doubt by the upward slant of the breeze.
They will often follow a steamer with wings perfectly
motionless, and the explanation of this, probably, is
that the passage of the vessel through the air causes

vii KLiCIIT 245

an upward current. I have tried to detect such a
current by means of small scraps of paper let fly from
the stern of a steamer. They showed that immediately
behind the vessel the air rushes violently downward
to fill the vacuum left as she moves onward. But
the rebound of this current from the water was by no
means so marked as I should have expected, though
there were unmistakable signs of a certain amount
of updraught.

It cannot be denied that when beneath a tropical
or sub-tropical sun a plain is heated in various degrees
at different places, according to the nature of the
surface, there are upward movements of air, and it has
been held by some writers that the up-currents thus
formed are sufficient to render soaring possible. It
is urged in support of this view that birds soar chiefly
during the great heat of the day, and more in summer
than in winter, and it is certainly remarkable that to
see soaring at its best you have to go far south. But
before we can believe in the adequacy of these up-
currcnts we want further evidence. The fact, too,
that soaring does not begin till 100 — 200 feet up
seems to me to tell against this view. 1

(2) Increase of the wind's velocity with altitude will
explain much, but the principle must be cautiously
applied. If this progressive increase extended to
altitudes of a mile or two, the grandest feats of soaring
birds might easily be explained, as Lord Rayleigh and
others have shown. 2 The Adjutant wheeling upward
would be only doing on a grand scale what Gulls may

1 Sec Nature, October 1st, 1891.

2 See Nature, May, -1883, May 9, 1889.

246 THE STRUCTURE AND LIFE OF BIRDS chap.

be seen doing when they advance at right angles to
the wind without moving their wings. Descending
with all the velocity of the upper current, he would
rebound from the slower one below, and the inertia
due to this undercurrent would make him rise like a
kite when he re-entered the more rapid stream at a
higher level. But, as I have shown, it is very improb-
able that there is a regular progressive increase in
the velocity of the wind at high altitudes. We know,
too, that it comes in gusts, and this would bring it
about that the bird, if he depended on the arrange-
ment of the air in strata of increasing velocity, would,
when the breeze happened to strengthen inopportunely,
find himself in a slower instead of a more rapid
current.

Whatever the difficulties that meet us when we
try to explain by this principle the phenomenon of
soaring, it is certain that near the surface of land or
sea the increase of velocity with altitude is rapid
enough and constant enough to assist a bird in rising
or in making progress at right angles to the wind.

(3) I believe myself that the irregularity of the
wind may supply the explanation of soaring. The
wind is a " chartered libertine," and, even when
steadiest, blows, as Professor Langlcy has shown, with
great fitfulness. A bird, when soaring, if this ex-
planation be sound, will face a strengthening breeze :
when it begins to slacken will turn and go with it,
until conscious it is freshening again, when once more
he will sweep round and face it, his aim being always
to feel the wind blowing in his face, sure evidence
that he has momentum that will lift him. To do

vii FLIGHT 247

this he would have to be perpetually feeling the
pulse of the wind, and, moreover, soaring would
be a much less regular progress upward than it
is supposed to be. Some turns of the helix would
be failures. There would be a loss instead
of a gain of elevation, or nothing more than a
maintenance of level would be achieved. But when
we watch a bird circling at a great height, what can
we tell of his progress during a particular minute ?
We only know that his general tendency is upward.
Gulls make many only partially successful turns when
they soar, and it is possible that similar failures in
a nobler performer like the Adjutant may remain
undetected.

Supposing the irregularity of the wind to be due to
eddies, the bird may still be able to avail himself of
it, turning to account the difference in velocity of the
outer and inner rings of the whirl. This supposes that
the eddy is of a convenient size. It may be either so
large or so small as to be useless to him. The com-
plications of the problem, when we introduce into it
the question of eddies, are almost insurmountable.
In the present state of our knowledge we can only
say that there are great irregularities of velocity, to
whatever cause they may be due, and that it is highly
probable that a bird in soaring turns them to account.
We are apt to speak of soaring as being effected
without an)- muscular effort. But it is by muscles
that the wings are held fixed and immovable, and the
strain must be considerable. Apparently, however,
there is hardly any limit to the time during which a
bird can continue an exertion of this kind. No such

24S THE STRUCTURE AND LIFE OF BIRDS chap.

thing as an ache or cramp disturbs him. The wing-
muscles are as unwearying as the leg-muscles that are
stretched when he sits upon his perch.

Steering.

A modern writer boldly asserts that birds " neither
do nor can use their tails as rudders." This shows
how rash it is to dictate to nature what she may do
or may not. I have already shown (p. 214) that the
necessary muscles are present for moving the tail
upward or downward, and for lowering one side
relatively to the other. And a little observation with
the naked eye, or, better, with a field-glass, will show
that numbers of birds actually do pull down the left
side of the tail when they wish to steer to the left,
and vice versa. Rooks make great use of the tail in
steering, the whole expanded fan of feathers being
sloped so as to make quite a different angle with the
horizon from that made by a line passing through
the two wing-tips. In Jackdaws it is almost equally
conspicuous. If, from below, you watch a Lark as he
rises, you can easily see that he keeps his head to
the wind by the perpetual play of his tail. The
Swallow, House-martin, Sand-martin, and Swift guide
themselves largely by the tail, as one might expect
from its great development. The divided tail seems
always to be much used as a rudder, though the loss
of the single long feather from one side makes no ap-
parent difference to a Swallow. Perhaps the domestic
Pigeon shows tail-steering more conspicuously than
any other bird ; he trusts much to it, and he is easy
to observe.

VII

FLIGHT 249

How, then, did the idea originate that the tail
was not employed as a rudder ? It was owing, I
believe, to the fact that a bird who has lost his tail
still manages to direct his course without great
difficulty. A Rook whom one of the many tussles
that go on in a rookery has left tailless is not a ship
without a helm. He can steer, but, as far as I have
observed, he cannot rest between the strokes of his
wings. The tail is a valuable parachute, and, bereft
of that, he must not loiter. His loss, too, reduces his
power of stopping suddenly. Moreover, steering is
not necessarily perfect steering, and I have no doubt
that a tailless swallow misses many gnats which he
might otherwise have caught.

Two things are quite clear, then: (1) the tail is
a rudder ; (2) there is some other means of steering.
I think I have seen gulls when flying fast adopt
another method which is in its nature the same-
let down a foot on the side towards which they wish
to go. More often they kick vigorously when they
make a sudden turn, or when they are about to settle,
working the feet together, and not alternately as they
do in swimming. Clearly, steering by means of the
feet must be limited to web-footed birds.

There is another plan very different from those we
have mentioned. If he wishes to steer to the left, the
bird flings himself on his left side, his left wing point-
ing downward and his right upward. The onward
course of the fore part of his body is retarded by his
outspread wings, the hinder part moves more quickly,
and this causes him to describe a curve. So far, it is
simple enough, but when we come to inquire how the

250 THE STRUCTURE AND LIFE OF BIRDS chap.

change of balance is effected the difficulty begins.
We might naturally suppose that one wing beats
harder than the other ; in the turn to the left the
right wing would work with greater energy, so as to
throw the body on to its left side. That being so, it
is puzzling at first, when we see the turn made, to
find that the right wing which should give the harder
stroke is held aloft while the left wing is lowered.
This, however, is not an insuperable difficulty. Each
wing makes the same angle with the body, so that a
line connecting the tips would either pass through the
shoulder-joints or be parallel to a line passing through
them. It is only the rolling over of the body on to
its side that causes them to point upward and down-
ward respectively. This being the position in which
we see the wings as the bird wheels, can the right
have struck harder than the left in order to effect the
turn? Possibly it may have. In ordinary flight, as
we have seen, the body at once rises in response to
the beat of the two wings together, and thus they are
brought home to their lower limit, without the stroke
having to be pulled through. The right wing, then,
if it gave the stronger beat, might instantaneously
raise the body on the right side and produce the
desired result, the change of balance ; when, very
possibly, all we should see might be the bird sailing
on with the right wing pointing upward and the left
downward. Still there would be an instant at which
the wings would form unequal angles with the body,
and what evidence of it can we obtain ? If a Gull be
watched as he slowly wheels, there seems to be ab-
solutely no inequality of angle ; when a rapid turn

vn FLIGHT 251

is made, the whole operation is like a flash, and
your opportunity has gone before it has come. If the
inequality were great, photographs would of course
supply irrefragable evidence ; but for or against such
a minute difference as the one in question their
testimony is not worth much. In this dearth of
evidence we can only say that, when other animals
have freedom of movement, it would be strange that
birds should have their wings tied to one another, and
that nothing but an unequal stroke can account for
the rapidity with which a Swallow dashes from right
to left. But, though I lean to this view, I believe that
a bird has other means of altering its balance for
steering purposes, which are very likely the sole
means employed in the slower turns and co-operate
in effecting the most rapid. I have already mentioned
that a bird's back is at this point by no means the
rigid rod that it is said to be. The waist is capable of
considerable movement not only up and down but
from side to side. I give what measurements I have
to show the amount of pliability, regretting that the
evidence is so meagre.

Angle

formed by backbone

bending sideways at waist.

Kestrel

... HH

Swallow...

... I 5 0°

Common Tern

... 155°

Kestrel (another specimen) ...

... 156

Domestic Duck

...

... 16s

A black-headed Gull and a Sand-martin both showed
great flexibility,but I have no measurements. From the
figures it appears that a Kestrel, the first specimen
measured, had a more flexible waist than any of the

252 THE STRUCTURE AND LIFE OF BIRDS chap.

others. The Swallow, like the Kestrel a first-rate
steerer, comes second. The Duck, who turns in a long
curve and with labour, has the stiffest waist of all. It
seems probable that a bird alters his balance in order to
change direction much as a skater does. Supposing
that he is curving to the right on the outside edge
on the right foot, the skater turns to the inside
edge (on the same foot), and curves to the
left by swinging the weight of his shoulders across.
And this is done by contracting the muscles at the
waist which lower the left shoulder. A bird may well
change balance in a similar way, swinging his hind-
quarters to right or left according to the direction
in which he wishes to go, and beneath the feathers
the process may not be visible to us. Certainly
the head is often turned at the moment of wheeling,
apparently to help the movement. Occasionally, how-
ever, like a good skater who despises conspicuous helps
to a change of equilibrium, such as a swing of the leg,
a Gull may be seen looking to the right while he slowly
turns to the left. It is these slow turns, when there is
almost certainly no beat of the wings, and during which
the head is occasionally turned the other way, that
make me think a bird alters his balance by a bend at
the waist as a skater does. In the more rapid turns
there may well be a momentary unequal beat of the
wings that defies detection.

Stopping. Use of the Bastaiti Wing.
When a Pigeon in mid -flight wishes suddenly to stop
he alters the inclination of his body, which has been
nearly horizontal, and, relaxing the muscles that had

VII

FLIGHT 253

held it in position, lets it hang almost vertically down-
wards. The wings are held extended, with just a slight
bend at the wrist, facing forwards, and so putting the
break on as strongly as possible. The tail forms the
largest fan it can spread to. For stopping it is perhaps
more important than for steering. If the Pigeon be
watched from underneath, another very curious point
may be made out. It will be seen that the bastard
wings are called into play to add to the spread of canvas.
They are often spoken of as quite rudimentary, or as
useful only in strengthening the wing, though how they
can act in this way is difficult to see. One ornithologist
imagines that in making a turn a bird extends one
bastard wing and revolves round it as on a pivot ! I had
long wondered what their use could be or how so many
muscles could be wasted on a mere rudiment, when I
saw a Pigeon, when checking his speed in order to
settle, lift the bastard wing so that daylight was visible
between it and the long feathers, this petty appendage
jutting out, and impudently spoiling the beautiful line
of the front margin of the wing from shoulder to tip.
If you stand at the entrance to the British Museum
(the Antiquarian Department at Bloomsbury) this
curious phenomenon may easily be seen, as the Pigeons
which are usually feeding in large numbers on the
o-ravel in front fly up and settle overhead on the
pediment. In two specimens of Kestrel Hawks which
I have examined, the extension of the wing necessarily
extended the bastard wing, the tendon within the
anterior membrane attaching not only to the metacarp
but also to the thumb. The purpose of this is not clear,
nor have I noticed anything of the kind in other birds.

254 THE STRUCTURE AND LIFE OF BIRDS chap.

In Mr. Muybridge's 1 photographs of a Cockatoo on
the wing, both bastard wings may be seen to be slightly
raised, for what purpose it is hard to say. In the
Pigeon too they project during a vigorous stroke, but
I have seen no other bird use them either for stopping
or striking.

Fig 65. — Drawn from a photograph by Ottomar Anschutz, showing bastard wing
extended during downstroke.

The Centre of Gravity.

In the bird the centre of gravity falls at a point low
down in the body : the heavier organs are accumulated
there, the lighter ones up above. Below are the great
breastbone and the ponderous muscles of flight (the

1 See Mr. Muybridge's Animal Locomotion, to be seen at the
Brit sh Museum.

VII

FLIGHT 255

pair that lower the wings sometimes weighing one
fifth of the whole weight of the body), and the entrails.
High up, just under the backbone, come the lungs
with their spacious air-sacks. This arrangement is no
doubt advantageous. Imagine a flying-machine with
wings springing from a point many yards above the
engine which supplied the motive power. It would
have a constant tendency to right itself if it capsized.
In the same way the bird is helped in balancing by
the fact that his centre of gravity is low down, but to
a much less extent, since the point lies only a little
below the wings when expanded horizontally. The
lower the weight lies, the greater the space through
which it must be raised before a capsize can take
place. And owing to the way the wings work, it must
lie, as I hope to show soon, mainly behind the shoulders.
But the power of recovering balance at any moment
by making the appropriate movements is quite as
important as the exact position of the centre of
gravity. A bicyclist never ceases to make the neces-
sary adjustments, though he may be unconscious of
the fact. And if a lark be carefully watched from
below as he rises he can be seen to be perpetually
moving his tail to left or right, thus maintaining his
balance and at the same time keeping his head to the
wind. If there is a dead calm, he trusts more to
movements of his head. Moreover, a bird uses, I
believe, his power of bending to right or left at the
waist and so shifting his centre of gravity. To recover
equilibrium, he might give a harder .stroke with one
wing or the other, but it is not certain that the wings
ever beat unequally. When at the end of the stroke

256 THE STRUCTURE AND LIFE OF BIRDS chap.

they arc mainly beneath the level of the body, this
last means of balancing is out of the question, and the
bird must trust to head, waist, or tail.

The question at what point between the head and
tail the centre of gravity lies is a very important one.
Probably if we divide the keel into three equal divisions
it will fall in the middle one, in a vertical line passing
a little behind the point at which the second rib joins
the trunk. It is impossible to decide exactly. It
has been supposed that birds can at pleasure shift the
centre of gravity backwards and forwards. To some
extent no doubt they can, but their power of doing so
has been much exaggerated. Any bird, but especially
the long-necked ones, can move the centre of gravity
forward by extending the neck to its full length. But
owing to the lightness of the skull this has not so
much effect as might be thought, except perhaps with
large-headed birds like ducks. Then again a long-
legged bird as he flies may move it in a backward
direction by trailing his legs full length behind him.
Many of them do habitually carry their necks or legs
stretched out during flight — their necks, possibly, in
order to shift the centre of gravity. The filling of
the air-sacks will have little effect, since they extend
before and behind the important point. The move-
ment of the wings forward or backward will, no doubt,
carry the centre of gravity to some extent with it.
These small changes, however, do not help us to
understand how the bird is able during flight to
maintain a horizontal position, for the main weight
still lies behind the points where the wings attach.

Of the many means that have been suggested for

vii FLIGHT 257

shifting the centre of gravity, none are, I believe, except
in quite exceptional birds, efficacious, and I doubt
whether any bird makes much use of them. The
problem, then, remains : when the body is suspended
from the wings and the centre of gravity lies at a point
farther back, how is a nearly horizontal position
maintained ? Mold up a bird, that has just been shot,
by the wings, and the hinder part of the body will drop
till the incline from the tail to the head (with the partial
exception explained on p. 227) is a very steep one.
This it is, no doubt, that has caused ornithologists to
look for some means by which the bird could at
pleasure move the centre of gravity forwards. The
true explanation is that during horizontal flight the
body is maintained in its position, with only a slight
upward incline, not by balance, but by muscular effort.
If you watch a Pigeon's movements and see how
instantaneously, without a motion of head or legs, he
changes the inclination of his body, you can hardly
doubt that it is the work of muscles. 1 The muscle
called the Latissimus Dorsi, which I have described
in connection with respiration, arises from the verte-
brae and attaches to the humerus (L. D. fig. 56).
During the downstroke it contracts, hauls upon the
wings, and thus raises the hinder quarters. The
enormous strength of the Great Pectorals prevents all
possibility of the wings yielding when thus pulled by
the Latissimus. The body must rise, since the wings
will not give. And, thus, indirectly, in addition to their
other work, the Great Pectorals help to bring the body
to the horizontal. When the wing rotates beyond a
1 See Bronn's Thicr-Reich, vol. u Aves," p. 229.

S

258 THE STRUCTURE AND LIFE OF BIRDS chap.

certain point, its lower face looking backward, the effect
will be the same, without the help of the Latissimus.
During the upstroke the hind-quarters drop no more
than the rest of the body. The centre of gravity
is always behind the shoulder ; in fact, when the
hinder part of the body is raised, it retreats still further.
It ought not to fall in a vertical line between the points
of the wings' attachments any more than a skater's
centre of gravity, when he describes a curve, ought to
be directly over his skate. The action of the wings,
nicely regulated by the Latissimus, is to raise the
hinder and lower the anterior part of the body, and, if
the bird were balanced horizontally between them,
would send him heels over head. The fact that the
main weight is behind prevents this, and enables him
to choose his course.

If an animal uses for progression at one time his
fore limbs only, at another his hind limbs, the centre of
gravity question cannot fail to be interesting, as equi-
librium must be maintained under two different sets of
circumstances. How well the point is situated for
flight, I have shown. In standing or walking, the
thigh-joint is a long way behind the main bulk of
the body, and the long toes are necessary to give
support at the required point. The horizontal or
nearly horizontal carriage of the body calls into play
muscles which, passing from the posterior end of
the pelvis to the thigh-bone, pull the hind-quarters
downward, and so raise the head and shoulders. The
more upright the bird, the farther back the centre of
gravity, and the less the pressure upon the toes.
When standing, a bird is in the position of a man who

VII FLIGHT

259

leans forward, bending at the hips, with the result that
the fore part of the foot bears all the weight, and the
heel tends to rise. This is not a comfortable position
to maintain long. But the stiffness of the bird's back,
due to the fusion of vertebrae, no doubt, makes the
attitude an easy one for him. 1

Force Exerted in Flight.

Borelli, a man of science of the seventeenth century,
calculated that a bird in flying employs a force that
exceeds ten thousand times his own weight. 2 This
astounding conclusion he arrived at by trying to
estimate the disadvantage at which most animal levers
work, the power being applied close to the fulcrum
while the weight is at the end of a long arm. By the
same methods he calculated that the force exerted
by a man in jumping exceeds three thousand times
his own weight. Obviously he has much overshot the
mark, but it is very difficult to devise any plan by
which correct results may be obtained. Professor
Marey has attempted to settle the question of a bird's
muscular power by experiment. 3 A Buzzard was
hooded, and so plunged into " a sort of hypnotism."
The Great Pectoral muscle was laid bare, the elbow-
joint was disarticulated, and all of the wing that lies
beyond was removed. A cord was fixed to the

1 That the centre of gravity must of necessity be where it is,
was first shown me by Mr. R. C. Gilson.

2 Borelli, De Motu Animalium, p. 191 : " Potentia musculorum
alas flectentium plus quam decies millies superat pondus avis
volantis."

3 Animal Mechanism, p. 214.

S 2

26o THE STRUCTURE AND LIFE OF BIRDS CHAP.

extremity of the humerus, and at the end of the cord
was placed a scalepan into which small shot was
poured. The muscle was then stimulated by electric
currents, and shot was added till the contraction of the
muscle was exactly counteracted. The weight raised
was just over 6J lbs. Troy. But in the lever the
power was much nearer to the fulcrum than the weight,
and, when this was allowed for, the effort was calcu-
lated to be equal to just over 33J lbs. Troy. What is
the value of this experiment ? Does the muscle under
the electric stimulus work up to its full power ? This is
very doubtful. Myself I cannot help altogether dis-
trusting the results. But the muscles of mammals
have been treated in the same way, and it seems that
they can develop at least equal energy.

Experiments might be made with Homer Pigeons
which would throw much light upon the question.
They might be weighed in delicate scales both before
and after a long flight, when the loss of weight would
help us to estimate the amount of energy put forth,
though it must be owned that to eliminate all causes
of error would be extremely difficult. Similar experi-
ments have often been made with men, their weights
being accurately taken immediately before and after a
race, or hard exercise of some kind. The comparison
of the loss of weight by man and birds in covering
equal distances would have great interest. It is much
to be regretted, therefore, that owners of Homer
Pigeons have as yet made no experiments that might
supply the necessary facts. The mere opinion that
the loss of weight is slight is of no value.

But, however much we may wish for definite evidence,

vii FLIGHT 261

there arc facts at our disposal which arc full of mean-
ing-, and which in a rough and ready way settle the
question. (1) Birds have large appetites and rapid
digestions, a great proof of vigour. (2) In proportion
to their size they give off from their lungs more car-
bonic acid than other animals, and this means greater
destruction of tissue, which implies a greater expendi-
ture of energy. (3) Their temperature is exceptionally
high, irrefragable evidence of the rate at which they
live. (4) They are capable of as great rapidity of
movement as mammals, and they tire less soon.
This points to superior quality of muscles.

Proportion of Whig Area to Weight in Large and
Small Birds.

This is a more difficult question than it might on
first thoughts appear to be. One elementary cause
of error may be disposed of at once : the doubling of
the size of the bird will not mean the doubling of the
length and breadth of the wings, for in that case the
wing area would be quadrupled. 1 This will explain
the following figures : the comparative wing areas of
a Herring Gull and a Great Tit are represented by
541 : 31, i.e., the Gull's expanse of wing is rather more
than eighteen times that of the Tit, whereas the length
is less than six times as much. 2 It is the areas, then,
and not the lengths or breadths, that we must compare,

1 See fig. 29.

2 I take the areas from Pieter Harting, quoted by Marey
(Animal Mechanism, p. 224), and the lengths from Howard
Saunders's Manual of British Birds. The results are too broad
to be affected by differences in different specimens.

262 THE STRUCTURE AND LIFE OF BIRDS chap.

and we should expect that unless some new factor
should come in and upset the calculation, the areas
would vary as the weight of the winged animal,
whether bird or insect. That this is not the rule is
shown by the following examples. 1

Wing area per kilogram.

Dragon-fly ... 44,032 sq. inches.

Swallow 1 ? 5442 51

Vulture 260 „

Australian Crane 139 ,,

Thus the Australian Crane has in proportion to its
weight only a little more than half the wing area of a
Vulture, rather less than Jy of that of the Swallow
and not quite ^ F of that of a Dragon-fly ! 2

How are we to account for these facts ? Before

1 These figures are those of M. de Lucy, quoted by Professor
Pettigrew, Animal Locomotion ; also by Professor Marey,
Animal Mechanism, p. 222. I have not referred to M. de
Lucy's work, but the quotations agree.

2 The following figures from L. P. Mouillard's U Empire de
I air are interesting. They give in grammes the weight sup-
ported by one square metre of surface, the whole undersurface of
the body as well as that of the wings being included. The
small birds, as a rule, have far more surface for their weight,
though the cock Kestrel heads the list and is an exception
difficult to account for. Most startling is the small amount of
surface that the Goose, Turkey, and Duck have to support
them.

Grammes supported by

Grammes suppoited by
1 sq. metre of surface.

1 sq. metre of sui

-face.

Kestrel (cock)

... 1,968

Golden Plover ... 3,565

House Sparrow

... 2,o66

Kestrel (hen) 3,773

Swift

... 2,073

Bustard ... ... 6,410

Turtle Dove ...

• •• 2,133

Goose ... 8,333

Kite

... 2,226

Turkey ... 9,345

Starling

• •• 2,932

Duck {Anas Ciypeata)

Raven ...

• • 3,° 12

,, hen 9,750

Stork

... 3,536

,, drake ... ... 11,050

VII

FLIGHT 263

giving what I consider to be the explanation, I shall
first try to show that what is accepted as the solution
of the problem by some noted ornithologists in reality
leaves it just where it was.

If you take two cubes, a side of one of which is
twice the length of a side of the other, the larger
one is in bulk eight times the smaller, but its surface
area is only four times as great (see figure 29, p. 113).
This will hold of other figures of three dimensions.
Magnify a bird till it is eight times its former size :
yet you will only have multiplied the surface area by
four. This is no doubt a true principle in geometry,
and it might be applied to the present case if
symmetry were the only thing under consideration.
We should then be doing right if we took the pro-
portion of bulk (not area) of wing to bulk of body, and
it would turn out that the build of big birds and small
is not very different. 1 But the present question is
really one of dynamics. The problem which nature
has to solve when she increases a bird's size is : " If
the weight of the bird be multiplied by so many, how
much will the area of the supporting surfaces have to
be increased ? " In other words, how much must be
added to the wing area, in order that the bird may be
able to fly ? And the answer is (except for modifying
circumstances which I shall pass on to soon), that if
the weight be doubled the supporting surfaces must
also be doubled.

There is an undeniable interest in the fact that
when we compare birds' bodies and wings (the legs are

1 Those who wish to apply this principle should take the cube
root of the bird's weight and the square root of the wing area.

264 THE STRUCTURE AND LIFE OF BIRDS chap.

often long beyond all justification) we find a certain
proportion maintained with no great deviation, that
when their bulk has been trebled their wing surface has
not at the same time been trebled, in defiance of the
laws of symmetry and geometry. But, looked upon
as flying-machines, why do small birds require larger
wings, in proportion to their bulk, than big birds ?

Since, allowing for his greater weight, the big bird
has less wing to support him than the small bird, we
must see what advantages he has that enable him to
do with less. To begin with, if we compare the sustain-
ing powers of two wings of different lengths, we shall
find that the superior power of the long one is altogether
out of proportion to its superiority in length. Suppose
the shorter one to be one foot, the longer two feet,
long. Then if the two are worked by muscles of
equally rapid contraction,the extremity of the latter will
move with a velocity that is far more than double that
attained by the extremity of the former. And since
the resistance of the air increases nearly as the square
of the velocity, it is clear that to judge even by the
comparative velocities of the extremities is to under-
estimate by a great deal the superiority of the longer
wing. Secondly, it has been found by experiment
that the air passes lightly off the margins of a plane
surface which moves through it, and so does not offer
much resistance to them. 1 The loss is proportionately
greater with small planes than with large ones, since
for each square inch of area they have more margin.
The resistance of the air in the case of a square plane
having an area of two square feet, moving in a direction
1 Marey, Vol des Olseaux, p. 214.

vii FLIGHT 265

vertical to its surface and at a certain pace,
would be more than double what it would be
in the case of a plane one foot square under similar
circumstances. Hence small birds get less support
from the air than large ones. It is true that, as
explained above (see p. 190), the front part of the
wing does most of the work during its lightning-like
downward movement, and therefore we find that
the wings of most of the best flyers have narrow
extremities. But during flight the bird's whole under
surface forms a parachute, and a gain in area means a
disproportionate gain in supporting power, even when
we make allowance for the principle just mentioned.
Moreover, a short wing necessitates a quick stroke,
and much power is lost by the small bird in constantly
raising the wing for a fresh effort. A few less
important advantages must also be mentioned. The
great flyers, even when we take into consideration
their greater size, have larger and stronger bones than
the best flyers among small birds, and the absence of
marrow has prevented an increase in weight that
might have been set against the increase in strength.
In large birds the coracoids, if we take the average,
are directed more outwards than in small birds, a
gain in strength without any drawbacks. It is
probable that in proportion to their total bulk they
require less flight muscle than small birds that
resemble them in other respects. Some interesting
measurements which go to prove this have been made
by Legal and Reichel. In the case of the Pigeon, as
I have shown (see p. 212), their figures do not agree
with the facts, but in most cases they are, probably,

266 THE STRUCTURE AND LIFE OF BIRDS chap.

more dependable. These two investigators found
that in expenditure on flight muscle a big bird is
more economical than a small one of the same family,
a big Tern than a small Tern, a big. Gull than a small
Gull. Even when birds of different families are com-
pared, the rule generally holds. The Eagle and other
birds of strong flight have, for their size, lighter muscles
than small birds. The heavy-flying Geese are an ex-
ception. Incalculable factors complicate the problem,
but, clearly, many big birds can save in muscle and
devote more vital energy to other organs. 1

Helmholtz undertook to show that a big flying bird
is an impossibility, since as the supporting power
increases the weight increases more rapidly. 2 This
runs quite counter to the figures just quoted. Even
if his calculation is right in the abstract, it ignores
some important facts that ought to make us hesitate
before we apply his conclusion to birds as they are.
It ignores the fact that the work done by a large wing
surpasses the work of a small wing by far more than
the superiority in area. It disregards what, if not
established fact, is yet not far from it, viz., that
pterodactyls, with wings measuring 25 feet from tip
to tip, were able to fly. It is now still further

1 In three Terns, the comparative weights of which are repre-
sented by the figures 53, 116, 174, the total weights were 5jjj,
6 to> 7/oo times the weight of the breast-muscles in the re-
spective specimens. See Jahresbericht der Schlesischen Gesell-
schaft fiir Vaterland-Cultur. Breslau, 1879.

2 The calculation was as follows : If the linear dimensions
increase as 1 : 4, then bulk and consequent weight (and presum-
ably strength) will increase as 1 : 4 3 , that is, as 1 : 64, but the
sustaining force must increase as 1 14^, that is, as 1 : 128.

vii FLIGHT 267

damaged by the fact that Mr. Maxim's flying machine,
weighing 3.^- tons (a very heavy bird !), has actually
risen from the ground. It is true that all the giant
fossil birds that have been discovered seem to have
been allies of the Ostrich and incapable of flight.
Since, however, the geological record is not only
imperfect but has, much of it, still to be read, we
cannot say that giant flying birds never existed.
Supposing that they once existed, they may have died
out for reasons not connected with flight. Many large
mammals and reptiles have become extinct, the
smaller having some advantage in the race of life.

When we come to compare the actual achievements
of big and small birds, it will be found that the
honours are divided. The small can rise with greater
ease, the line of ascent being far nearer to the vertical.
This may in many cases be due to the greater stiffness
of the shoulder-joint in large birds, which prevents
the wing turning its under surface backward, as it
must turn for ascent up a steep incline (see above on
upward flight). But in one of the big birds that I
have examined, the Eagle, 1 this stiffness is not found.
If we concede that small birds rise with less effort,
on the other hand, the big are, I believe, even allowing
for their greater bulk, better weight-carriers. An
Eagle will carry a young lamb ; for a House Sparrow a
very small piece of bread is a heavy burden.

In long-distance flight the two classes are about
equally matched.

Small birds never soar, and it is generally supposed

1 It can rise with greater ease, I think, than many big sea-
birds that are stiff at the shoulder.

268 THE STRUCTURE AND LIFE OF BIRDS chap.

that their weight is insufficient to give them momentum.
This may be the true explanation. When we speak
of a soaring bird as a kite, the momentum is the
string, and the small bird with a spread of wing, for
him, so large, may be like a kite whose string is too
weak to hold it. Those who maintain the superiority of
the small to the big, would perhaps say that they do
not get the wind to help them, because they have no
need of its help.

Velocity.

Many and various are the methods by which
attempts have been made to measure the velocity of
birds' flight. Audubon found rice in the crops of
Pigeons which, judging by its condition, he estimated
had been eaten six hours before. This rice they could
only have obtained in Carolina, which was 300 — 400
miles distant. These data give at the lowest estimate
a velocity of fifty miles per hour. The Frigate Bird
is often seen flying over mid-ocean, and it is said that
he never travels at night, and never sleeps upon the
sea. Hence, a very rough calculation of the pace of
his flight may be made.

Such methods, however ingenious and interesting,
are most unsatisfying. We want indisputable
measurements, and it is only in the case of one or
two species that they are obtainable. The racing of
Homing Pigeons is a popular amusement in England,
Belgium, and other countries, and " times " are
accurately taken. In 1892 a pigeon, according to
the published record, accomplished a flight of 114
miles at a rate of eighty miles per hour. This is so

vii FLIGHT 269

much in excess, not only of anything achieved in any
other race during that year, but of the velocity of any
of the birds competing in the same race, that I cannot
help hesitating to accept it as trustworthy. The
next best record for the year was seventy-one miles
per hour in an eighty-two mile race. 1 Even when the
conditions are most favourable, when there is a tail-
wind blowing — i.e., a wind carrying the birds towards
their destination — sixty miles an hour is a very
exceptional pace in a race of 100 miles and over.
When the weather is all that could be wished, fifty
miles per hour or slightly more is a velocity more
often recorded. In 1883 the average velocity of the
winning birds in eighteen of the races of the United
Counties Flying Club was thirty-six miles per hour,
the fastest having maintained a rate of fifty-five miles
for a distance of 208.

In France, the experiment has been made of
employing Swallows in place of Homing Pigeons.
The idea is a very ancient one, for Pliny tells us that
a certain Roman knight who wished to let his friends
at Volaterra; in Tuscany know who had won the
chariot races used to take with him to Rome — a
distance of 130 miles — some Swallows which he let
loose after dyeing them the colour of the winner. 2 Of
the experiments in France I have not been able to
obtain any account at first hand. One flight is
reported to have been a very grand one, far surpassing
anything credited to a Homing Pigeon. A Swallow
was taken from Roubaix to Paris, a distance of 258

1 See Burgess's Homing Pigeon Fanciers' Annual {or 1892.

2 Pliny, Natural History, x. 34

270 THE STRUCTURE AND LIFE OF BIRDS chap.

kilometres, or 160 English miles, and in ninety minutes
from the time of its liberation at Paris it was back
again. It had maintained an average pace of 106
miles per hour ! 1 This may appear startling, but the
figures may possibly be correct. It must be remem-
bered that the Swallow is better built for rapid flight
than the Pigeon. Of the velocity attained by the
Swift, who in his flight is very like the Swallow,
though probably more than his match, many people
have arrived at a much higher estimate.

A few years ago some experiments of indisputable
accuracy were made in a range constructed for experi-
mental shooting. Two " screens " formed of very
fine threads were put up at a distance of forty yards
from one another. These screens were connected
with electrical apparatus, by means of which the time
occupied by the bird in traversing the forty yards was
registered. The highest speed attained by any of the
twelve Pigeons experimented on was at the rate of
33S miles per hour, the lowest at the rate of 26* 1.
This is much lower than we should have expected,
considering that the " screens " were placed at the
farther end of the gallery in order to allow the birds
to get up pace. The velocity of four Pigeons was
measured in the open on a calm day by persons
stationed at a certain distance from one another, who
marked carefully the moment at which the birds came
opposite to them and registered it with a stop watch.

1 See an article quoted from the Globe in the Zoologist for
1889, p. 397. In the Hoinifig News for September 13, 1889, is
an account, apparently, of the same flight, the distance being
given as 250 kilometres.

vii FLIGHT 271

The fastest travelled at the rate of 279 miles per hour.
Pheasants were experimented on in the same way in the
range and in the open : in the former case the greatest
velocity was 33-8, in the latter 361 miles per hour. 1

These experiments, made in a gallery where the air
was perfectly still, or in the open, when there was little
or no wind, lead us to conclusions very different from
those which we draw from the records of the races of
Homing Pigeons. All the best of these records are,
as I have said, obtained on very favourable days, when
there is a tail-wind blowing, the velocity of which has
to be added to that due to the bird's own exertions.
But the difference seems to be too great to be ac-
counted for in this way, even if we make no allowance
for the fact that the Pigeon expends time and strength
in circling to a height before starting for home, and
for the probability that his course is not an absolute
bee-line. It is possible that it is easier to attain great
pace in rarefied air at a great height ; at an altitude
of 6,000 feet, the density of the air, as may be seen by
referring to a mountaineer's aneroid barometer, is only
four fifths of what it is at the sea-level. It is true that
such air will afford the bird less support, and that,
therefore, the minimum pace that is necessary, if he
is to maintain his level when gliding, is greater than
in the denser air near the earth. But supposing that
he can support himself, he will advance with greater
rapidity since he will meet with less resistance. We

1 See Charles Lancaster's Illustrated Treatise on the Art of
Shooting, p. 175. Sir R. Payne-Gallwey {Letters to Young
Shooters, p. 152) mentions similar experiments made by himself
by aid of stop watches.

272 THE STRUCTURE AND LIFE OF BIRDS chap.

have somehow to account for what seems to be a well-
established fact — viz., that the American Golden Plover
as it travels southward in autumn accomplishes over
1,700 miles in one flight. Even if we assume an
average rate of sixty miles per hour, the birds would
be over twenty-eight hours on the wing, and this is a
long time to be without food.

Further experiments on the velocity of the flight of
birds of different species under varying conditions are
much to be desired.

A pace of over 30 miles per hour is maintained by
race-horses over a short course. Ladas' time over the
Derby course, 1 J miles, gives him a velocity of 32 \.

A Note on Flying Machines.

It is difficult in writing of flight to leave unmen-
tioned the subject of flying machines. At the same
time, an elaborate account of them would be out of
place here. Not many years back it was supposed that
only in balloons was aerial navigation possible for
men, and the problem that was for ever being debated
was, How is it possible to steer a balloon ? With a
vehicle that travels with the air and has no velocity of
its own, steering is practically an impossibility. Since
this has been realised, attention has been diverted
from balloons to flying-machines, which are, necessarily,
heavier than the surrounding air, since, if they are to
do as they are intended to do, they must develop in
themselves energy sufficient to lift them from the
ground, and drive them forward, when required, in the
teeth of the wind. Naturally, the first idea has been

vii FLIGHT 273

that the propelling force must act by means of wings.
Quite recently Herr Lilienthal has tried to navigate
the air with an equipment resembling a bird's. But
though his wings have enabled him, starting from
an elevation, to sail over 800 yards before sinking to
earth — a truly wonderful feat of pluck and skill —
it is improbable that any one could ever succeed
by the help of such appliances in rising and main-
taining himself in the air ; and nothing short of this
can be called flight. Mr. Maxim has advanced a great
deal further. He has seen that we ought not, in
trying to rival a bird, to imitate it slavishly, as the
first sewing-machine is said to have imitated the
hand-sewn stitch, and he has, therefore, employed
screws for the propulsion of his flying-machine. In
animal mechanism, where the different parts cannot
be separated, screws are, of course, out of the question.
In man-made machinery a screw is better than a lever.
It has no idle intervals, whereas a wing during the
upstroke would be no better than a parachute.
Besides this, it is doubtful whether machinery would
not be too clumsy to effect all the turns that a wing
must make even in straight-ahead flight. As soon as
Mr. Maxim's machine was allowed to run along its
line of rails at a rate of thirty-six miles per hour, it
rose in the air, and but for contrivances designed to
restrain it from mounting more than a very little, there
is no knowing what heights it might have reached.
This great aeroplane, weighing 8,000 lbs., imitates a
sea-bird rising from the water : it presents to the air
a surface inclined slightly upward, and this inclined
surface causes it to rise when it travels fast. But

T

274 THE STRUCTURE AND LIFE OF BIRDS ch. vn

important questions of balance have yet to be decided.
Moreover, it cannot as yet leave the earth without iron
rails on which to get up pace. Wherever it might
alight, it would remain helpless as a stranded ship.
But the man who has got so far towards solving the
problem of flight may well get further.

Conclusion.

This ends my account of flight. Much, I hope,
has been made clear, but much remains that is
inexplicable. Mathematici?ns will, no doubt, some
day arrive at a formula of flight that will claim to
be a complete solution of the problem. Nevertheless
birds will still excite the wonder of men. Even those
who can quote the formula at a moment's notice will,
when they look at a Swift doing his sixty miles an
hour for mere play, or if they happen to see a soaring
Adjutant, relapse for a moment into blank astonish-
ment, the mental state of the Pacific islander when a
steamship first invades his lonely seas and claims a
place in his philosophy. It will always be difficult to
forget for long together, that, however much is learnt
on such a subject as flight, a great deal more remains
to be learnt.

Some of the Literature of the Subject.

Marey's Vol des Oiseanx.

Marey's Animal Mechanism (International Sci. Series).
Alix's Appareil Loco?noteur des Oiseanx.
The article on " Flight " in Newton's Dictionary of Birds.
Pettigrew's Animal Locomotion (International Sci. Series).
Books and papers referred to in the footnotes in the course of
this chapter.

CHAPTER VIII

THE BIRD WITHIN THE EGG

IN writing of birds, as of other subjects, it is logical
to begin ab ovo. This system, however methodical
and German it may be, I have deliberately avoided,
since in practice it is unwise to begin with what is
least intelligible. Now that the reader understands
the circulation of the blood, he will more easily under-
stand some important points in the development of
the embryo. If the pages that describe the circula-
tion are not fresh in his memory, he is recommended
to look them up before reading this chapter.

If a sitting hen be watched upon her nest, she

may sometimes be seen to raise herself a little, and

stir the eggs with her feet. Many people have

imagined that the sole object was to turn them

over. This may be an advantage, since the eggs

require damping, and, by this means, each side in

turn is moistened by the ground. 1 No turning is

required to ensure that the embryo which lies just

1 E gg s that are much exposed to damp in the nest have a
waterproof layer.

T 2

276 THE STRUCTURE AND LIFE OF BIRDS chap.

within the thin membrane that envelopes the yolk is
sometimes at least next to the breast. Break a hole
about half an inch square in the side of a hen's egg,
and on the yolk will be seen a round, whitish spot.
If now the hole be walled up with sticking plaster so
that the white will not run out, and, the egg having
been turned over, another similar hole be made on the
other side, the whitish spot will again be seen. The
yolk has rolled over, as it always does when the egg
is turned. The white spot is the place where the
embryo lies, and it is always at the top, close to the
body of the bird. If the eggs under a sitting hen
are marked and examined the day after, the main
object with which she moves them will soon become
apparent. It will be found that she has shifted the
outside ones to the middle, and the middle ones to
the outside, so that all may have a turn at the warmest
place and consequently hatch about the same time.
For several days I marked the eggs in two nests, in
which they lay in a circle with three in the middle.
As a rule only two of these three (for no system
works perfectly) had been replaced by others from the
outer ring.

Now for the cause of the yolk's rolling over when
the egg is turned. When you break a raw egg, you
can hardly help seeing two twisted cords, of an
opaque white, which at their ends spread out and lose
themselves in the albumen. It has been supposed,
and in some recent books on natural history it is
still maintained, that these cords are the machinery
by which the revolution is brought about. But since
they only float in the albumen, it is difficult to see

VI II

THE BIRD WITHIN THE EGG

277

how they can help towards it. They have a use, for
they act as buffers and save the yolk from violent
concussion when the egg is shaken. The turning is
due simply to the fact that the yolk is lighter on that
side on which the embryo lies. If you examine a
hard-boiled egg, you will see that the yolk is not
uniform in colour ; most of it is yellow, the rest is
whitish yellow, the two being different not only in
colour, but in microscopic structure. The white yolk,

Fig. 66. — Egg, after Fester and Balfour, a, air-chamber between two layers of
shell-membrane ; CH, chalaza; ; E, embryo ; is, internal shell membrane ; WY, white
yolk ; yy, yellow yolk.

as it is called, lies under the round white spot, and,
swelling out like a flask, descends to the centre of the
sphere. When the egg is turned, the yolk becomes top-
heavy, and therefore rolls over. As yet this is only
a rough description. Besides the flask-shaped mass
of white yolk, there are four very thin layers dividing
the yellow into five.

There is another feature that belongs only to an
egg that is not perfectly fresh — the chamber at the
large end. This appears after a time, even if the egg

278 THE STRUCTURE AND LIFE OF BIRDS chap.

is put under a hen, and gradually increases in size as
the white shrinks by evaporation.

To follow in detail the kaleidoscopic development
of the embryo from day to day, till at last by the
help of the "egg-tooth" at the end of his beak he
pecks his way out, is quite beyond the scope of
this chapter. I merely wish to make clear one
or two of the most interesting points. To the
right understanding of these, some preliminary re-
marks are necessary. It is generally held that the
embryo goes through in a short space of time many
of the various stages by which in the course of
ages the species has become what it is. Thus the
progenitors of the butterfly were wingless crawling
creatures, and the change from the caterpillar to the
perfect insect is a brief abstract of the history of long
ages of slow development. In the same way when
we find in the embryos of reptiles, birds, and mam-
mals organs that are the same in nature and origin as
the gills of fishes, we infer that these three classes
of animals were once water-breathers. The heart
and the blood-vessels, as they advance step by step to
the perfected form that we find in the mature bird,
show us clearly that if we trace upward the avian
and reptilian pedigrees we shall come at length to a
point at which they meet, and that if we proceed
further, we shall find the line joining another, wh'ere
the common ancestors of birds and reptiles drew
apart from the more primitive types that continued
to make their home in the water, the progenitors of
the fish of the present geological period.

This being so, it is well beforehand to get some

vin THE BIRD WITHIN THE EGG 279

idea how the blood of a fish circulates. The heart is
a very imperfect one, and mixes the pure and impure
blood. It has one ventricle and one auricle. From
the former the blood is driven through a great artery
to the gills, and from them is gathered into another
great artery which branches and distributes it all over
the body. Thus, as a necessary consequence of the
absence of lungs, there is only one circulation.
The blood on leaving the heart is purified in the gills
and does not return to the heart before doing its work
in the body and limbs. Every one is familiar with
the gills of fishes. On the right and on the left sides
are arches of gristle which spring from the top of the
back part of the mouth and stand out, like bent bows,
on either side. Their frames of gristle or cartilage
are covered with delicate red fringes through which
the blood flows, separated only by a thin membrane
from the water which contains the oxygen it stands
in need of. In principle, therefore, they are the same
as lungs. The great artery, the aorta, which brings
the blood from the heart sends off branches to each
of the gills. Thus there are aortic arches as well as
gill arches (G, fig. 67).

To return now to the chick. On the third day of
incubation, there are clear signs of arches such as I
have described. There are four clefts on each side,
and each cleft has a fold on its front border. The
fourth cleft has a fold both before and behind, and
thus there are five folds and four clefts. These folds
and clefts are homologous to the gill arches and clefts
in fishes. It is true they are now functionless. The
chick breathes throughout the twenty-one days of

28o THE STRUCTURE AND LIFE OF BIRDS chap.

incubation, but not through its gill arches, nor through
the lungs, which are not for some time connected with

Fig. 67.— (After Duval.) Embryo of sixty-eight hours, in, forebrain, to become
optic lobes ; B2, mid brain ; H3, hind brain, to divide into cerebellum and medulla
oblongata ; ch, cerebral hemispheres ; E, eye ; ea, internal ear ; g, gill arches, with
clefts between ; h, heart ; n, notochord, to pass into backbone ; ml, nasal pit.

the heart, and do not set to work till respiration begins
with the hatching of the bird. The system of breath-

VIII

THE BIRD WITHIN THE EGG

ing is peculiar to the embryo, and is carried on by means
of a wide extension of blood-vessels, rendered possible
by its protected situation within the shell. The round
spot, called the blastoderm, at the top of the egg, grows
all round its circumference, and, its edges at length
meeting, it becomes a bag, with various pockets, which
envelopes all the yolk. Out of the folds of the blasto-

Fig. 68. — («) after Gadow. Transverse section through embryo during third day ;
AL, allantois : later it spreads out and lies, full of blood-vessels, close under the shell ;
am, amnion, a fold formed from embryo and enveloping it ; M, spinal marrow ; N,
notochord, to pass into backbone ; ys, yolk-sack. (6) Diagram of the circulation of
the yolk-sack at the end of the third day of incubation. The veins are marked in
outline, and the arteries in black, h, heart (after Foster and Balfour).

derm the chick is formed ; it lies like a crease in the
walls of the bag, and sends out its blood-vessels into
two of the pockets, one called the yolk-sack, the
other the allantois, which forms a branch of the
alimentary canal. The first few days the work is
done mainly in the yolk-sack ; later on the allantois
plays the more prominent part. The shell, of course,

282 THE STRUCTURE AND LIFE OF BIRDS chap.

is porous, and allows of the passage of gases out
and in.

All the pairs of arches except three, the first, second,
and third, become obliterated in the mature bird. The
first pair become the lower jaw : branches sent* out
from them form the upper
jaw. The hyoid bone, which
lies at the bottom of the
mouth and helps to support
the tongue, represents the
second and third pairs. The
first cleft, the only one which
does not get obliterated, be-
comes connected with the or-
gan of hearing, and forms the
eustachian tubes. 1 The bran-
ches of the aortic artery form
pairs of arches corresponding
to the folds visible on the
surface. The first pair of
aortic arches lies within the
first fold, the second pair in
the second, and so on. There
are five folds and five pairs of
aortic arches within them. At
a very early stage there is another pair between
No. 4 and that just spoken of as No. 5, which must,
therefore, rank as No. 6 (Fig. 70- ° n the fourth
day the first two pairs also disappear, leaving
only three, and we must carefully trace the history
of these three and their connection with the heart.
1 See p. 134-

Fig. 69.— Hyoid bone of Crow.
2, second arch ; 3, third arch.
The jaws are the first.

VIII

THE BIRD WITHIN THE EGG

283

By the end of the fifth day the ventricle of the heart
has divided into right and left chambers. On the
seventh day the artery leading from it follows suit and
divides into right and left branches, which are quite
separate. And of these two arteries, that from the
right chamber connects with the sixth pair of arches,
while that from the left connects with the third
and fourth. Meanwhile the lungs have come into
existence, being in their origin pouches formed in
the gullet. The sixth pair of arches open a connection

Fig. 70.— (After Foster and Balfour.) Diagrams illustrating arterial circulation
(a) on third day— the first three pairs of arches, Nos. 4 and 6 not yet formed, No. 5
not represented ; (b) on the fifth or sixth day. The first, second, and fifth pairs
have been obliterated.

with them, and here we have the final arrangement,
the artery from the right side of the heart leading to
these arches, and, so, onward to the lungs. Mean-
while the third pair of arches has been growing weaker,
and the fourth stronger. Finally, only one of the fourth
pair is left, that on the right side ; it is the great aorta,
which may be seen curving to the right, one of the
three single survivors out of five pairs, the other two
being the pulmonary arteries, which lead to the left
and right lungs and represent the sixth pair. Occa-

284 THE STRUCTURE AND LIFE OF BIRDS chap.

sionally the left arch, opposite to the aorta, may be
seen in a rudimentary form. I have once seen it in a
chicken. In mammals it is the left arch, instead of the
right, of the fourth pair that survives, and forms a
most important distinction between them and birds.
Reptiles generally have only the fourth pair of arches
surviving, but in many lizards both the third and fourth

Fig. 71.— Diagrams— after Boas— illustrating (a) lizard's heart ; (6) bird's heart ;
(c) human heart, viewed from the front ; 6, 5, 4, the 6th, 5th, and 4th pairs of aortic
arches.

are found. For a time the bird is, judged by his
heart, a reptile. The crocodile's heart, which in several
points comes near to that of birds, may be called a
noble failure. It has four chambers instead of, like
most reptilian hearts, only three. But it has two aortas
instead of one. Of these, one springs from the left
ventricle and carries pure blood ; the other from the

VIII

THE BIRD WITHIN THE EGG 285

right ventricle and carries impure. Outside the heart
the two arteries cross and there is a small passage
from one to the other, so that the pure and impure
blood are mixed. The fourth pair of arches have
become separate except at this one point ; but what
should have followed, has not — viz. the reduction of
the right aorta to a rudimentary state. Otherwise
the crocodile might have become a warm-blooded and
altogether finer beast.

Some of the Literature of the Subject.

(1) Foster and Balfour's Ele7ne7its of Embryology.

(2) The article on " Embryology " in Newton's Dictio?iary of
Birds.

(3) Bronn's Thicr-Rcich, vol. "Aves." [Dr. Gadow is the
author of both 2 and 3.]

(4) Duval's Atlas D'embryologie.

CHAPTER IX

YOUTH, MATURITY, AND AGE

THE lives of most young birds are fairly familiar to
most of us. Young chickens, not long after they are
hatched, run after their mother, picking up what she
tells them to eat, and listening to her warning cry.
Young ducks soon take to the water, and the method
of their bringing-up may be easily seen. Young
Blackbirds stay long in the nest and wait open-mouthed
to be fed. All this is an old story. There is one bird,
the Hoatzin, the bird of British Guiana already men-
tioned, whose infant life and infant powers it is difficult
to describe without exciting incredulity in the mind of
the reader. But the interest does not end with the
strangeness of the bird and its ways. As we watch
him we may feel sure that we are learning much
of the build and of the habits of many ancient birds
that have made room for more highly developed
types.

The Hoatzin is more nearly related to the Fowls
than to any other birds. But he diverges from them
so far that he is regarded as the sole representative of

CH. ix YOUTH, MATURITY, AND AGE 287

a distinct group. All his near kindred have passed
away, and he stands solitary, a living fossil, the only
survivor of a number of families that have either
disappeared, too primitive to hold their own, or have
advanced to a higher organisation.

Apparently about as large as a rather under-sized
Pheasant, the Hoatzin is really considerably smaller ;
his long tail, his large wings, and his crest suggest a
larger bulk of body than he really possesses. He and
his kind are born from eggs that are usually smaller
than a hen's, but which vary much in size. His breast-
bone has a very slight development of keel, and that
only in its hinder part, and the clavicles, from the
point where they meet, send back a long bone which
looks remarkably like a reptilian interclavicle. Among
the trees and bushes that form a jungle-growth along
the banks of the Berbice and often overhang its waters,
Hoatzins are plentiful. When a boat passes they will
generally remain concealed among the leaves. They
seldom fly, their large wings having but weak muscles
to move them. The longest observed flight was no
longer than forty yards, and that with a considerable
descent, from a high growth on one bank to a lower one
on the other. Mere jumps or the very shortest flights
are more usual when the crack of a gun disturbs them.
The food consists, as far as is known, entirely of leaves :
they are crushed in the crop, which is formed of thick
walls of muscles with outstanding ridges. The nest
is formed of a few sticks intertwisted. When the
birds'-nester comes, cutting a way for his boat
through the bushes, or wading thigh-deep through the
mud, the old bird makes off upon the wing. The

288 THE STRUCTURE AND LIFE OF BIRDS CHAP.

nestling, unless very recently hatched, crawls off on
all fours, making much use of not only his enormous
feet, but of the great claws that grow at the ends of
the two first digits of his hand. If you try to pull
him from the nest by the legs, he holds fast by means
of these wing-claws and his beak. Often a young
bird may be found clawing his way onward some
distance from the nest. If he tumbles into the water
he proves to be a born swimmer and diver. All this
is so astonishing that some people are inclined to
hesitate before they accept it as true. But the paper
by Mr. J. J. Quelch, from which I derive this account
of the bird's habits, is based upon patient and careful
observation. Nor does the Hoatzin, so far as his
infant mode of life is concerned, stand absolutely
alone. The young Ionornis, a Florida bird, has been
seen to climb out of his nest by means of his wing-
claws, 1 and besides this the anatomy of the young
Hoatzin bears witness to his habits. The wing-digits
are enormously developed. I have already mentioned
that the embryo has a distinct rudiment of a fourth.
More remarkable is the great length of its first.
Though No. 2 is of wonderful dimensions, yet No. I
is more than half its length. And each of these two
carries at its end a very big claw. Contrast with them
the digits of a Swift's wing ; there we find the first
hardly more than a quarter of the length of the second,
and there is no sign of a claw on either. 2 In the

1 Shufeldt, Ibis, vol. ii., 1890.

2 The hand of the embryo Hoatzin is here compared with
that of an adult Swift. The hand of an embryo Swift would
probably present a very great, though not so great, a contrast.

IX

YOUTH, MATURITY, AND ACE

28c

nestling Hoatzin the hand is longer than the forearm ;
gradually it grows shorter, while the other parts of the
wing lengthen, till, in the fledged bird, the forearm
surpasses it (Fig. 72 a, b). The feathers, too, adapt
themselves to changing circumstances ; in the nest-
ling the growth of the two outermost primaries is
completely arrested, so that the use of the claws
may not be impeded ; when it is fledged and can
fly, they begin again to grow and attain their full

^Ar^l

Fig. 72.— Wing of Hoatzin — (a) young, (/>) mature — after Pycraft.

length. With maturity, too, the claw on digit No. I
grows small, while that on No. 2 is lost altogether.
The pause made by the growing quills was first
noticed by Mr. Pycraft, and he has further pointed
out that in the common chick we have traces of a
similar development. There too we find the hand of
the young bird longer, though only by a little, than
the forearm ; the second digit has a claw only in the
embryo stage ; the first is much reduced, but retains

U

290 THE STRUCTURE AND LIFE OF BIRDS chap.

its claw. But the most wonderful point of all is that
the three outermost primaries have their growth arrested
while the others advance in length ; later on they
grow to their full size. This can be well seen in the
series of young chickens in the hall at the South
Kensington Museum. And so the chicken, a thor-
oughly modernized relative of the archaic Hoatzin,
still retains this queer trace of its ancient life.

The anatomy and habits of Hoatzin may help us to
understand what may have been the manner of life of
Archaeopteryx. Mr. Pycraft suggests that it used its
claws mainly during its infancy. May it not also have
had recourse to them when moulting ? A poor flyer
at best, and its leg-muscles possibly weak, it might
well, when bereft of some of its big feathers, or,
through an imperfect system of moulting, bereft of
all at once, appreciate fully the value of wing-claws, to
help it to climb out of the reach of reptile enemies.

We will now pass on to a comparatively common-
place subject. As a rule a young bird differs in
plumage from a mature one. The cock bird as he
grows up often puts on fine plumes unknown to his
youth. If there is a difference in brightness between
the old birds and the young, it is always the latter
which are characterised by the more sober dress.
But beyond this it is impossible to find a rule that will
apply to all. When the cock is more conspicuous
than the hen, the young usually resemble the mother.
The young cock Blackbird, however, is easy to dis-
tinguish in the nest by his darker tint. In the Painted
Snipes and the few other species in which the female
bird is more gaudy than the male, it is the latter that

ix YOUTH, MATURITY, AND AGE 291

the young birds resemble. If the two sexes are alike
when mature, the young birds may, as with the Robins,
have a distinct plumage of their own ; or, as with the
Kingfisher and Jay and many sober-hued birds, young
and old may be alike. Still the resemblance is often
not exact. When, as in the case of the Dunlin, both
sexes put on a distinct and richer dress in spring, it is
the old birds in their winter garb that the young
resemble.

The age at which maturity is attained varies much
in different species. Our small birds are ready to nest
the first spring after their birth. In some cases there
is no difference of plumage ; the young swallows
arrive later than the old ones, but when they have
arrived they are indistinguishable. Gulls, however, do
not appear in quite mature plumage till their third or
fourth year, or even their fifth — i.e., till the moult at the
end of their fifth summer. The young Lesser Black-
backed Gull keeps his first plumage till the summer
following. Then comes a moult, after which we find
him still brown but paler than in his first year. At the
end of his third summer his colour becomes an ashen
gray, and the tail is in some specimens white, in others
mottled. None of the primary wing-feathers, or, at
any rate, only one of them, has a white spot or
" mirror." The moult after his fourth summer brings
him to the plumage of his maturity — back and wings
black, breast white, head white with dusky streaks
that will pass away and leave pure white in spring. A
Gannet, which may be described as grown up, has not a
single black feather except the primaries ; all the rest
of him is white except the fine cream-coloured head

U 2

292 THE STRUCTURE AND LIFE OF BIRDS chap.

and neck. The young bird is blackish, speckled with
white, and at each moult some of the incorrect feathers
are weeded out. These changes, it is believed, give a
rapid recapitulation of the history of the Gannets. At
one time they had throughout their lives these blackish
feathers ; in the course of generations they attained to
their present spotless white. It is probably not till
the fifth autumn that the .plumage reaches the per-
fection of maturity, and the next spring the bird
makes his first nest. In the photograph at South
Kensington of Gannets sitting on their eggs, there is
apparently not one whose plumage shows any sign of
immaturity. The youth of a Golden Eagle lasts ten
years, or even longer. The rate of mortality is great
among young birds, and but few Golden Eagles, prob-
ably, survive to build a nest and rear young. And
this leads us on to the subject of the age attained by
birds, for there is no doubt that there is a connection
between this and the age of maturity. There is a good
deal of evidence that Eagles and their allies live to a
great age — to ioo years and more ; and even if parti-
cular cases are doubtful, yet on the principle that a
number of weak sticks make a strong faggot, I think
we may accept it. Suppose that an Eagle lives to
only sixty years, and becomes mature at ten, then a
pair will produce ioo eggs in the fifty years, the
number laid being usually two, and of the hundred
birds hatched only two will live to grow up. This
calculation, which I quote from Professor Weissmann,
even if only roughly correct, is valuable as bringing
together three facts which must be viewed in con-
nection— (i) the great length of an eagle's natural

ix YOUTH, MATURITY, AND AGE 293

life ; (2) his late attainment of maturity ; (3) the
high death-rate among the young. The ages to
which birds may attain are as a rule but vaguely
known. Our small singing birds sometimes live to be
ten years old ; a Magpie has lived twenty years in
captivity, Parrots upwards of 100. Humboldt's story
of a Parrot, whose words the Indians said could not be
understood because it spoke the language of an ex-
tinct tribe, is an amusing myth sprung from an old
belief. Some idea of the age attained by Guillemots,
Razorbills, and Puffins may be formed from the fact
that they lay only one egg, and, though the young are
exposed to great dangers, yet the numbers of the
species do not diminish. Even if they sometimes have
two nests in the year, yet unless their span of life were
a fairly long one they could not keep up their numbers.
Ravens have lived nearly 200 years in captivity, and a
White-headed Vulture captured in 1706 is believed to
have died at the Zoological Gardens at Vienna in
1824.

Literature bearing on the Subject.

Darwin's Descent of Man (see vol. ii., p. 187, and onwards).

Weissmann's Essays on Heredity (see p. 1 1 and onwards).

Professor W. K. Parker on the " Morphology of Opisthocomus
Cristatus," Transactions of the Zoological Society, vol. xiii.,
part 2.

Mr. J. J. Ouelch on the " Habits of the Hoatzin," Ibis, vol.
ii., 1890, p. 327.

Mr. W. P. Pycraft on " The Wing of Archaeopteryx," Nat.
Science, Nov. 1894.

(I am also indebted to Mr. C. M. Adamson's Some more
Scraps about Birds, printed for private circulation.)

CHAPTER X

BIRD POPULATION

BRITISH ornithologists have good reason to be
depressed, and to wish that they had lived in the
days of their grandfathers. One bird after another
passes away exterminated by sportsmen, collectors,
drainage, or increase of population. The Bittern
when he comes to us is shot before he can nest ; the
Great Bustard only occasionally strays here ; the
Golden Eagle is only to be seen in remote moun-
tainous parts of our islands, and the Bearded Tit
lingers on in but two or three counties. And the
evil seems likely to increase. Nothing, I believe, but
the establishment of protected districts, on a larger
scale than has hitherto been accomplished by private
persons, can check it. Parliament may perhaps some
day grant charters to societies of naturalists allowing
them to maintain such oases, and punish as a thief
any one who steals eggs or birds from the sacred
precincts.

The question of diminution of species is, however,
quite different from the question of diminution of

chap. X BIRD POPULATION 295

population. There are probably as many birds, when
all told, in the British Isles as there were in the last
century. Cultivation supplies them with abundance
of food almost everywhere, and if you wish to find
the greatest possible number of birds' nest in a day,
there is no better place to search than a garden.
There are far more small birds there than in a big
forest, or an open moor. In a day's walk in Suther-
landshire you may see nothing but some Grouse, a
Raven or so, a Buzzard, and perhaps an Eagle. A
rickyard simply swarms with birds. The preservation
of game has led also to a preservation of Warblers.
They are saved from birds'-nesters, and Hawks are
kept down. Now that Scotch firs and larches are
common trees, we have more Golden-crested Wrens.
With the spread of plantations the Robin and the
Blackbird have extended their range further north.
The Missel Thrush is now found as far north as
Caithness, and though unknown in Ireland before
1800 is now common there, even as far west as
Connemara. In Scotland Chaffinches are on the
increase, and in many parts Starlings, never seen
some forty years back, are now familiar birds.
The Peewit, whose nests grow rarer and rarer in
England, breeds in greater numbers than formerly
in the north of Scotland. The conditions, in short,
have changed, and under the new conditions some
kinds thrive and multiply, others dwindle and vanish.
But new species do not come to us, except in very
rare cases, to replace those that pass away, the
tendency of civilization being to reduce more and
more the amount of variety upon the earth. There is

296 THE STRUCTURE AND LIFE OF BIRDS ch. x

one bird, the House-sparrow, who is always at home
amid the bustle of human life, and who, if England
were to become one big city, would out-do Sir Boyle
Roche's famous bird and be, not in two, but in all
places at once. Happily he is not the only bird
which takes to town life. In Germany there is the
Stork, and in London we have the Domestic Pigeon,
the descendant of the Rock-dove, at Westminster,
at the National Gallery, at the British Museum, at
Liverpool Street Station, and many other places.
And in Regent's Park in January I have heard the
Thrush and the Robin singing in a thick and choking
London fog. There are a great many birds that
will live happily and without fear amid the noise of
human life, or in a comparatively quiet town-garden
or park, where, however much they may be looked at,
they are free from actual molestation. Even the
Wood-pigeon is now a Londoner.

Books on the Subject.

Facts bearing on this question will be found in almost all
books on the natural history of birds. See Mr. Hudson's Birds
in a Village.

CHAPTER XI

COLOUR, SONG, AND ALLIED PHENOMENA

Nature of Colours

While others were discussing for what purpose the
brilliant colours of birds existed and what part they
played in the life of the species thus adorned, some
German investigators with characteristic thoroughness
set to work to discover the nature of the colours them-
selves. They found that the colours in birds' feathers
might be divided into two classes. There were first
those which appeared the same from any point of
view ; secondly, those which changed as the bird, or as
the person who watched it, moved. To these classes
have been given the names of objective and sub-
jective colours. The first class had to be subdivided
into two— viz., those which were due to pigment
alone, and those which were due partly to the pig-
ment and partly to the feather's structure.

Thus we have — (I.) objective colours, (i) due to
pigment ; (2) due to pigment, plus structure ; (II.)
subjective colours.

THE STRUCTURE AND LIFE OF BIRDS chap.

(I.) Objective Colours.

Colours due to pigment alone may be : (a) black ;
(b) brown ; (V) red ; (d) yellow or greenish-yellow ;
(e) very rarely green ; in fact, only in the feathers
of the Touracou.

White pigment has been found in the pineal eye
of the Lamprey and in one butterfly. 1 But it does
not, as far as is known, exist elsewhere in nature.
Black, brown, and red are always due to pigment
alone, yellow sometimes. These are occasionally
modified by another layer of pigment overlying
them.

Blue and violet belong to the second division of
objective colours ; pigment and structure combine
to produce them. Take any blue feather and hold it
up to the light, so that the rays pass through it. It
is no longer blue, but a dull black or gray. Hammer
it, and all the blue vanishes. Green, except in the
one case mentioned, is also a pigment-structural
colour. Hammer a green feather from a Parrot,
and it becomes yellow, the colour of its pigment.

No blue pigment has been found; that which a
blue feather contains is black-brown to yellow. The
colour which it presents to the eye is due to the
structure of the horny feather coating which encases
the pigment. It has been found that under a thin
outer sheath, there are a number of small polygonal
cones ; from the surfaces of these cones project ex-
tremely fine ridges, and it is believed that to these
1 See Beddard, A?iimal Coloration, p. 4.

xi COLOUR AND SONG 299

ridges, with the very narrowest interstices between
them, the colour is due. No doubt they somehow
break up the light rays, but how they act in com-
bination with the underlying pigment I am unable to
explain. Violet and green feathers have no cones
like these, but long thin ridges lying close together
have been found, so that in them too the cause of the
colours is the same.

The underlying pigment in the case of the green
feathers is yellow. There is one structural colour
which seems to be produced without the help of

Fig. 73. — (After Gadow). Cone with fine ridges, found in blue feathers, s, thin
surface layer overlying cone ; R, R, rays of light.

pigment, or possibly the pigment has not been dis-
covered. However this may be, the resulting colour,
yellow, is found in the feathers of the Pitta or " Ant
Thrush."

(II.) Subjective Colours.

Take a feather which has a metallic lustre. The
Bronze-winged Pigeon's will do fairly well ; Humming
Birds, I think, supply the best. Hold it horizontally
on a level with the eye, and look along it from either
end or across it, and it looks simply black ; but look
down upon it, and move it to and fro, and it will

300 THE STRUCTURE AND LIFE OF BIRDS chap.

show some of the colours of the spectrum following
one another in their right order. Beginning with
bronze-red it will change to golden green, to green,
and thus, in some cases, on to blue and violet. There
is no brown or gray, but only the colours of the
spectrum ; another fact that makes it probable that
they are prismatic. The light when it falls on the
feather is broken up by a number of prisms in the
same way as drops of rain break up the light and
form a rainbow.

" In the spring a livelier Iris changes on the burnished dove."

These colours, then, are properly called iridescent.

But it never happens that all the colours of the
spectrum are visible. Of this various explanations
have been suggested, for instance, that two prisms
overlap, and that the complementary colours produce
white light. Iridescent colours are seen also in
reptiles. The reticulated Python at the Zoological
Gardens shows wonderful changes when the sun falls
upon it.

White stands by itself. It is due to the rays of
light being broken an infinite number of times and
reflected. It requires no pigment. To this breaking
of the rays is due the whiteness of powdered glass
and of snow.

The lustre of feathers as distinguished from colour
is due to the reflection of the light from the polished
surfaces.

xi COLOUR AND SONG 301

Variety of Colours in the same Bird.

This variety is often most striking. The Beautiful
Grass Finch (Poephila mirabilis) displays blue, yellow,
green, and red. There is a kind of Parrot, the Red-
sided Eclectus from New Guinea, which has a black
beak ; a thin ring of blue skin round the eyes ; the
head, the under side of the neck, and the upper
part of the breast a rich dark red ; a half ring of
indigo blue on the back of the neck; the lower
part of the breast also indigo blue; the wing
coverts and the back claret colour, and the tail
scarlet. There is none of the limitation of colours
which we find in flowers. It is rarely that we find
among flowers a species, a genus, or even an order in
which red, blue, and yellow are represented. A blue
rose has not yet been invented. All the many butter-
cups are yellow or white. In the multitudinous
British compositor, red is unknown and blue is rare.
In the order which is more remarkable than any other
for elaborate specialisation, the orchids, there is,
among all the British representatives, no decided blue
or yellow. Thus we find individual birds which show
more variety of colours than many orders of flowers.
Two well-known genera, the anemones and the
hyacinths, have blues, yellows, and reds, and this
makes them quite remarkable. Besides the splendour
of feathers birds often have grand combs and wattles,
which latter are appendages on the throat. This will
bring the Turkey-cock to any one's mind. Tetrao
Cupido, a kind of Capercailzie, has an orange-coloured

302 THE STRUCTURE AND LIFE OF BIRDS CHAP.

sack on each side of its neck. The Umbrella Bird
has a kind of fleshy dewlap thickly covered with
scale-like blue feathers. The Cassowary carries a blue
horny elevation at the top of his head.

Patterns.

On first thoughts the patterns in which we find the
colours distributed on the surfaces of birds and butter-
flies seem to show an infinite variety. But on in-
vestigation it proves not nearly so great. There is
one law which always operates in wild animals, the
law of bilateral symmetry — i.e., the right and left sides
are always coloured very nearly alike. In birds, the
head is often of one colour, the breast of another, and
so forth ; but this can hardly be called a pattern. As
a rule the patterns are varieties of lines or spots, and
this is true, not only of birds, but of other classes of
animals ; among mammals, for instance, of the deer
and the great carnivora. The ocellus, or peacock eye,
is a spot in its most perfect development ; the centre
is surrounded by one or more rings of different colours
or of different shades. The delicate shading is more
beautiful in the ocelli of the Argus Pheasant than in
those of the Peacock, though the colours are not so
brilliant. And in this bird one and the same feather
shows how the ocellus grows out of a line through
the transition stage of a vague ellipse. As we look at
it, the work of Nature's " 'prentice hand " and mature
skill seem presented to us at once, suggesting the
gradual stages by which the species has developed
its magnificence. There was a time when Argus

XI COLOUR AND SONG 3°3

Pheasants had no ocelli. Slowly, in thousands of
generations, they have attained to their present beauty,
and the record of the process is written on their
feathers.

To show how similar patterns run through nature,
a writer in the Spectator 1 points out that the peacock
eye is found not only in the Peacock, the Peacock
Butterfly, the Eyed Hawkmoth, and other allied
insects, but also in a small fish (Guppy's Cypri-
nodon) and in a kind of Iris. The fish in question,
recently to be seen at the insect house at the Zoo-
logical Gardens, is very minute, hardly more than an
inch long, but the males are adorned with several un-
mistakable and very beautiful " eyes." Thus we have
the same form of decoration in birds, insects, fish,
and flowers.

Perhaps the beauty of birds is not due mainly
either to their brilliant colours, or to the patterns in
which they are arranged. Some sober-coloured birds
are far more beautiful than some of the most gaudy.
The lines of the figure have often a grace almost un-
equalled in Nature. Take for instances, the Crane's
neck, the gentle curve where it joins the body, or the
lines of the Gull's expanded wings as he floats in the
air. Colour and fine symmetry are, of course, often
combined, but the less gaudy birds seem to have the
advantage in figure, unless it is that over-brilliancy of
colour distracts the attention from other charms. And
even in the dullest the eye is always bright. A wild
bird must be in a bad way if he has a lacklustre
look.

1 June 3rd, 1893.

304 THE STRUCTURE AND LIFE OF BIRDS chap.

Protective Coloration.

On one point every one is in agreement — viz., that
many birds are protectively coloured. They escape
the hungry eyes of birds of prey, reptiles, and other
enemies, from the fact that they are of the same colour
as their surroundings. The birds of the Sahara are
most of them sandy-coloured. Pallas's Sand-grouse,
a native of desert tracts north of the Himalayas, must
be familiar to most people. The sober browns and
grays of our common birds are well suited to their
life in woods which half the year are leafless, whereas
the evergreen tropical forests afford concealment for
more conspicuous plumage. The Ptarmigan drops
his dark summer dress for one of a pure white that
may make him almost invisible on the winter snows.
The Canary Islands have two very remarkable
instances of this adaptation to surroundings. There
is a Chaffinch (Fringilla teydea) that lives among
the pine trees on the high slopes beneath the peak of
TenerifTe, and to harmonise with the glaucous hue of
the pine trees, he has become a beautiful blue-gray.
There is no doubt that this bird is a descendant of
European Chaffinches. Teneriffe is a volcanic island,
thrown up from the bottom of the ocean, and it has
only upon it such animals as have found their way
over sea, though these have often been curiously
modified. The Trumpeter Bullfinch is of a uniform
bright salmon colour, but on the ochre-coloured
rocks of the nearly desert island of Hiero he is ex-
tremely difficult to see. In the Natural History

xi COLOUR AND SONG 305

Museum at South Kensington there arc some admir-
able illustrations of protective coloration.

Sexual Coloration, Song, Antics, Combats.

Where there is a difference between the two sexes
it is almost always the cock-bird that is the more
brilliant of the two. The vocal powers also of the hens
are very limited. The superior endowments of the
males are accompanied by two allied characteristics,
the love of displaying their fine colours and voices,
and great pugnacity. The Peacock spreads out his
splendid sea of eyes, and there can hardly be a doubt
that during the display he is filled with pride and
vanity. Sometimes he will back against a wall ;
whether to hide the less brilliant back view, or to
escape from the wind is not clear. The Turkey-cock
puffs himself out, spreads his tail, and drag's his wing-
tips on the ground, no doubt with a view to effect.
The Barndoor-cock looks often the personification of
pride. Many of our small birds are constantly singing,
and there is no doubt that love of display, emulation,
and excitement are among the motives that actuate
them. Bird fanciers match birds against each other to
see which will sing the longest, and a really keen one
will sometimes sing till he drops dead. Where Nightin-
gales are common they often seem to be singing against
each other. Bird-catchers attract Cock Chaffinches by
exposing a stuffed bird to view while a first-rate singer
carols in a cage out of sight. To decide for certain
what the motive in a bird's mind may be, is, no doubt,
impossible. Even to know what is in the mind of

X

306 THE STRUCTURE AND LIFE OF BIRDS chap.

another man with whom we are familiar is a difficult
problem ; we are each of us so isolated from every one
else. When it comes to reading the thoughts of a
bird, we must expect sometimes to arrive at very
wrong results. Poets in ancient times invariably
represented the Nightingale as a melancholy bird who
poured forth a dirge. They read their own thoughts
into the bird's song, like a German commentator
who reads profundities into the simplest line of
Shakespeare. And then the legend of Philomela
made permanent a notion which otherwise might
have given place to a more natural one. In this case
the modern view can hardly be wrong, that, though no
doubt there is a strong admixture of other motives, the
Nightingale sings to give vent to his exuberant spirits.
There are some birds whose song, if it expresses any-
thing at all, expresses the wildest jubilation. Such is
the song of the Lark. The Robin, it is true, seems
even in spring time to put into his note something
of the melancholy of autumn. But what seems
melancholy to us need not be so to him. The
hideous cry of the Peacock is, no doubt, charming to
himself and, very possibly, to the Pea-hen.

Society has on many birds an exhilarating influence :
Rooks when they come home for the night fly round
and round, making a babel of cawing, before they
settle down to rest. Mr. Hudson 1 describes several
great flocks of Crested Screamers singing alternately
— a splendid and orderly concert. Wild birds have
superabundant health and spirits from the simple fact
that illness nearly always means for them speedy
1 Naturalist in La Plata, p. 227.

xi COLOUR AND SONG 307

death. In fact, happiness is the general rule in the
animal world, and chronic melancholy is unknown.

In all these displays of plumage and song, in
every exhibition of high spirits, it is the cock-birds
who play the leading part. The nesting season is the
time of jollity and hilarity, in whatever way expressed.
It is in springtime that there take place among some
species those elaborate performances that go by the
name of love antics ; in spring, too, the cock-birds
engage in their most desperate fights. Colour-dis-
play, singing, antics, and fighting are all allied
phenomena ; and all are manifested at the time of
pairing and nesting.

In many species there are special preparations.
The cock-birds have a partial moult, after which
they don far gayer plumes than they have worn
during the winter. The Linnets, Red-polls, Dunlins,
Golden Plovers, Gulls, brighten up their old dresses
or adorn themselves with new. Ducks and their
kin get ready over-early ; the male Teal, having
been since July as dull as the hen-bird, in October
blossoms out in the black, chestnut, green, buff, and
white which he wears in spring. The Ruff puts on a
noble breastplate. Even the Cormorant appears with
a white patch on each thigh that breaks the monotony
of his attire. So different in many birds is the spring
plumage from that of the rest of the year that when
you shoot a specimen in autumn or winter it is often
quite unlike the pictures in the books which, as a rule,
give the birds at their best. In many cases, however,
for instance in that of the Peacocks and Pheasants,
the grand plumes are worn throughout the year.

x 2

308 THE STRUCTURE AND LIFE OF BIRDS chap.

When the birds have arrayed themselves for
spring, there is heard that outburst of song that gives
to an English wood or shrubbery a charm that is said
to be often wanting in a tropical forest. In England,
however, we have not the performances that are made
up of music, partly vocal, partly instrumental, and
dancing that is sometimes sedate and sometimes
madly wild. The best account of these is given in
Mr. Hudson's Naturalist in La Plata, a book which
should be read by all who wish to enter into the lives
of animals. I give a few instances from other sources.
In the Nineteenth Century} speaking of the dances of
a kind of grouse Mr. John Worth writes : " One of
the cocks lowers his head, spreads out his wings
nearly horizontally, and his tail perpendicularly, dis-
tends his air-sacs 2 and erects his feathers, then rushes
across the floor, taking the shortest of steps, but
stamping his feet so hard and so rapidly, that the
sound is like that of the kettle-drum ; and at the
same time he utters a kind of bubbling crow, which
seems to rise from his air-sacs, beats the air with
his wings, and vibrates his tail so that he produces
a loud rustling noise, and thus becomes a really
astonishing spectacle. Soon after he commences,
all the cocks join in, rattling, stamping, drumming,
crowing, and dancing furiously ; louder and louder
the noise, faster and faster the dance becomes, until
at last they madly whirl about, leaping over each
other in their excitement." The Australian Bower

1 April, 1893.

2 I.e., the coloured sacks on his neck. The "floor" is the
spot where the birds meet for these performances.

XI COLOUR AND SONG 309

Birds are equally wonderful. A number of these
combine to erect a bower, which they decorate with
shells, feathers, and anything that commends itself
as ornamental, the cock-birds doing most of, but
not all, the work. In these bowers go on antics of a
much more gentle and sedate kind than those just
.described. It is much to be regretted that the Bower
Birds at the Zoological Gardens seem recently to have
had no spirit for architecture or for elaborate sports.
Among our English birds, as I have said, antics on a
grand scale are unknown, perhaps because they have
a richness and variety of song sufficient to express any

Fig. 74. — Snipe's outer tail-feather (after Darwin).

emotion. But we have occasional instances of instru-
mental music in our domestic and wild birds. The
Peacock rattles his quills. The "drumming" noise
made by the Snipe, as he descends with wild speed
from the sky, is now known to be caused partly by
the curiously curved outer tail-feathers. If one of
these is held in the hand and waved rapidly through
the air, the "drumming" is actually heard. The
wings probably assist. 1

Fighting is very often combined with antics, notably
by the Blackcock and Capercailzie.

In some cases the fighting itself seems merely to be
of an antic character, the males sparring with little
1 See Darwin's Descent of Man, vol. ii., p. 64.

310 THE STRUCTURE AND LIFE OF BIRDS chap.

danger to each other before the females. The Ruffs,
who, unhappily, no longer breed in England, supply the
best instance of this. They have regular places at
which they, assemble, known to fowlers by the grass
being trampled down. Here day after day they con-
gregate and go through their fantastic performance.

Fighting pure and simple unaccompanied by antics
is common. Humming-birds are said to fight des-
perately. The males of the Common Waterhen have
great battles, standing nearly upright in the water,
and fighting with their feet. Robins are great fighters.
The males of gallinaceous birds are often very pug-
nacious and in their spurs they have a formidable
weapon. The courage and ferocity of the Gamecock is
proverbial. As a rule it is among polygamous species
that we find the combats most desperate and the
antics most elaborate.

Theories to explain these Phenomena.

Darwin was first in the field with a definite
theory, to which he gave the name of Sexual Selection.
The cock-birds fought and the hen-birds were the
prizes. There is no doubt that this theory rests on a
foundation of fact. Selection by battle goes on in
many classes of animals, notably among deer and
cattle. But an explanation of the brilliant plumage
was more difficult to find. The theory which we owe
to him is very ingenious and plausible, but the evidence
on which it rests is insufficient. The female bird, he
maintained, selected the handsomest male bird, or the
best singer, or the one that performed the most strik-

XI COLOUR AND SONG 311

ing antics. Those who were selected left descendants
behind, and thus grand plumage and voice became
more and more developed. He found support for
his view in the habits of those species in which the
hen is more brightly coloured and larger than the cock-
bird. With them it is the hen-birds who fight among
one another ; the males sit upon the eggs and care for
the young. Among these birds is the Indian Turnix
Taigoor, the females of which are kept, like Gamecocks,
by the natives for fighting. The cock-birds, it is said,
undertake the whole duties of incubation and nursing,
the hens absenting themselves and collecting in flocks
as soon as they have laid. The Painted Snipe,
another Indian bird, is an instance of the same thing.
Ostriches and their allies have similar habits. The
cock-birds are responsible for incubation and the
care of the young. But the female is, at any rate not
in all the allied families, the statelier bird. The male
Cassowary is larger and more brightly coloured about
the head than his partner, and the cock Ostrich is
larger than the hen-bird whose eggs he hatches.

Darwin saw in the Indian Turnix and Painted
Snipe a confirmation of his theory. The brilliant
colours were due to selection. But here it was the
brilliant hen-bird that was selected by the male. One
of his strongest arguments was the fact that the
splendid plumes were often a serious inconvenience
and even a danger to the bird which carried them.
Instead of natural selection lopping off these cum-
bersome appendages, female taste stepped in and
preserved what, according to the great law of the
survival of the fittest, ought to have passed out of

312 THE STRUCTURE AND LIFE OF BIRDS chap.

existence. The portentous feathers carried by the
Argus Pheasant render him nearly incapable of
flight: he keeps to the jungle and trusts to his running
power. The comb and gills of a Gamecock put him at
a great disadvantage with an antagonist who has had
them trimmed off. The Bird of Paradise with his
forest of plumes, and the Lyre Bird with his far-
spreading burden of beauty, seem very ill-fitted for the
struggle of life. Of Peacocks a good authority writes :
" Peafowl run very fast, but the old cocks, burdened
with tails six feet in length, are poor flyers ; and I have
frequently seen my men run them down during the
hot hours of the day by forcing them to take two or
three long flights in succession, in places where they
could be driven from one detached piece of jungle to
another." x The question, then, was to prove (i) that
the cock-birds largely outnumbered the hens so that
some would necessarily remain without mates ; (2)
that the hen-bird actually did exert a choice. The
evidence on the first point is still insufficient. Dr.
Guillemard mentions in the Cruise of the Marchesa,
that his collection of birds included 584 males, 285
females, and 1 1 1 of undetermined sex. This seems
to show a large preponderance of males. But the
male is the more conspicuous, and therefore is more
likely to be shot. The question, however, is not so
important as it might appear, since a large number of
the species in which the cock-bird has the grander
plumage are polygamous, and in them the number
of males is obviously excessive. Even in the case
of monogamous species Darwin was able to argue
1 Hume and Marshall's Game Birds of India, vol. i., p. 88.

xi COLOUR AND SONG 313

plausibly that the selected males would pair earlier,
and thus have time for a second or third brood. It
must be owned that the evidence of female preference
is deficient. Sometimes when the Peacock is making
the most of himself, the Peahen looks the other way.
Sometimes he shows off to men, or even, it is said, to
pigs. Most of the instances of a hen-bird falling in
love with a particular cock-bird seem to show caprice
rather than a regulated taste for the beautiful. A male
Blackbird and a Thrush pair together. Out of a flock
of twenty-three Canada Geese one pairs with a
solitary Bernicle Gander. A male Wigeon, living
with others of the same species, has been known to
pair with a Pintail Duck. We have also the damaging
fact that some Pigeons dyed with magenta were not
much noticed by the others. But there is some really
valuable evidence from Audubon's Ornithological Bio-
graphy that in the United States hen Woodpeckers,
Red-winged Starlings, and Nightjars actually do make
a choice among several suitors. Again, persons who
have had great experience of Canaries maintain that
a hen-bird will choose out the best singer. But, when
it is all added up, the direct evidence does not amount
to much ; and there is adverse evidence such as this,
that the cock of the farmyard, who has beaten his
rivals in battle, does not lose caste though all his
plumes are draggled. The arguments often brought
against this theory are not necessarily fatal — (1) that
it presupposes a highly cultivated aesthetic taste in
the hen-birds ; (2) that selection by battle and by
female preference cannot go on at the same time ; (3;
that some birds sing at all times of the year. It is not

314 THE STRUCTURE AND LIFE OF BIRDS chap.

required the hen should admire each particular
ocellus, or the delicate pencilling of each feather, but
only the grandeur of the whole display, since the
theory assumes not that her admiration is the cause
of the fine plumes, but only the cause of their not
being weeded out by Natural Selection. The con-
stancy of the colours, and the markings of the feathers
as one generation succeeds another, is no doubt a
difficulty, since one kind of brilliancy might be as
pleasing to the hen-bird as another. But the patterns,
as I have said, are simple, and much may be explained
by what is called correlated development, of which
a good instance, illustrating our present subject, is
given by Darwin. In all breeds the males have the
elongated feathers called hackles on the neck and
loins. In cases where both sexes have a topknot,
that of the cock-bird alone consists of hackle-shaped
feathers. Thus there is in the cock-birds a tendency,
due to causes as yet unknown, to produce hackle-
shaped feathers in certain parts of the body. We
need not then assume that in these species female
taste demands hackles in one place and not another.
With regard to the second objection, it may well be
imagined that where grand plumage and pugnacity
are combined the hen-birds admire the splendour of
feathers as the natural accompaniment or corollary
of warlike prowess. The third objection was well
met by Darwin. That the Robin sings nearly the
whole year round does not prove that the power of
song was not originally developed to charm the hen.
Animals take a delight in the exercise of their powers.
A Gull, for instance, delights in its evolutions in the

xi COLOUR AND SONG 315

air, though its power of flight is due solely to the fact
that it could not live without it ; and the horribly
cruel instinct that leads a cat to play with a mouse
before killing it may be, perhaps, accounted for in
the same way.

Dr. Wallace seeks to explain the facts by an
entirely different theory. But before entering into
this, it is necessary to clear the ground a little. In-
organic things — e.g. gold — have brilliant colours. Mere
brightness of colour, therefore, requires no theory to
explain it. In animals the pigments are probably
waste products derived from their food, and if not
employed for this purpose would be of no service
at all. It is easy, then, to account for the bright
colours of many deep sea animals, where there is no
light except from the occasional phosphorescent lamps
borne by some of the fishes. Even the bright colours
of a butterfly may be no tax upon its strength, for,
very probably, they can be as easily produced in the
animal system as a dull neutral tint. The difficulty
consists partly in the constancy of the colours and
patterns, but, chiefly, in the long plumes, the annual
production of which must tax the bird's strength, and
which are a source of danger to it. The moulting
season brings with it an increased rate of mortality
among the birds at the Zoological Gardens. How
the Argus Pheasant is over- burdened by its plumes, I
have already shown.

With regard to the constancy of colours Dr. Wallace
has pointed out that much may be explained on the
principle of protective coloration. The hen-bird is
exposed to greater danger than the cock-bird, since

316 THE STRUCTURE AND LIFE OF BIRDS chap.

she sits upon the nest. We might expect, then, that
she would be more soberly coloured than her partner.
And this is what we do find, with the remarkable
exception which so admirably supports Dr. Wallace's
theory, that among birds which nest in holes, so that
the hen as she sits is concealed, the bright colours
are very frequently common to both sexes. This is
the case e.g. with the Kingfisher. The hen Wood-
pecker is brilliantly coloured, though less so than the
cock. The conspicuously- coloured Pigeons sit exposed
upon the nest ; but Natural Selection only requires
that a species should have some means of maintaining
itself. The particular means which we find in opera-
tion is due to unknown causes. Thus the Pigeon's
great power of flight, and the ease with which he finds
food, may render protective coloration unnecessary
to him.

There is good reason, then, why in most cases the
hen should be dull-coloured. But this affords no
explanation of the enormous plumes of the Peacock,
the Argus Pheasant, the Lyre Bird, and the Bird of
Paradise.

Dr. Wallace's views on this subject I will give
in his own words : " The fact that they (long plumes)
have been developed to such an extent in a few
species is an indication of such perfect adaptation to
the conditions of existence, such complete success in
the battle for life, that there is in the adult male, at
all events, a surplus of strength, vitality, and growth
power which is able to expend itself in this way with-
out injury." x This is very strange as coming from the

1 Darwinism, p. 293.

xi COLOUR AND SONG 317

greatest living champion of the theory of Natural
Selection through the struggle for existence. Else-
where, Dr. Wallace speaks of this struggle as un-
ceasing. As with artificial selection, every advan-
tageous variation is selected, every disadvantageous
one is weeded out. In Domestic Pigeons—^., in
the Dragons and the Pouters— peculiarities have been
artificially produced which are not well suited to
wild birds. If such breeds of Pigeons are not kept up
by constant selection on the part of the breeder, they
will soon tend to return to the characters of the
wild Rock-dove from which they have sprung. If
highly-bred horses are allowed to run wild they
gradually lose many of the points which were pro-
duced by artificial selection. But the Peacock and the
Argus Pheasant, whose plumes render them ill-fitted
for wild life, are, according to Dr. Wallace's theory, so
full of vigour and vitality that Natural, Selection ceases
in their case to operate. These birds, it seems, having
won their laurels, are not compelled any longer to
enter the lists, and are at liberty to expend their
vigour on finery which, however beautiful to us, must
be pernicious to them. Of the dancing and antics,
however, the theory gives a satisfactory explanation,
and perhaps even of the elaborate vocal performances
of the Thrushes and Nightingales. These do not, as
a rule, endanger the birds in question, or draw largely
upon their vitality. ' Health and high spirits are the
result of the working of Natural Selection.

Dr. Wallace's view has been developed by Professor
Geddes. In nearly all species of animals, the males
are the more vigorous ; in them most variations are

318 THE STRUCTURE AND LIFE OF BIRDS chap.

produced, and if the sexes show a difference in adorn-
ment, it is almost always the male that is more
brightly coloured. In many butterflies, and in some
fishes and crustaceans, there are such differences.
Among sea-urchins and starfish there is thought to be
in some cases a superiority in point of colour in the
male over the female. 1 It is very important, if we
are to come to a right conclusion upon this question,
that all the facts should be considered, and one of the
most important facts is this which Professor Geddes has
emphasized, that in many of the lower animals, and as
a rule among the higher, we find the male possessing
some superior adornment. Here we have an undoubted
tendency, however we may seek to account for it.
But to explain the Peacock's enormous plumes we
require something further. We want to know why
the regulating law of the Survival of the Fittest has
not reduced their growth. If we ask why a Peacock
is encumbered by a train that may easily lose him
his life, it is no answer to be told that in a certain
species of crustacean {Squilla sty lifer a for instance)
the male is rather more brilliantly coloured than the
female.

Mr. Stolzmann has, I believe, supplied the clue to
this puzzle. According to him it is advantageous to
the species that the number of cock-birds should be
kept down, and their grand plumage helps towards
this end. This is an extension of Dr. Wallace's view
that it is the female which mainly needs protection.
Mr. Stolzmann maintains that the cock-bird in many
species not only needs no protection, but that it is
1 See Beddard's Animal Coloration, p. 255.

xi COLOUR AND SONG 319

desirable that he should fall a victim to a bird or beast
of prey. He takes as proved, what I have said still
requires proof, that the cock-birds largely outnumber
the hen-birds. But even if this is not so there must
in the polygamous species (which are chiefly in ques-
tion, for in them the male is most the slave of his
plumes) be an excess of male birds. Where this
excess exists, the hen-bird, when she is sitting, will be
liable to be disturbed by cock-birds which have not
found a mate. Certainly, many game-preservers
maintain that you get more young pheasants, if you
keep down the numbers of the old cocks. Thus the
gorgeous plumes and the desperate fights of the
pairing season have for their object the reduction of
the number of males.

It may well be thought that there is something of
an over-statement here. It might have been safer to
formulate it thus, that since the males are largely in
excess, Natural Selection ceases in their case to work.
They run riot in plumage, because they are of little
importance to the species, and it is only for the pre-
servation of the species that Natural Selection cares.
If a cock-bird's loud song attracts his enemies and so
causes his death, there are plenty more to take his place.
In the same way, the drones of the beehive, of which
there is a monstrous superfluity, are liable to be easily
caught, and have no sting to defend themselves with.
However great the death-rate among them, the life
of the hive goes on and the species continues. Natural
Selection does not kill them, but lets them die.

Thou shalt not kill ; but nced'st not strive
Officiously to keep alive.

3 2o THE STRUCTURE AND LIFE OF BIRDS ch. XI

Summary.

Coloration can be in many cases explained as pro-
tective. Dr. Wallace's theory of exuberant vitality
goes a long way to account for bright colours, song,
and love antics. Patterns are in the main simple, and
depend on laws of growth not yet understood : to one
of these laws the name of correlated variation has
been given. The grand plumes are explained by Mr.
Stolzmann's theory that the males are largely in
excess, and that, therefore, Natural Selection in their
case does not act, or even tends to make them less
adapted for the struggle of life.

Some of the Authorities on the Subject.

Gadow : " The Colour of Feathers affected by Structure,"
Proc. Zool. Society, 1882.

Gadow : Article on Colour in Newton's Dictionary of Birds.

Gadow : Bronn's Thier-Reich, vol. " Aves."

Darwin : Descent of Man, vol. ii.

Wallace : Darwinism.

Beddard : Aniinal Coloration.

Geddes and Thompson : Evolution of Sex.

Stolzmann : Proc. Zool. Society, 1885, p. 421.

Hudson : Naturalist in La Plata.

See also references given in footnotes in the course of this
chapter.

CHAPTER XII

COLORATION OF EGGS

The eggs of reptiles are white, and the reptilian
origin of birds would lead us to the conclusion that the
eggs of primitive birds were like them. But the eggs
of existing birds present a great variety of colours,
though it is seldom that very bright tints are found,
and the question is how these colours are to be
accounted for. There can hardly be a doubt that
the coloration is in some cases protective, but we
must be careful not to extend this explanation too
far. Dr. Wallace has pointed out that a very
large proportion of the eggs that are white or light-
coloured enough to be conspicuous are found in
domed nests or nests built in holes, so that they
cannot be seen. For instance. Kingfishers and Puffins
nest in holes in the ground ; Woodpeckers. Hoopoes,
and Owls in holes in trees ; Wrens and the Willow-
warblers build domed nests. Ducks and Grebes lay
their pale eggs in open nests, but they have the habit
of covering them up : these exceptions are, therefore,
a very first-rate illustration of the principle. There

322 THE STRUCTURE AND LIFE OF BIRDS chap.

are other exceptions which require a different explana-
tion. Swans, Herons, Pelicans, Cormorants, and Storks
lay whitish eggs, and leave them exposed ; but they
are strong birds and can protect themselves from
enemies, the more easily if, like the Herons, they nest
in colonies. On the other hand, the Plover and the
Snipe, who have open nests upon the ground, and who
could not beat off enemies, lay eggs so like the sur-
roundings that it requires a very good eye to see
them. Blue eggs, such as the Hedge-sparrow's, Dr.
Wallace maintains, are not at all conspicuous among
green foliage.

I shall now mention some of the difficulties that we
encounter if we try to press the theory. Pigeons
lay white eggs, and, whatever may be said about their
being concealed among the leaves, they are often
most conspicuous. The Short-eared Owl leaves its
eggs on a tump of grass or heather, and they are white
like the eggs of other Owls which nest in holes. The
case of the Barn-door Fowl counts for nothing.

The eggs of Gulls are protectively coloured, and as
the birds nest in crowds together, they probably need
no protection. The same may be said of Rooks'
eggs. Moreover the rookery would be visible enough
to any egg-seeking enemy, and it is hardly likely
the eggs would be saved by any dulness of colouring.
The eggs of Hawks and Eagles are not conspicuous,
and no enemy is likely to invade their nests. The
Redstart's blue eggs are laid in holes, where, according
to the theory, we might expect them to be white.
Moreover wc often find among birds that are nearly
related, a suspicious family likeness, regardless of the

\ii COLORATION OF EGGS 323

fact that different species have different nesting
habits. The eggs of the Raven, Crow, Rook, and
Jackdaw are by no means unlike in colour. The
Gulls arc nearly allied to the Plovers, Curlews, Ruffs,
Sand-pipers, and Avocets ; and this fact, probably,
supplies the explanation of the supposed protective
colouring in the case of Gulls' eggs, which require
no protection. In the same way we can account for
the white eggs of the Short-eared Owl. Occasionally
we find a species whose eggs are differently coloured
from those of its allies, without there being any
difference of nesting habit to explain it. The Little
Bittern, for instance, lays eggs of a dull white, while
those of other Bitterns are of a brownish olive colour.
It seems, in fact, that Natural Selection does not
work either universally or very promptly upon the
colours of eggs. If the Wood Pigeon's eggs were of
an inconspicuous tint, it might be a slight gain to the
species. But the birds are so vigorous, so strong on
the wing, and so able to find food, that a slight imper-
fection such as the whiteness of their eggs does not
materially reduce their numbers. The large number
of eggs laid by the common chicken may allow for
thc destruction of a good many nests, so that con-
cealment can be dispensed with. And it must also
be borne in mind that when a bird lays only one or
two eggs, sitting begins soon, so that there is more
need for the concealment of the birds than the eggs.
Though it may seem a paradox to say so, it is some-
times because the eggs arc many and sometimes
because they are few that protective colouring is
unnecessary.

Y 2

324 THE STRUCTURE AND LIFE OF BIRDS CHAP.

Natural Selection does not bring about absolutely
perfect adaptation, but only so far moulds a species
to the conditions of life as to enable it to live and
thrive. White eggs without protective colouring may
be only a slight source of danger and, so, continue,
just as we often find an organ continuing in a useless
rudimentary state, not eliminated in the struggle for
existence, because, if useless, it is also almost harmless.
It must not be supposed that the production of colour
puts a great strain upon the system. It is merely a
waste product of the body, which otherwise would be
made no use of. The pigments are very similar to
those of the bile, though they are not exactly identical.

Natural Selection has no doubt in many cases
turned colours to account. It is impossible to deny
that they are in some cases protective. But imagine
the case of a species which at one time needed pro-
tective colouring for its eggs, and which afterwards
changed its nesting habits, or was relieved of the
presence of some enemy that preyed upon its eggs,
so that the protection became unnecessary ; then it is
highly probable that the old tint and markings would
continue because of their harmlessness, though they
had ceased to be beneficial. This may be the reason
of the protective coloration of the eggs of Gulls ; it
explains, too, the want of it in the case of the eggs
of the Short-eared Owl. The old style of egg common
to the family has been maintained because, though in
this species some colour might be a slight advantage,
the question has not become a burning one.

If Natural Selection is so slow to act upon them,
how is it that the colours of eggs arc as constant as

xii COLORATION OF EGGS 325

they arc ? Guillemots seem never to lay two eggs
that arc similarly coloured. The ground colour
varies from blue through man)- stages to white, and
the flail-like marks vary much or arc omitted
altogether. Out of thirty Guillemots' eggs it is often
impossible to pick out two that arc really alike.
I low is it that we do not find in other eggs a similar
tendency to variation ? To answer this question is
difficult, but perhaps not so difficult as it might at
first appear. Eggs have either a plain wash of colour,
or else a ground colour marked with spots or dashes.
All the markings, it is believed, are originally circular,
but as the egg moves down the oviduct, they become
smeared or lengthened out. Sometimes they take
a spiral form — for instance, in some birds of prey — and
this would seem to show that the egg rotates as it
moves forward. Thus there is nothing that can
properly be called a pattern to account for, and, as
the pigment is simply a waste product used up, its
constancy does not perhaps require a very profound
explanation. There is really more variation in the
eggs of many species than one is apt to think. But
in a small egg the change in the shape of spots and
blotches does not attract attention as it docs on a
larger egg, such as the Guillemot's, where they too arc
on a large scale. In that, the most striking example
of variation, the remarkable phenomenon is the range
of the ground colour from what is almost a deep blue
to white, and this can be paralleled. With birds that
lay only two eggs it often happens that nearly all the
colouring matter is deposited on one only, sometimes
on the first, and sometimes on the second of the two,

326 THE STRUCTURE AND LIFE OF BIRDS ch. xn

I shall speak of the variations in Cuckoo's eggs in
the chapter on Instinct and Reason.

Some Authorities on the Subject.

Wallace : Darwinism and Contributions to the Theory of
Natural Selection.

Newton : Article on "Eggs* in Dictionary of Birds.

CHAPTER XIII

INSTINCT AND REASON

If a frog's spinal cord be divided at the neck and a
drop of strong acid be placed on his thigh he will
bend his leg and rub it off with his foot. The brain
can give him no help, for the connection has been
severed. Only some lower nerve centre is called into
play, and there is no consciousness. It is such action
that we call reflex. If we accidentally touch hot em-
bers, then suddenly draw back, the action of drawing-
back is as reflex as the frog's movement of his leg. So
far all is easy. But no one can approach the subject
of instinct and reason without feeling that it is an
extremely difficult one. An instinctive action is
different from a merely reflex one in this, that it
originates with the brain, and is probably accom-
panied by consciousness, though there is no conscious
working towards an object in view. When a hungry
Blackbird sees a worm he at once proceeds to eat it
without going through a process of reasoning. But
he is probably conscious, all the while, what he is
doing. It is, therefore, an instinctive and not a

328 THE STRUCTURE AND LIFE OF BIRDS chap.

reflex action. Next let us look for some examples
of reasoning power or intelligence in birds. If they
learn by experience that men in their neighbourhood
do not shoot on Sunday, and if, in consequence, they
are much less cautious on that day than on week-
days, the)' are showing intelligence. In the same
way Pheasants learn by experience to distinguish a
rifle from a shot-gun. The former has no terrors for
them, and they will feed quietly while the bullets
pass over them. I have seen the same complete
indifference to the noise of rifle-shooting in the
Great Spotted Woodpecker. To learn wisdom by
individual experience is of the very essence of
reason. Without intelligence or reasoning power of
a kind, a Redpoll could hardly learn to pull up his
bucket of water when he is thirsty. Probably he
does not consciously connect the means and the end.
He is like the man who puts a penny in the slot and
takes his piece of chocolate without any knowledge
of the machinery, the working of which has given
him what he wanted. He connects the penny and
the chocolate, but does not know by what process the
one produces the other.

There is no doubt that some actions are purely
instinctive, but it is comparatively seldom that a
" little dose " of reason is absent. Intelligence often
modifies instinct. A caterpillar who weaves a small
web of silk from which to suspend his chrysalis will,
if he finds himself in a box with a muslin lid,
economise in silk and hang his chrysalis from the
muslin. A bird will modify the form of his nest
to suit changed circumstances. Instinct is in fact

xin INSTINCT AND REASON 329

plastic. A particular action may be partly instinctive
and parti)' intelligent. Professor Lloyd Morgan has
contributed a good deal to the understanding of the
question. 1 lie took some eggs from under a sitting
hen and put them in an incubator, and when the
chicks emerged from the eggs, he experimented on
them. They pecked at almost everything, no doubt
by instinct. But they had to learn to peck straight,
and to learn to judge distance, so as to know whether
a piece of food was within range of their beaks.
Experience taught them that burning cigarette-ends
were not good for food. When pieces of dark
crimson worsted-wool were first substituted for the
worms they had so much enjoyed, they were
swallowed greedily, but afterwards they were viewed
with much distrust and generally rejected. All his
life long a bird is learning. An old Heron is far
more knowing than a young one. The young Curlew
has to learn much from his seniors and by experience
before he attains to the proper Curlew standard of
wariness. On the other hand, the Cuckoo is to a
great extent able to dispense with experience and
instruction. For it can hardly be supposed that an
old bird takes a young bird in hand and teaches her
what to do with her egg, or that the young bird goes
through a process of learning to find a nest and
entrust her egg to a foster-mother. Birds which arc
hatched from the egg by the sun, must be born with
some ready-made knowledge of the world. Even
teaching and experience can only awaken powers
that are born in the bird. We often find him at the
1 See his article in the Fortnightly Review for August, 1893.

CHAP.

end of his tether. In South America there is a little
bird {Furnarius cunimlarius) which makes its nest
in a horizontal burrow in the ground, said to be often
nearly six feet long. These birds have been known
to burrow again and again into a mud wall with a
view to nesting there, and were no doubt surprised
when they came to daylight on the other side.
Yet they had flown over the wall, and had had man)'
opportunities of seeing, its small thickness before
they set to work. 1 In the same way with a stupid
persistence one of Mrs. Brightwcn's pet starlings
continued to search for grubs in every corner of the
drawing-room. The intelligence, however, comes
oftener to our notice than the stupidity.

Song.

Daines Barrington took three Linnets, when quite
young, from the nest, and put them with different
foster-mothers, selecting three with easily recognis-
able notes — the Skylark, the Woodlark, and Titlark or
Meadow Pipit — and each, he maintained, learnt and
adhered to the song which it heard in the days of its
early youth. The Linnet educated by the Titlark was
afterwards put with other Linnets, but it never unlearnt
the Titlark song. Daines Barrington was one of the
correspondents of Gilbert White, of Selborne, and
these experiments were made in the latter half of
the last century. More recent investigators have
not, as a rule, been led to the same conclusion. Mr.

1 See Darwin's Journal of Researches (Minerva Library
edition), p. 69.

XIII

INSTINCT AND REASON 331

A. G. Butler 1 took a Skylark from the nest which
"sang its own wild song, but introduced into it the
song of the Persian Bulbul." l< Chaffinches," he says,
" unless absolutely isolated, readily pick up the wild
song, but if kept in the same room with Canaries, their
song is lengthened (and thus improved), though not
altered in character." A Missel Thrush which he
reared sang only two notes. A Blackbird sang the
first line of "Villikins and his Dinah, and another
the first line of a Psalm tune." A Cock Starling
li sang a jumble of sounds mixed with the guttural
call-note of the Missel Thrush." In fact a bird, if
isolated, sings his own song, if any ; as a rule the
power that is in him requires awakening. If he
hears one of his own species carolling, he is very
soon able to imitate it. 2 If he hears only other birds,
he no doubt learns to imitate them, but the process is
a comparatively long one, and often the foreign notes
are only an addition to his own proper song, which
can still be clearly made out. Many of the tame
Thrushes in bird-fanciers' shops have been taken
early from the nest, and they sing the Thrush's
song. Sometimes they may have heard no bird
sing, in which case their music must be due to pure
instinct, or they may have heard the songs of many
birds and singled out that of their own species. The
Cuckoo is not taught by his sire. If instinct does
not teach him, how does he know the one cry amid

1 " The Songs of Birds reared from the Nest : ' in the Zoologist
for 1892, p. 30.

2 Romanes {Mental Evolution in Animals, p. 227) says,
" The singing of birds is certainly instinctive,"

332 THE STRUCTURE AND LIFE OF BIRDS CHAP.

all the chorus of the woods that he is to pick out and
imitate ? Mr. Witchell, who has written much upon
the subject, holds that all birds learn their songs
from their parents or from other birds. Every one of
them, according to him, is a mimic, and is constantly
imitating others. We are thus reduced to hopeless
confusion. In an elaborate song we have to pick out
the bird's ancestral music from all the superadded
variations. This is easy with a caged bird, because
we can learn his proper song from his kinsmen in
the woods. But if there is no limit to imitation
among wild birds, chaos must result, and it would be
far more difficult to learn the distinctive song of each
species than it is. Confronted with the fact that
nearly related birds living widely separated often
have a similar song, Mr. Witchell is able still to
cling to his theory. But if birds have to learn their
notes by imitation, surely the American Ferruginous
Thrush would by this time have picked up a different
song from our common Thrush ; the Shore Lark of
America would not sing like our Skylark, and the
American Snipe would have a different cry from
ours. 1 It is a remarkable fact that many birds that
are good mimics have little song of their own. This
is the case with Parrots, Jays, Jackdaws, Starlings,
and Bullfinches. It would seem as if it were an
advantage to the mimic to have no old family
music for the acquired song to drive out or modify,
and this tells strongly against the notion that singing

1 See " Bird Song and its Scientific Teaching," by C. A.
Witchell, in the ProC. Cotteswohi Naturalists' Field Club, vol. x.,
part iii., p. 238.

xni INSTINCT AND REASON 333

is taught by each pair to their offspring. But
it must be owned that there are exceptions. The
White-banded Mocking-bird of Patagonia not only
imitates every other bird, but has a glorious song of
his own that surpasses all that he mimics.

I have already mentioned the remarkable fact that
some birds that have little or no song in the wild
state have highly developed song-muscles which they
can turn to account when subjected to instruction
in captivity. The Bullfinch, is perhaps, the most
remarkable example of this. His finely equipped
organ of voice suggests that Bullfinches were once
great songsters, but that they have lost the art of
singing. If this is so, the theory that song is instinc-
tive is not affected, since it is quite possible that in a
musical species individuals might be born who had
no impulse to sing ; and if the species did not suffer
through this, there is no reason why the song should
not have become obsolete, while the organ of voice,
being so small as to draw but slightly on the' bird's
vital energy, might remain.

The conclusions, then, that we come to are — (1) That
song is instinctive. (2) That in many birds it requires
to be awakened : they must hear their parents sing,
but they pick up the song so quickly that to speak of
their learning it by instruction is absurd. (3) That
when a good singer learns another bird's song his own is
generally traceable still. The several songs of Daines
Barrington's Linnets may have been Linnets' songs
with variations.

334 THE STRUCTURE AND LIFE OF BIRDS chap.

Nest-building.

Nest-building is generally held to be entirely the
work of instinct. But Dr. Wallace has tried to show
that this too is an acquired accomplishment. 1 He was
at first inclined to believe that the young birds when
still in the nest learnt the principles of architecture.
This is as if an infant in arms on seeing a steam-engine
should at once understand how it is made. Giving up
this theory he suggests other possibilities — that, when
they first have to build they see another pair at work
and so learn, or that a young bird always pairs with
an old one. These views will hardly bear examination.
If we wish to get at the true explanation, we must
realise that instinct is plastic and can be modified by
reason. Birds frequently, as Dr. Wallace says, show,
when they are building their nests, that they are not
mere machines. They adapt themselves to new
situations. The Swallow and the House-Martin have
availed themselves of barns and houses. The Palm
Swift in Jamaica till 1854 always built in palms. But
in Spanish Town when two cocoanut palms were
blown down, they drove out the Swallows from the
Piazza of the House of Assembly and built between
the angles formed by the beams and joists. In
America the Tailor-bird now uses thread and worsted
for its nest instead of wool and horsehair, and wool and
horsehair may originally have been substitutes for vege-
table fibres and grasses. In Calcutta an unconven-

1 See Dr. Wallace's Contributions to the Theory of Natural
Selection^ p. 211.

xiii INSTINCT AND REASON

335

tional Crow once made its nest of soda-water bottle
wires, which it picked up in a backyard. In districts
liable to floods, Moorhens often build in trees. In
New Zealand the " Paradise Ducks," which usually
build on the ground near rivers, have been known
when disturbed to build on the tops of high trees, and
to bring down their young on their backs to the water.
But all this does not show that birds have not an
instinctive knowledge how to build. It only shows
that their instinct can be modified by reason and
experience.

Many nests are works of very great skill. In Eng-
land we have the Long-tailed Tit's nest, wonderful for
its neatness and its beauty. Some of the commonest
nests, such as the Chaffinch's, are works of art. The
Magpie's, if not beautiful, is a formidable fortress.
Among foreign birds there are marvellous builders,
such as the Tailor, Weaver, and Oven birds. For
fine architecture the feet must have a power of grasp.
No web-footed bird builds a really clever nest. But
a long fine beak is not, as Dr. Wallace maintains,
necessary. Of the four commonest Tits, the Long-
tailed is by far the best builder, and his beak is
remarkably short, much shorter than that of the
other three. The Chaffinch, too, has a short bill and
makes a good nest. Some birds — e.g., Ducks — have
beaks that could never turn out very good work ; but,
speaking generally, skill is more important than a
beak of a particular form. And to say that a bird
learns how to build a nest from the casual sight of
another pair at work is almost as much as to say that
she ahead}' knows how to do it. The power must

336 THE STRUCTURE AND LIFE OF BIRDS CHAP.

be inborn, only requiring to be awakened, or, as
Professor Morgan says, requiring " only the touch of
the trigger to fire off the complicated train of activities,
the ability to perform which is innate." The Razor-
bill affords a good illustration ; he is a born diver, and
yet cries plaintively when his mother coaxes him to
take the first plunge. The principle will become clear
if we imagine an attempt to teach a bird to build
anything but its own particular nest, to imitate the
Bower Bird, for instance, and construct an elaborate
arbour, or an attempt to teach the Chaffinch to build a
domed nest like the Long-tailed Tit's, or a House-
Sparrow or a Wood-pigeon to build a neat nest of any
kind. If it were ever successful it would at any rate
require much time, whereas just a hint, if even that
is required, is enough to set a bird off building as its
parents have built before it. Any one who has taught
boys must have noticed what is not very dissimilar.
A boy — some vara avis — will perhaps master Euclid as
if geometry were born in him. In classics much
teaching, and much work on his part may produce
very little result. In short, all faculties are innate,
and, supposing them to exist, the only question is,
whether it requires any teaching or practice, and if
so, how much, to awaken them.

Birds have, compared with man, very few and very
limited powers, and they differ from us, besides, in this,
that it requires comparatively very little stimulus to
bring their faculties into full working order. A few
suggestions from an older bird on a particular subject,
and a younger one at once advances the greater part
of the way towards the furthest point to which his

xiii INSTINCT AND REASON 337

tether will allow him to go. Hut within his narrow
limits he still continues to gain by experience. It is
generally said that one bird builds a nest just as
another of the same species does, hence intelligence
cannot come into play at all. It is probable, though,
that there are differences which escape our notice ; at
an)- rate, as I have shown, individuals are capable of
adapting themselves to new circumstances, and some
authorities hold that a bird's first nest is decidedly
inferior to her later ones.

We conclude, then, that nest building is instinctive
but that intelligence to some extent works up-
on and modifies the instinct. It is no argument
against this that birds in captivity often build a very
poor nest or are incapable of building one at all.
Among domesticated animals instincts are apt to go
wrong. There is a breed of hens that never sit upon
their eggs. Among the lowest class in our big towns
unnatural conditions of life not unfrequently lead to
the decay of an instinct on which the continuance of
the race depends, the affection of mother for child, an
instinct which is never deficient in savage races.

The Cuckoo Instinct.

The habits of the Cuckoo are so marvellous that if
we were to come fresh to the subject, we should be lost
in astonishment at them. But, as Lucretius says,
even the sun ceases in time to be an object of wonder.
The Cuckoo lays many eggs, and we can hardly be
wrong in seeing a connection between this fact and
the parasitic habit. They are laid, some ornithologists

z

338 THE STRUCTURE AND LIFE OF BIRDS chap.

believe, at intervals of several days, so that if she were
herself to undertake the incubation, she would have
to leave one for some time unsat upon, or else have
eggs and young in the nest together at the same time.
A German naturalist, Karl Eimer, gives a rather
different account ; the Cuckoo lays two eggs in a clutch ;
that is, if she made a nest herself, she would lay only
two eggs in it. And as she generally migrates
before August, she would not, if she herself nested,
get many young ones reared in the course of the
summer. In favour of the former view it may be
urged that the American Cuckoo, who almost always
builds a nest for herself, does have young birds and
eggs in process of hatching in the nest at the same
time. In any case we must look to the bird's great
laying powers for the explanation of the cuckoo
instinct. The egg of the Cuckoo is wonderfully small
considering the size of the bird. It is less than an
inch long, and J inch broad. A Hedge Sparrow's egg
is about i inch long and a little more than J inch
broad. Thus there is no very great difference in bulk.
But the Cuckoo is 12 inches long and the Hedge
Sparrow only 5 J, a monstrous disparity even when wc
allow for the length of the Cuckoo's tail. The dimin-
utive size of the interloper's egg no doubt deceives
the toster mother, and is necessary if it is to hatch as
early as those of the rightful owner. Moreover, if it
were not so small, how would the bird after laying it
be able to take it in her beak and deposit it in the
nest where it is to be left ? The egg sometimes varies,
approaching in colour those in the particular nest
chosen, so that it is bluish when laid in a Hedge

xin INSTINCT AND REASON

339

Sparrow's nest. Many suggestions have been made
to account for this fact, the best, perhaps, being that
there arc varieties of Cuckoo, each of which has its
favourite nest for laying in. When hatched, the young
Cuckoo has not a vestige of clown, and is perfectly
blind. His back from the shoulder blades downward
is very broad, and has a depression across the middle
which fills up after the twelfth day of life. This
remarkable form of back is very useful to the still
blind young bird. Using it as a shovel, he ousts the
other fledglings or an unhatched egg from the nest
sometimes before he has completed his second day,
when his victims may be picked up round the nest.
When once you have seen this blind young demon with
his shovel-like back to help him in his murderous
career, you can never forget him. The foster-mother
devotes all her energy to the murderer of her young.
Only one egg (rarely two) is laid in each nest, so
that the young bird may get plenty of food.

The instinct of the Cuckoo and all the accompany-
ing modifications have been brought to perfection — the
diminutive size of the egg, only one egg (or at most
two) in each nest, and laid, moreover, before incubation
has begun, the occasional approximation of the egg in
point of colour to that of the foster-mother chosen,
the hollow back, and the self-asserting disposition of
the young bird. It is only in the system of migration
that we find imperfection. The old birds, most of
them, leave by August ; .the young ones sometimes
remain as late as October, and have to find their way
to Africa, even to South Africa, alone.

The cuckoo or parasitic habit is not limited to one

z 2

34o THE STRUCTURE AND LIFE OF BIRDS chap.

order of birds ; it is found in various stages of develop-
ment in the different species of the American Molothrus,
a bird allied to our Starlings. One South American
species {M. bonariensis) always lay their eggs in other
birds' nests, and never sit upon them themselves, but
the number of eggs laid in one nest is so great that it
is impossible all can be hatched. They sometimes lay
in old forsaken nests, or in a nest of the year where in-
cubation has already begun, or before the building is
finished, so that their eggs are covered by the thick
lining and never hatch. Many are dropped upon the
ground. The parents, too, will often peck holes in
numbers of their own eggs. Sometimes several
together set to work to build a nest for themselves,
but it is clumsily constructed, and, as far as is known,
is never made use of. Another species {Molothrus
rufoaxillaris) is also parasitic and apparently not so
foolish as the last mentioned, though not so accom-
plished a parasite as the Cuckoo. Another South
American species {Molothrus badius) is probably never
parasitic. But they sometimes go to the length of
seizing another bird's nest and building their own
upon the top of it. All these interesting facts we
owe to Mr. Hudson, who has carefully observed the
two South American birds that have the cuckoo
habit.

In North America there is a Molothrus which never
lays more than one egg in one nest.

In this genus, then, we see the instinct in its various
stages of development. Molothrus badius is a pirate
and not a parasite. M. bonariensis is foolishly prodi-
gal of its eggs. . M. rufoaxillaris shows a greater

Xiii INSTINCT AND REASON 341

spirit of economy. The North American species lays
only one egg in one nest. Our Cuckoo in the perfec-
tion of the adaptation of its structure and habits seems
to surpass them all.

Piracy.

The White-headed Eagle watches the Osprey, and,
when the latter has secured a fish, pursues and
threatens him till he drops his prey, which, making a
swoop downward, he catches as it falls. The Robber
Tern lives wholly by the plunder of other birds. The
British Avifauna boasts four pirates, two that breed
here, besides two that visit us. All these are Skua
Gulls, and by far the commonest is the Arctic or
Richardson's Skua, intermediate in size between a
Kittiwake and a Herring Gull. The Great Skua,
which breeds in two islands of the Shetland group
and nowhere else in the British Isles, is a much larger
bird. When a Gull or a Tern has secured a fish, the
Arctic Skua will pursue him with a velocity that makes
escape impossible. When he has overtaken the
fugitive, he flies over and under him with a menacing
air. It is evident that he will brook no refusal, and his
victim drops the fish or allows it to be taken from his
beak, sometimes crying plaintively the while. Whether
the Skua ever finds it necessary to resort to actual
violence, I do not know. As far as I have been able
to see, threats arc sufficient, but the whole scene
passes with such rapidity that it is difficult to make
out the details of the action. It is certainly probable
that he uses his beak with effect, since he, is known to

342 THE STRUCTURE AND LIFE OF BIRDS chap.

prey on wounded birds. He will condescend, too, to
pick up worms and molluscs, but I do not know that
he ever catches fish for himself. The Great Skua I
have never seen upon the warpath. Besides robbing
Gulls of their fish, he is known to attack and eat
Kittiwakes and even Gulls of larger size.

When it is migrating southward, the Arctic Skua
may often be seen upon our coasts. But it is worth
while going to Shetland to see both kinds in their
breeding haunts. A great part of the day is devoted
to gyrations in the air, the smaller bird often accom-
panying its movements with its peculiarly rasping
twangy note, the greater one croaking less harshly.
The Great Skua is a bird of majestic flight, ascending
high, when there is an upward wind off the cliffs,
in easy spirals without a motion of the wings.
Though both birds have a grim look and though they
live by plundering the weak, a gull who has not just
caught a fish shows no fear, at any rate of the Arctic
Skua. There is no panic when he appears, as there is
among small birds in the presence of a Hawk.

Nesting Habits of the Rhea.

The nesting habits of this bird are so remarkable
that it is difficult to pass them over. The hen lays a
great number of eggs, so that if she were to leave
them till she herself could sit.upon them, many would
become addled. Several, therefore, lay in one nest,
and when a good many have accumulated, a cock bird
comes and undertakes the incubation, and not only
that, but cafes for the young when they are hatched.

xin INSTINCT AND REASON 343

But the system has not been brought to perfection, for
a number of eggs are dropped anywhere about the
country. It is believed that Ostriches, too, make a
nest that is common to several hen birds. 1 Certainly
the cock bird sits on the eggs and tends the young,
and this is also true of the Emeu and the Cassowary.
The New Zealand Apteryx, however, lays only one egg
and sits upon it herself. 2

The Death-feigning Inst i net.

The death-feigning or wound-feigning instinct is
very well developed in some birds. The Canadian
Ruffed Grouse rises with a loud whirr, then tumbles in
front of the pursuing dog, who never thinks of the
young and goes after the mother whom he imagines
wounded. If the Willow Ptarmigan be approached
she crouches to the ground among her brood, and if
she sees that she cannot escape notice, she rolls and
tumbles along as though mortally injured. 3 The
Spotted Tinnamou, or common Partridge of the
Pampas, when captured, after a few violent struggles
to escape, drops his head, gasps two or three times, and
to all appearance dies. 4 The Corncrake is very good
at the art. He has sometimes been put in a sports-
man's pocket, apparently quite dead, and when his

1 See Darwin's Journal of Researches, chap. v.

2 See a paper by Mr. P. L. Sclater, F.R.S., in the Proceedings
of the Zoological Society, June 9th, 1863.

3 See an article by Air. John Worth in the Nineteenth Century,
April 1893.

4 See Hudson's Naturalist in La Plata, p. 204.

344 THE STRUCTURE AND LIFE OF BIRDS chap.

chance has come, has run away and escaped. 1 Mr.
Hudson in his Birds in a Village tells of a Reed
Bunting, which, in alarm for the safety of its young in
the nest, flew out on his approach, " but only to drop
to the ground, to beat the turf with its wings, then to
lie gasping for breath, then to flutter on a little further,
until at last it rose up and flew to a bush." A good
naturalist has just been describing to me very similar
behaviour on the part of a Whitethroat. The Opossum
and the Fox excel in the art of "shamming dead."
Among beetles and spiders the instinct is more com-
monly found than among mammals or birds.

We must not put this behaviour down entirely to
good acting. The animal is actually afraid, often
even paralysed by fear. In time it recovers itself, and
seizes any opportunity of escape that offers. But the
natural stunning effects of fear have been turned to
account, and the temporary paralysis caused originally
by a violent shock to the nerves has by long ages of
natural selection been developed and improved so that
now we may look upon it as due to a valuable pro-
tective instinct, though helped in most, if not in all,
cases, by actual alarm. It is very remarkable that
this instinct should be found in creatures so remotely
connected as Spiders, Beetles, Birds, and Mammals, and
among birds in species belonging to widely separated
families, e.g. in the Reed Bunting and the Canadian
Ruffed Grouse.

1 See Romanes' Mental Evolution in Animals, p. 305.

INSTINCT AND REASON 345

Origin of Instincts.

It is natural to think of instincts as habits that
have been handed down from generation to generation
till at last they have become petrified. It is impos-
sible, in spite of the dearth of direct evidence, to deny
that acquired habits may be transmitted, but it is
not difficult to show that instincts sometimes have
a quite different origin. In a beehive it is the worker
bees alone that make the hexagonal cells, shaping them
with almost mathematical exactness, and fitting them
together in a way that involves the least possible
expenditure of wax. These workers arc undeveloped
females and leave no descendants, the eggs from
which the young bees are born being all laid by the
queen bee, whose sole duty is to lay eggs and who
never helps in the work of cell-building. Any habit,
then, that is formed by the workers cannot possibly be
handed down to the next generation. We must,
therefore, look elsewhere for the origin of the instincts
o( the hive bee. The explanation which Darwin
gave was the very simple one that communities of
bees which had these three classes, the drones or males,
the queens, and the neuter females or workers, throve
greatly and multiplied rapidly, whereas in hives in
which all the females were both egg-layers and
workers, the population gradually dwindled, so that at
last the race became extinct. This idea might seem
far-fetched had not gardeners produced a similar
result with stocks. These flowers are generally
duublc and produce no seed, but among them there is

346 THE STRUCTURE AND LIFE OF BIRDS CH. XIII

occasionally a single-flowered plant. The seed from
this produces plants, most of which bear double, but
a few of them single flowers. The barren double
flowers correspond to the neuter bees or workers, the
single ones to the queen.

Some of the Literature of the Subject.

The chapter on " Instinct 1 ' in Darwin's Origin of Species.

Romanes' Mental Evolution in Animals and Animal Intel-
ligence.

Wallace's Contributions to the Theory of Natural Selection.

Sclater and Hudson's Argentine Ornithology (on " Parasitic
Birds.")

Various books and papers to which references have been given
in the course of this chapter ; see footnotes.

CHAPTER XIV

MIGRATION

For ages past the mysterious going and coming of
birds has