THE RIDDLE OF
MIGRATION

RIDDLE OF

MIGRATION

BY

WILLIAM ROWAN

Department of Zoology^ University of Alberta

BALTIMORE

THE WILLIAMS & WILKINS COMPANY

1931

Copyright, 1931
THE WILLIAMS & WILKINS COMPANY

Made in the United States of America
Published October, 1931

Composed and Printed at the
WAVERLY PRESS, INC.

FOR

The Williams & Wilkins Company
Baltimore, Md., U. S. A.

To

My Mother

GERDINE ATALIA ROWAN

CONTENTS
Introduction xi

Prologue 1

CHAPTER I
The Living Bird 5

CHAPTER II
Environment, Past and Present^ 45

CHAPTER III
The Evolution of Migrations 90

CHAPTER IV
Annual Migrations HI

CHAPTER V
Summary 140

Epilogue 145

Index 147

4149S

LIST OF ILLUSTRATIONS

The Reflex Arc 15

Brain of a Cat and of a Crow 20

Sections through Cerebral Hemispheres of Cat and Crow. . 22

Attempts at Building of a Pair of Dippers 25

Crustacean "Ear" and Internal Ear of Crow 30

Diagram of Human Ear 31

A Part of Section of Testis and Ovary of Junco 39

World-Map with Southern Hemisphere Inverted and Im-
posed on Northern 52

Geological Table 66

Map of Northern Hemisphere 70

Compulsory Exercise Cage 124

/

INTRODUCTION

Biological problems can be approached from a
number of angles. The specialist views them
through the glasses that he habitually wears, with
his special interests magnified and in the foreground,
a profitable proceeding but likely to suffer from dis-
torted perspective. The gathering up of the view-
points of the various specialists is a certain way
of restoring proportions and arriving at a compre-
hensive and at the same time an analytical picture
of the whole.

The migrations of birds have been a subject of
interest for centuries. They have been examined
from two avenues of approach. One of them has
been worn wide and smooth: the other remains
almost untrodden. Field observations and specula-
tions based thereon exist in sufficient volume to fill
a library, but few and far between are the attempts
of the technically trained biologist — the anatomist,
biochemist, biophysicist, physiologist, etc. — to
apply his special knowledge to the problem.

It requires little imagination to see that both are
crucial aspects of the subject. An illustration will
make this clearer. Every bird student is familiar

xii INTRODUCTION

with the existence of a preen-gland just above the
root of the tail in most species. It, Hke migration
itself, has received a great deal of attention from the
field worker, and speculations as to its possible uses
are frequent. One author maintains that it oils and
waterproofs the plumage: another denies it. Yet
another, by the use of blotting- and tissue-paper,
attempts to prove or disprove the presence of oil
and so on. The accumulated observations as to
how various species make use of the gland are of
interest and value, but the suggestions as to its
functions are merely speculations.

The preen-gland has also received attention from
an entirely different source. The papers published
have never appeared in ornithological journals. In
all probability they have not even been reviewed
there, leaving the field man quite unaware of their
existence. They are the product of the laboratory
and embody the findings of the trained biochemist.
The test applied for fats is not blotting-paper but
the most precise chemical analysis. Moreover the
biochemist, not content with the more or less casual
nature of simple observation, systematically extir-
pates the gland from birds kept under controlled
conditions of food and lighting, records by means
of scientific methods the precise effects on plumage,
nutrition and vitality, compares his analyses with

INTRODUCTION XllI

those of other analogous animal products and finally
determines the possible functions of the preen-gland
and its secretions on a firm basis.

Here, then, is tangible, concrete information on
the biochemistry of the preen-gland. Anatomists
have provided us with knowledge of its structure
and morphology. Combining these sources of in-
formation with field observations and considering
them together, we can arrive at an understanding
of its fundamental nature. The cooperation of the
laboratory specialist is essential.

Field observations only can acquaint us with the
facts of bird migration, but their interpretation can-
not be undertaken without the use of information
accumulated by the trained biologist, facts of struc-
ture and of function, of biochemistry, physiology,
biophysics and so on. For migration is an example
of animal behaviour, and a very fascinating one of
extreme complexity.

It is the purpose of this little volume to examine
the subject from this viewpoint — through the eyes
of the biologist. It deals particularly with the pos-
sible mechanism of migration, or rather with certain
aspects of it, those in which the author has taken a
somewhat particular interest. If it convinces the
ornithologist that the laboratory and microscope
may prove real adjuncts to a solution of the prob-

xiv INTRODUCTION

lems of migration and if it encourages him to con-
tinue more enthusiastically than ever in the collec-
tion of facts and in analysis of the field aspects, it
will have achieved its object. It is the writer's
belief that not only can biology assist ornithology
to elucidate the problems of bird migration, but
that ornithology can assist biology in its attempt to
discover something more of the true nature of
animal behavior.

The various field aspects of migration are referred
to only as necessity arises. Full summaries of what
is known today of the subject are to be found in
several excellent works, notably Wetmore's very
readable little book The Migrations of Birds (Har-
vard University Press, 1927. Price $2.50) or
Landsborough Thomson's more detailed volume
Problems of Bird Migration (Witherby, 1926.
Price 18/-).

THE RIDDLE OF MIGRATION

PROLOGUE

The sun has set. Rifts of gold and crimson in the
western sky glow through a heavy pall of cloud
that shrouds the moorlands in a sullen canopy of
pending storm. Around us the listless silence is fit-
fully punctuated, now by wail of Curlew calling to
its partner, now by plaintive mew of incubating
Plover or the staccato keking of the mated Merlin.
Half a mile away, from the reservoir at the base of
the fell on which we are seated, rises the murmur
of a gaggle of pink-footed Geese. They are feeding.
The wildness and isolation of the rugged moors, the
crags and wind-swept fells, the munificence of this
bountiful wilderness, will surely persuade them that
here they too, with Curlew and Peewit, Sandpiper
and Wild Duck, Grouse and Raven, can find a
paradise for the rearing of their young. Surely they
will stay. They must stay. But as the darkness
deepens, the murmur increases to an excited cack-
ling, the flock shifts uneasily back and forth and in
a minute we hear the pounding of pinions on the
glassy surface of the lake. We see them rising, up
and up, at first a straggling line grotesquely mir-
rored on the rippled water beneath ; but before they

1

2 PROLOGUE

fade from sight, an orderly V heading into the
north, into darkness, into storm. For Yorkshire is
not for them. For them it is the bleak face of Spitz-
bergen with its ice-bound lagoons, its snow-covered
cliffs and precipices, its ocean crossing, its hazards,
its hidden perils. No aviator's instruments are
theirs; no navigator's compass can help them. But
what of that? Though they fly by night, though
they encounter deterring storm or tempting moor-
land or leagues of ice-flecked ocean, only a few
more days and they, too, will be breeding, but a thou-
sand miles away. They will arrive upon schedule
and according to ordinance, as thousands of their
generations have done before them.

The structure of the atom, the birth of a child,
relativity, the tunnelling of a mountain, the broad-
casting of messages through the ether from conti-
nent to continent, such things we understand. Or
we think we do.

But the annual, the precise, the infallible return
of migrating wild fowl?

It is autumn in Alberta. A tang is in the sunlit
air. The poplars and birches are brilliant reds and
oranges and yellows. The placid lake before us is a
deep, sparkling blue. Even the broad mudflats
appear to be shimmering with color. And certain it

PROLOGUE 3

is that they are teeming with life that belies their
apparent barrenness. Here a small group of Long-
billed Dowitchers from Alaska is rubbing shoulders
with a fussy flock of Stilt Sandpipers from the Barren
Lands. There a flight of two hundred Golden
Plovers is indulging in spectacular manoeuvers over
golden sheaves of grain in company with Black-
bellied Plovers and Knots from the very purlieus of
the North Pole itself. Further out, preening, bick-
ering and swirling, a flock of a thousand Phalaropes
in massed formation. Everywhere there are shore-
birds. It appears to be the bird Mecca of the West,
a rabble from all corners of the North. As we scan
them, an extraordinary fact strikes us. Every
wader we can see is a bird of the year. Not one has
travelled before. This is their virgin migration.
Yet here they are, southward-bound in their hordes,
making no mistakes. Inexperienced, untutored,
mere infants, here today in a stupendous throng,
seemingly inextricably mixed; gone tomorrow, to
regions where the trials of a northern winter, from
which they are fleeing without knowing it, will not
concern them. But they are mixed no longer.
One species to the Argentine, another to the Falk-
land Islands, others to the Indies, yet others to
California or Florida or Peru or Patagonia. They
are travelling into the unknown without guidance,

4 PROLOGUE

without previous experience, without knowledge of
Hfe, ultimately to winter on predestined grounds of
the very existence of which they are completely
ignorant.

Few achievements in the animal kingdom parallel
this, the most striking aspect of bird migration. As
we turn the pages of our book, we shall see some-
thing of the many difficulties that confront us when
we attempt to explain it in terms of modern sci-
entific knowledge. But the attempt is worth while
for it takes us far beyond the world of birds and
their migrations. It brings us face to face with the
fundamental problems of Life. A solution of but a
single phase may unexpectedly widen the present
boundaries of biological knowledge.

CHAPTER I

The Living Bird

Before we can speculate on the possible capabili-
ties of a bird, mental or physical, it is essential that
we acquaint ourselves with the general principles of
the avian constitution. It is useless, for instance,
to bestow on a bird human powers of thought if the
structure of its brain obviously precludes any such
possibility. Whether we wish to credit it with
powers of long-sustained flight or with such a simple
thing as the appreciation of color, we are not justi-
fied in so doing merely on the strength of personal
opinion. The least we can do, before committing
ourselves, is to study the elementary facts of the
anatomy and physiology of a bird and ascertain if
our assumptions are justified. If on these grounds
they are not, there is no choice but to modify our
hypothesis to suit the facts. We may therefore
profitably start our consideration of migrations by
making a brief survey of bird structure and function.

Quite the most striking thing about a bird to the
comparative anatomist is the large number of fea-
tures that impress him as being essentially reptilian.
This is far removed from what we would naturally

5

6 THE RIDDLE OF MIGRATION

expect, for no living creature offers so great a con-
trast to the cold, sluggish reptile of popular concep-
tion as the excessively active, hot-blooded bird.
Yet when we warm up a reptile by heating its sur-
roundings we frequently get an unexpected display
of activity. Many tropical reptiles, particularly
lizards are, as a matter of fact, among the earth's
speediest small animals. But a drop in temperature
distinctly crimps their style. Birds supply their
own heat (except in their extreme youth) and
remain constantly active. Their body tempera-
ture averages even higher than that of mammals
and their rate of metabolism is greater. It is the
source of their extraordinary activity and char-
acteristically restless nature and may, quite con-
ceivably, coupled with their power of flight and
potential disregard for barriers, be one of the chief
factors in inducing migrations. It is certainly one
of the important items that make migration a
possibility.

Anyone who has kept birds in aviaries is ac-
quainted with their superabundance of energy.
Except when feeding, or during the breeding season,
they appear never to sit still but to fly incessantly
back and forth from one end of the aviary to the
other. Their daily mileage, although confined to
the limits of a cage, probably equals that of a normal

THE LIVING BIRD 7

day's migration. Many wild birds appear to fly
merely for the sake of flying. Thus male white-
winged scoters {Oidemia deglandi) in Alberta, on
their favorite breeding lakes such as La Nonne,
Wabamun, St. Anne, etc., from the middle of June
to the end of July take a flight of two to two and a
half hours every evening. Their cruising speed is
approximately forty miles an hour. For five or six
weeks they thus do a daily dozen — mostly in circles
round the lake — of some eighty to one hundred
miles, or a total in six weeks of about four thousand
miles. The same amount of flight directed in a
straight line south would place them at the equator,
far beyond their normal wintering limits in Florida
and lower California. Yet this expenditure of
energy takes them nowhere. They finish where
they began. It is but a pastime, a congenial
method of killing a summer evening.

The mere production of heat, however, is not
sufficient to account for the results. When we talk
about a ''warm-blooded" bird or mammal, we are
using the careless terminology of everyday lan-
guage. What we actually imply is even-tempera-
tured, a very different matter for it means not only
the production but the conservation of heat. Even
reptiles produce heat, but only some (e.g. the fe-
male python when incubating eggs) can con-

8 THE RIDDLE OF MIGRATION

serve it. Their temperature vacillates with that
of their surroundings. They are slaves to environ-
ment. Even the conservation of heat is not all-
sufficient, for when an animal exerts itself it gener-
ates heat at a greatly increased rate. Unless such
excess is eliminated as speedily as it is produced,
super-heating, fever and death result. Birds pos-
sess a non-conducting layer of feathers that ac-
counts for the conservation; they cannot curb the
output at times of extreme activity but they rid
themselves of the overflow through their lungs and
air sacs about as fast as it is produced. They en-
tirely lack the common mammalian safe-guard of
sweat glands. The essential point is that they suc-
cessfully maintain a relatively high temperature
(100° to 112°F.) under all conditions as long as the
food supply remains adequate. Most mammals, at
such temperatures, would be suffering a dangerous
fever. Unlike reptiles, birds thus become inde-
pendent of what would otherwise be an insurmount-
able bar to distribution. As things are it makes
little material difference to them if the surrounding
temperature is 150°F. in the tropical sun or -60°F.
(below zero) during the arctic winter. Not all birds
can tolerate this extreme range, the parrots and
their relatives, for instance, being able to resist but
comparatively slight degrees of cold. But to this

THE LIVING BIRD 9

point we shall have occasion to return in more detail
later. Obviously, this stability of body tempera-
ture leaves birds free to move whither they will,
other things being equal. It removes a specific
restriction and gives them that unlimited freedom
they could not otherwise enjoy.

With temperature limitations virtually removed,
birds have been able to utilize to the full their chief
anatomical peculiarity, exceptionally perfect organs
of flight. Flight is not confined to birds and inci-
dentally not all birds can fly, but, apart from
insects, no other group of animals can boast mastery
of the air on a wholesale scale. Among other
vertebrates, bats only can truly fly, although certain
members of all other groups, fish, amphibia, reptiles
and mammals have learned to glide. Flying rep-
tiles, extinct long since have, however, existed in
the past. The earliest birds were little more than
this themselves, but they no doubt already had the
advantage, to some extent at least, of controlled
temperature.

It is hardly necessary to go into details of wing
structure. Sufiice it merely to point out that while
in bats the fingers of the hand are enormously
elongaged with a patagium spread over them which
reaches back to the tail, in birds the fingers are
reduced in number as well as size, and numerous

10 THE RIDDLE OF MIGRATION

Individual feathers borne by them (remiges) replace
the patagium of the bat. The entire arrangement
is not only more effective but more efficient. The
feathers of the tail (rectrices) give the bird yet a
further advantage. Both bats and birds are thus
warm-blooded creatures capable of sustained flight.
Birds are undoubtedly the more successful but the
two share many advantages in common. Bats, for
instance, even as birds, are found on oceanic islands
and in territory from which all other higher mam-
mals have been excluded simply because barriers,
otherwise effective — expanses of water, deserts,
mountains, etc. — mean little more than a risk to
animals that can traverse the air. But the spec-
tacular migrations characteristic of birds are un-
known to bats which are, without doubt, the weak
brothers of the vertebrate air force. Some species,
moreover, like some other mammals, overcome the
difficulties of winter by hibernation.

In addition to the possession of wonderfully per-
fect wings, birds are yet further peculiarly adapted
to life in the air. There are no doubt limits, both
mechanical and hereditary, to size of wing and if
really long journeys are to be undertaken a maxi-
mum reduction of body weight would obviously be
a direct advantage. Such reduction has actually
taken place and in various ways. The shafts of the

THE LIVING BIRD 11

feathers themselves, made of material that is itself
quite light — keratin — are built (like the famous mast
of the Enterprise) on the hollow cylinder principle,
combining lightness with strength. The bones
show the same plan and are individually lighter
than corresponding bones of equal size in either
reptile or mammal. Not only are they hollow but
air passages, directly connected with the lungs, pass
into many of them. The curious tendency for
various bones to fuse with each other, a character-
istically avian trait, goes to increased rigidity, as
important as lightness and strength. But a bird's
respiratory mechanism is perhaps its most wonder-
ful adaptation to life in the air. Flying is strenuous
exercise — some birds, e.g. various swifts, may
achieve a speed of more than 200 miles an hour — the
tissues demanding a constant and rapid supply of
oxygen via the blood stream. The rather solid and
heavy appearance of birds' lungs does not look
promising when one opens the body, but micro-
scopic examination reveals the structure as a vast
series of ramifying tubules, all intermingling with
one another and together producing an enormous
area of respiratory surface. Connected with the
lungs is a series of air passages and sacs traversing
most parts of the body and as has just been ex-
plained, extending actually into many of the larger

12 THE RIDDLE OF MIGRATION

bones. These do not add materially to the bird's
buoyancy, but they bring about an inrush of air
through the lungs and then out again, providing the
most perfect respiratory mechanism known in the
animal kingdom. Added to all this is the corre-
lated construction of the ribs, breast-bone and bones
of the pectoral girdle which together provide a con-
trivance that may be said to pump air in and out of
the lungs automatically with each wing-beat.

From these comments, to which much might be
added, it will be apparent that the actual perform-
ance of continuous flight on the part of a bird is in
the nature of a pastime rather than a labor. From
the viewpoint of adaptive construction the typical
bird is undoubtedly perfectly equipped for life in the
air, more so than any of its fellow creatures. But
that, of course, is not all that is required. A
migrating bird, not only in the actual performance
of migration, but on the thousand and one occa-
sions of incidental events, exhibits reactions to its
environment that we know must depend on the pos-
session of a brain and nervous system, and before we
proceed further it is necessary that we take a
cursory survey of the possibilities and limitations of
the avian nervous system and related sense organs.

The nervous system is made up of several com-
ponent parts. The dominating aggregation of

THE LIVING BIRD 13

nerve cells is termed the brain. Its substance is
continued backward to the root of the tail as the
spinal cord inside a protective canal perforating the
chain of vertebrae which form the axis of the bony
skeleton. The 7ierves that supply all organs and
parts of the body — the peripheral nervous system —
take origin either from the brain or the cord, twelve
springing from the brain as in reptiles and mam-
mals. Distribution of all but the last three of
these is confined to the head whose muscles and
sense organs they supply, as well as the teeth in
reptiles and mammals. The spinal nerves supply
the skin and muscles of the trunk and limbs. In
addition to these elements there exists what is
termed the sympathetic or involuntary nervous
system, innervating the viscera. The sense organs,
since they keep the central nervous system in touch
with the outside world, the animal's environment,
are obviously as important as the brain itself and
we shall have to give them particular attention.

Nerve fibres are of two kinds, sensory and motor.
The former carry impulses from the outside in, i.e.
from the external centres of perception to the
internal central nervous system; the latter send
impulses outwards from within, inducing responses
in the musculature which bring about appropriate
movements in reply to the sensory stimuli pre-

14 THE RIDDLE OF MIGRATION

viously received. Any given nerve is made up of a
very large number of fibres and every fibre, be it
short or long, possesses a nucleus and cell-body, of
which the fibre is merely a process. Fibre and cell-
body together constitute the complete nerve-cell
which is termed a neurone. An individual fibre can
transmit an impulse in but one direction, never in
two. It thus comes about that nerves containing
only fibres of one kind are either pure sensory or
pure motor. If they contain both, they are mixed
nerves and, as such, can transmit impulses either
in or out. All the spinal nerves, but only some of
the cranials, are mixed.

In its simplest form a nervous response is as fol-
lows. A sense organ somewhere on the body receives
a stimulus. The sensory fibres supplying it convey
the message inward to the central nervous system.
Here minute processes on the tip of the sensory fibre
arborize and come into contact with similar fibrils
of a motor fibre or fibres. (Fig. 1, A.) The impulse
is thus transferred to the motor fibres and so re-
transmitted outward to a muscle (or gland) which
makes a suitable response. We can illustrate this
by a simple experiment. If a frog is killed by having
its brain destroyed, and a drop of irritating fluid is
then placed somewhere on its body, one of the hind
legs will go through the appropriate action of kick-

THE LIVING BIRD 15

ing off the offending droplet. The simple reflex arc
involved obviously has not required the intermedia-
tion of the brain which was already destroyed. But
in spite of this, another aspect of the experiment is
bound to strike us. The response was actually a
very complicated rather than a simple thing. The

Fig. 1. The Reflex Arc

A: in its simplest form; B: with association neurones inter-
posed. Af, afferent (sensory) neurone; As, association neurone;
Ef, efferent (motor) neurone; Gl, gland; M, muscle fibre; Sk,
tactile corpuscle in skin.

entire leg with most of its muscles, as well as various
muscles of the trunk and other limbs (balance ad-
justments) participated. The solution is readily
explained. The simple arc referred to above illus-
trates the principle rather than the actual fact for
interposed between the termination of the sensory
fibre and the commencement of the motor, there

16 THE RIDDLE OF MIGRATION

nearly always lies an intermediate link termed the
association neurone and there may be many of them
rather than one. They greatly multiply and com-
plicate the connections in the cord. The local
stimulus, such as our drop of acid, may thus call
into action motor neurones up and down the length
of the spinal cord. (Fig. 1, B.)

Where then, it may well be asked, does the brain
come in? An example will make the answer plain.
When we sneeze we are exhibiting a simple reflex
response to nasal irritation. Under normal circum-
stances we heartily indulge ourselves. But should
we be seized with a desire to sneeze in the middle
of a prayer at church, we suppress the impulse and
refrain from sneezing. The brain, or rather a cer-
tain section of it, overrules a perfectly natural reflex
action. Possession of it enables us to exercise judg-
ment and control. This implies an understanding
of the circumstances and reasoned, intentional ac-
tion to suit it. This is really what the term intelli-
gent behaviour means. The greater the develop-
ment of this part of the brain, the higher is the
standard of intelligence that can be expected from
the individual. It has unquestionably attained the
greatest degree of perfection in man and reaches a
remarkably high level in some other mammals,
notably in the higher apes. Our next aim must

THE LIVING BIRD 17

therefore be to examine briefly the vertebrate brain
and to see to what extent we are justified, on ana-
tomical grounds, in crediting a bird with inteUigence.
Just as the spinal cord is a centre for reflex ac-
tions, so is that part of the brain (originally the
anterior end of the spinal cord) known as the brain-
stem. It represents the oldest part of the brain in a
phylogenetic sense and in the lowest vertebrates
there exists but little else. In the more highly dif-
ferentiated of the lower vertebrate brains, say that
of a fish, various regions can be recognized in the
brain-stem, each of which is particularly concerned
with one of the senses of touch, taste, smell, sight or
hearing, respectively. In these centres, as in the
spinal cord proper, reflex arcs are organized and
completed. The response to an auditory stimulus
comes in the main from the auditory centre, a visual
stimulus elicits a response largely from the optic
centre and so on. They are not entirely independ-
ent, however, for they are connected by association
neurones but their connections are relatively scanty
fibre-tracts which do not lead to elaborate coor-
dination. Hence we find that fish, generally speak-
ing, are capable of only simple and rather stereo-
typed responses to external stimuli and they are,
naturally enough automatic, not intelligent, per-
formances. These connections can be improved by

18 THE RIDDLE OF MIGRATION

multiplication and the more numerous they become,
the more intimately is one sense put in touch with
another and the more elaborate become the
responses.

But another and better mode of association exists
in what is termed a correlation centre, best repre-
sented in the brain of fishes (and all higher verte-
brates) in that part of the brain known as the
cerebellum. This is really the unit that regulates
bodily activity. It is not the seat of any individual
sense but receives messages from all the senses.
They are here coordinated. In the fish's brain we
find exactly what we should expect, a comparatively
large cerebellum, for the fish is a relatively active
animal. The same is true of birds. In the sluggish
reptiles it is small.

The most important part of the primitive brain
was the paired olfactory lobe constituting most of
the anterior end. It is very large in fish but tends
to become subordinated in the higher vertebrates.
One reason for this is that other brain tracts keep
encroaching on its space so that we slowly get it
replaced to varying extents by other units. There
has also been a tendency to build over it, i.e. non-
olfactory tracts have not only invaded the precinct
of the olfactory section and actually reduced it, but
they have been superimposing new material simul-

THE LIVING BIRD 19

taneously and so enlarging the anterior end of the
brain. Ultimately, in the vertebrates above the
fishes, we get true cerebral hemispheres established,
which begin to overshadow the old olfactory sec-
tions. This arrangement reaches its climax in
mammals where the hemispheres are not only enor-
mous, but the top layer develops convolutions
which greatly increase the surface area. An exami-
nation of the hemisphere of a mammal gives us a
picture of a thick outer layer of nerve cells which
constitute the well-known grey matter of the brain,
or cortex. Here, as neurologists have demonstrated
in a thousand ways, is the centre of the higher
mental processes and, in man, the seat of the mind.
Here then in mammals we find the master organ of
the vertebrate brain. Except for the characteristic
arrangement of its cell-layers — there are many
millions of neurones in the cortex — and the fact
that they occur at the surface, in close juxtaposition
to the richly vascular pia mater, a most advantage-
ous site that also permits expansion, there is nothing
remarkable about the cortex, but there is something
remarkable in its relationship to the rest of the brain.
The various primitive connections of the fish's
brain (already described) still exist, but there is a
far larger number and the arrangement of them is
such that every other part of the brain and the

20

THE RIDDLE OF MIGRATION

entire spinal cord is now dominated by and under
direct control of the cortical centres. Moreover,
the mammalian brain possesses, in the corpus cal-
losum, a huge sheet of transversely running fibres

Fig. 2. Brain of Cat and of Crow

A: Brain of cat: dorsal aspect. B: brain of crow: dorsal
aspect. B^: same, ventral aspect. Cb, cerebellum; Hm, cerebral
hemisphere; Md, medulla; 01, olfactory lobe; Op, optic lobe.
(Scale of B and B^ slightly greater than A .)

that puts the two hemispheres into the most inti-
mate relationship with each other. Even the cere-
bellum, already an important correlation centre in
fishes, is now supplied with a new and additional

THE LIVING BIRD 21

switch-board, the pons varolii (pontal fibres exist in
birds, but they are relatively few), giving it added
importance but also transferring its previously
autocratic and automatic control to the more or less
voluntary hemispheres.

We are now in a position to understand the brain
of a bird. Once again, we are constantly reminded
of the reptile, not in its macroscopic appearance,
but in its constitution. The first thing that strikes
us when looking at the exposed brain (Fig. 2) is
the size of the cerebral hemispheres. We might
readily conclude that here we have a brain that
compares favorably with the mammalian, but the
illusion is dissipated when we examine its make-up.
In place of a thick cortex, the roof is thin and the
cortical cells are but meagrely represented, (they
are even scarcer than in some reptile brains) the
substance of the hemisphere consisting of an enor-
mously developed corpus striatum, a much older
part of the brain than the cortex. There is no
corpus callosum to unify the two halves : the pons
varolii is poorly represented: the pyramids are en-
tirely wanting. The corpora striata, large aggrega-
tions of nerve cells, differ not only in structure from
the cortex, but likewise in function. They are not
much more than enlarged, elaborated and somewhat
improved editions of the old primitive brain -stem

22

THE RIDDLE OF MIGRATION

centres — the seat, not of discriminating actions, but
of instinctive, reflex behavior. (Fig. 3).

The structure of a bird's brain thus leads us to
expect high development of instinctive behavior and
a limited intelligence. There is specific as well as

Fig. 3. Sections through Cerebral Hemispheres

yl , cat: B, crow. C. Ca, corpus callosiim; C. St, corpus striatum;
Co, cerebral cortex; Ft, fibres; La, lateral ventricle. (Scale of
B somewhat greater than A.)

individual variation but the general conclusion is
in keeping with observed facts for the key note of
avian behavior is undoubtedly instinct (see p. 87).
When a bird builds a nest it is not repeating some-
thing that it has learnt in the ordinary sense. It

THE LIVING BIRD 23

had not even been laid as an egg when the nest in
which it was reared was under construction nor did
it stop to study its architecture before struggling out
of it. Yet a year later, on its own initiative it pro-
duces a wonderful fascimile, correct to the minutest
detail. A canary reared in a machine-made nest of
felt will unhesitatingly construct its own from grass
and moss the year following if forced to do so. It
will even built it in a bush like a wild bird if no
frame is provided. This is instinctive behavior in
its most highly developed form. But even in-
stinctive behavior is subject to modification under
the guidance of experience and it is usual to find
old birds much more deft and individualistic in
nest building than the immature. Practice un-
doubtedly makes perfect even among birds. Yet
when real intelligence is called for, birds fail. Thus
the European jackdaw (Coloeus monedula), which
often builds in hollow trees, will occasionally find
a hole without a bottom. Instead of spending the
usual few hours dropping in sticks for the base of
the nest, jackdaws may persist for days without
discovering that they are attempting the impossible.
Dippers {Cinclus cinclus) occasionally select the
girders of small bridges in Yorkshire for their nests.
The repetition of convenient spaces side by side, is
generally more than they can comprehend and it is

24 THE RIDDLE OF MIGRATION

quite usual to see a number of nests alongside each
other as though each space had provided a separate
stimulus to the nest-building instinct. (Fig. 4).
The little south American bird, (Furnarius cuni-
cularius) which makes a horizontal nesting burrow
in the ground sometimes six feet in length, has been
observed to burrow into a mud wall, only to break
through at the other side before the burrow had
attained its requisite length. Yet repetition upon
repetition fails to teach it the limitations of the
wall or the futility of its perseverance.

So strong is instinctive behavior in birds that it
may actually lead to destruction . Many birds have
the habit of removing the faeces of the young from
the rim of the nest. As shown by Howard, if the
droppings are removed by human agency during
the absence of the parent birds, they will insist on
their return in removing something and if there is
nothing else it will actually be the material of the
nest. They might thus, by persistence, be induced
to destroy the very nest they have made for their
young while these are still in occupation.

It has been experimentally shown that birds have
very retentive memories. Pigeons that have been
taught to thread a maze, even though the teaching
be slow, will remember the key for a year or more.
Canaries switched from an aviary A to an aviary B

THE LIVING BIRD

25

26 THE RIDDLE OF MIGRATION

and returned a year or even two years later to A,
will unfailingly use the food and drinking appliances
and roosts in a way that leaves no doubt that they
remember details of their earlier home. The return
of wild birds to nest in the exact tree they used the
previous year, or to the same nesting box, indicates
a remarkable topographical memory.

The important sense organs are found on the
head. This is to be expected since this is the end
of the animal that comes first into touch with its
environment. They make direct connections with
the brain, the centres involved tending to become
more and more elaborated and intercorrelated as the
sense organs themselves improve. With its increase
in size and complexity adequate protection becomes
more imperative and hand in hand with the evolu-
tion of the brain goes the development of the skull, a
protective contrivance. In the lowest of the verte-
brates it remains cartilaginous throughout life but
in the higher forms cartilage is replaced by the
harder material, bone, the skull being cartilaginous
only during a transitory embryonic period. The
bird's skull is bony but it has had to adapt itself
not only to the requirements of a brain and sense
organs but also to the carrying of materials, nest
building, climbing, etc. In other words, in large
part it has had to provide a substitute for the fore-

THE LIVING BIRD 27

limbs, which have been entirely devoted to flight.
(In the Hoatzin the forelimbs are still used for
climbing, in ancestral fashion, during nestling life).

This combination of requirements occurs nowhere
else in the animal kingdom and in consequence the
bird has a skull that is peculiar and unique.

In its essentials the nasal organ of birds resembles
that of other vertebrates. The acute sense of smell
in mammals is a familiar fact. The relatively great
distance between the large, sensitive nostrils, placed
at the end of a ''snout," and the olfactory lobes of
the brain, may not be so familiar but it constitutes,
nevertheless, an important corollary for it is in this
intervening space that the large tract of sensory epi-
thelium, responsible for picking up the olfactory
stimuli, is situated. Reduction of the cavity inevi-
tably means reduction in smelling ability. The
olfactory mucous membrane in mammals (exclusive
of the whales and their immediate relatives) is very
extensive, the available surface in the anterior end
of the skull being enormously increased by the
development of thin sheets of scrolled bone, (turbi-
nals), covered with the nasal epithelium. Thou-
sands of special sense cells here receive olfactory
stimuli at each intake of breath, the sensations being
transmitted to the olfactory regions of the brain via
exclusive nerve fibres. In the case of birds, in

28 THE RIDDLE OF MIGRATION

striking contrast to that of mammals, the nostrils
are not fleshy and sensitive but openings in a horny
bill, pushed backward to the base, a place of second-
ary importance. The cavity behind the nostrils
boasts but few turbinal bones, entirely insignificant
in comparison with mammals, the whole arrange-
ment suggesting that the nasal organ in birds is now
largely decadent. Some birds (e.g. pelican, gannet)
actually have the nares entirely closed in the adult.
In the New Zealand kiwi alone, a flightless bird of
nocturnal habits, are the nostrils found at the tip of
the bill. Carefully devised experiments have dem-
onstrated a sense of smell — and a poor one at that —
in only a few species of birds. Even the vulture, so
widely accredited with a particularly acute olfactory
sense, is incapable of discovering meat in an ad-
vanced state of decomposition if it is wrapped in
paper, even though the bill may actually come in
contact with the parcel. On the right side of the
wind, on the other hand, a carnivorous mammal
would speedily find the packet at a great distance.
Taste is without doubt a poorly developed sense
in birds. The fact that certain birds will refuse
their normal food if it has been coated with unpleas-
ant chemicals demonstrates a power of discrimina-
tion, while the undoubted selection exhibited among
many insectivorous birds no doubt reflects the same

THE LIVING BIRD 29

thing. The bill is horny in many species and func-
tions rather as an organ of touch than of taste.
Perhaps the most peculiar taste apparatus is that
found on the tips of the bills of many shore birds
that probe in mud for a living. It probably com-
bines the senses of taste and touch. Taste buds
outside the oral cavity, unusual as they may seem
to us, occur in other groups and are well developed
in many fishes, e.g. on the barbels of the catfish.

With its covering of feathers, scales and horny
plates a bird has but little opportunity to develop
tactile organs. The tongue is perhaps the most
highly developed that any birds possess. Yet the
fact that the feather bases are in connection with
nerve endings gives a bird an indirect sense of touch
over most of its body, in the same way as whiskers
serve a cat. The cere (bare skin at the base of the
bill) of hawks, parrots, etc., is a localized tactile
centre.

The ear of birds is of particular interest from
several viewpoints. The original function of what
is popularly known as the ear is not hearing at all,
but equilibration. In its most primitive form it
consists of a hollow sac lined with hair-like sensory
cells, on the tips of which play a particle or particles
of loose material. (Fig. 5i\). Whichever way the
animal turns, a different set of processes receives

30 THE RIDDLE OF MIGRATION

the pressure of the particles and it is this simple
little piece of apparatus that gives an animal its
sense of orientation with regard to gravity. The
ingenious experiment on the crayfish Palinurus
demonstrates this in a simple manner. So many
times a year, when a crustacean sheds its coat of

Fig. 5. Crustacean "Ear" and Internal Ear of Crow

A: Crustacean ''ear": Ch, chitin; Gr, granules, resting on tips
of hair processes; N, nerve; S, sensory hair cells. B: left internal
ear of crow from left side: Am, ampulla; Ca, semi-circular canals;
La, lagena and cochlea; Ve, vestibule.

chitin which also lines the cavity of the "ear,"
it has to find new particles to replace those thrown
out. By providing an animal at this time with
nothing but iron filings, it can be induced to utilize
them. Gravity acts in the usual manner unless a
stronger magnetic field is artificially introduced and

THE LIVING BIRD

31

Fig. 6. Diagram of the Human Ear

A, outer ear; B, middle ear; C, inner ear. 1, the ear-trumpet
(pinna). 2, external ear-passage (meatus) running to 3, the ear-
drum (tympanum). On the inner side of this is the middle ear,
containing air, and communicating with the cavity of the mouth
by the Eustachian tube, 7. It contains the three auditory ossicles,
4, 5, and 6, which transmit the vibrations of the drum to the mem-
branous window, to the right of 6, in the wall of the inner ear.
The inner ear is entirely embedded in bone. It contains a fluid,
the perilymph; this surrounds the "membranous labyrinth," 8, 9,
10, a series of membranous organs containing another fluid, the
endolymph. 8, the utricle with the three semicircular canals
arising from it; the organ of balance. P, the sacculus, leading to
10, the spiral cochlea, the organ of hearing. Above 11 is a second
membranous window which is pushed outwards when the first
window is pushed inwards, and vice versa. (Haldane and Huxley,
Animal Biology, Oxford University Press.)

32 THE RIDDLE OF MIGRATION

then the fiUngs are displaced. The crustacean
immediately turns itself sideways into the position
dictated by the magnet, i.e., the position in which
the iron filings press on the hairs that are normally
stimulated by the downward pull of gravity. With
an "ear" deprived of any particles whatever, the
animal loses all sense of stability.

This same apparatus, but greatly elaborated,
exists in the vertebrate ear. It is no longer merely
a spherical sac but consists of a sub-divided major
compartment {vestibule) from which run three semi-
circular canals in three planes (Figs. 5B and 6)
Two are vertical and at right angles to each other.
The third is horizontal and at right angles to the
other two. If one of these is injured the bird w^ill
periodically fall over and show obvious inability to
walk normally. If all three are injured the bird
completely loses control over its movements. These
balancing canals, with their common chamber, are
the oldest part of the ear, still subserving the
original function of equilibrium. The canals are
very delicate structures both filled with liquid
(endolymph) and surrounded by it (perilymph) , the
latter in its turn contained in bony canals duplicat-
ing the true (membranous) canals.

Sound waves are received from the outside by the
ear-drum (tympanum) and are transferred from it

THE LIVING BIRD 33

by delicate bones (see figure) stretching across the
air-filled chamber of the middle ear (Fig. 6) to the
perilymph of the inner ear and so to the endolymph.
Projecting from the vestibule is another compart-
ment, known in mammals as the cochlea. It is
spiral in structure, relatively broad at the base and
narrow at the tip, provided with peculiar cells.
The apparatus may be likened to a series of strings
lying side by side, those at the base longer than
those at the tip with every intergradation between
them. If they are assumed to show sympathetic
vibrations to sound waves of various lengths coming
in from the drum, a very perfect arrangement exists
for analyzing a great variety of sounds. In mam-
mals the nerve supplying this organ terminates in a
highly developed (paired) brain-centre of its own
(posterior colliculus) , connected both with the cortex
and the cerebellum while at the same time function-
ing as a reflex centre. It is this intricate piece of
apparatus that makes speech in man comprehen-
sible and so possible. In the lower vertebrates the
cochlea is represented by the primitive lagena. In
birds a cochlea exists, but it is relatively simple and
lacks some of the essential features of the mam-
malian. Moreover, the colliculi do not exist as
such. It seems certain, in fact, that birds, in spite
of their great variety of song cannot experience the

■"•-■.:• ^^.W^^. \*\'»^,

34 THE RIDDLE OF MIGRATION

mental appreciation of sound distinctions possible
in a mammal. Experiments readily demonstrate
this general conclusion to be correct for despite
obviously acute hearing, birds are remarkably indif-
ferent to the character of the sounds produced.

There is one interesting feature about the vesti-
bule and canals of birds which will be further dis-
cussed below. This section of the ear shows struc-
tural differences in various groups, being most
highly developed in birds that may be termed
"good" flyers, e.g. swifts and swallows.

It is hardly necessary to dwell long on the struc-
ture of the eye, an organ that has reached remark-
able perfection in birds. In general principle it
somewhat resembles a camera possessing a dia-
phragm {iris) which controls the amount of light
admitted, a lens for focussing (in which the cornea
is also partially concerned) and at the back of the
chamber a sensitive nervous layer {retina) for the
reception of the image. It is obvious that the lens
cannot be wracked back and forth as in a camera
and an alternative principle is therefore adopted.
The convexity of the lens is altered as required by
appropriate muscles for close or distant focus. In
the higher mammals the retinal layer is made up of
two kinds of cells, the rods and the cones. They
differ in structure and in their connections with the

THE LIVING BIRD 35

other nervous elements of the retina and it seems
certain that the perception of color is a function of
the cones alone. The rods build up images without
color and are, phylogenetically speaking, the older
element. In birds the cones are relatively limited,
a fact which suggests a limitation of color vision.
Experiments, although they present certain difficul-
ties, seem to support this inference. At all events,
it appears to be well established that birds are
practically blind to blues and violets but as fully
appreciative of the yellows as man himself.

Two points of particular interest must be men-
tioned in connection with birds' eyes. One is the
universal presence of a structure known as the
pecten in the posterior chamber of the eye-ball.
Both its constitution and function have been vari-
ously interpreted. It is generally assumed to be
associated with a bird's exceptional powers of rapid
accomodation and is unknown in other eyes save
those of certain reptiles. The solving of the prob-
lem should offer an attractive, as well as profitable,
field to the anatomical investigator.

The second point is the presence of more than one
fovea in the eyes of many birds. The mammalian
(as well as avian) eye is so constructed that the rods
and cones lie behind the other constituent parts of
the retina and through them the light rays must

36 THE RIDDLE OF MIGRATION

first pass on their way to the rods and cones. But
over a certain very small area of the retina these
layers are so thinned as to expose the rods and
cones almost direct to the incoming rays. This
area is termed the yellow spot or fovea and is the
centre of perfect vision. Many birds possess two
such foveae, while the best flyers, — terns, swifts,
etc. — possess three.

Before leaving the eye it is of interest to note that
the ring of bones so characteristic of the outer wall
of the avian eye-ball (particularly well developed in
hawks, owls, etc.) is a typical reptilian feature,
while apart from birds, only certain reptiles can
boast striped muscle fibres in the make-up of
the eye.

The function of the nervous system of birds, like
other nervous systems, is to coordinate the activi-
ties of the many organs and systems that go to
make up an animal's body. Like the Federal
Government of the States or Canada, it welds to-
gether and unifies a collection of independent and
often antagonistic units, converting to the general
welfare of the whole the separate interests of the
component parts. But in this it is not entirely
alone and to the accessory mechanism we shall now
have to turn.

Situated in various parts of the vertebrate body

THE LIVING BIRD 37

there occur tissues and organs which are known as
ductless glands, owing to the circumstance that they
do not pour their secretions on a free surface by
means of a duct but liberate them directly into the
blood stream. The internally secreting organs are
sometimes collectively referred to as the endocrine
system but morphologically they do not compare
with other systems, (e.g. the nervous, or muscular)
since they are structurally independent units differ-
ing greatly from each other in constitution. How-
ever, more is continually being discovered about
their function and the fact that many of them are
intimately correlated physiologically speaking, i.e.
in terms of function rather than structure, permits
us, perhaps, legitimately to look upon them as a
chemical system of co-ordination. Without going
into details, suffice it to point out that their effects
on the animal organization are various and ex-
tremely profound. They may influence not only
growth (bones, etc.) and development of various
organs (comb, wattles, antlers, etc.) but also devel-
opment of the ''mind" and may even control
behavior. They are of vital importance to the
general welfare of the individual, whether immature
or adult, and the upsetting of a single factor may
lead to disturbances of the entire system sufficiently
serious to cause death. The "active principle" of

38 THE RIDDLE OF MIGRATION

some of these glands has been successfully isolated
and it has been shown that in case of certain diseases
due to the cessation of production of one of these
vital substances, the deficiency can be made up by
administration of extract. Thus the condition
known as diabetes, due in the first place to degener-
ation of the islet cells of the pancreas which results
in the disturbance of the sugar metabolism of the
body, can be alleviated by use of the extract known
as insulin. It matters nothing if the preparation
is made from the pancreas of hog or steer, it is
equally efiicacious in cats or men. The active
principle appears to be the same.

The true endocrine organs include thyroid, para-
thyroid, suprarenal, pituitary, pineal, pancreas and
gonad. It is to the last that we shall confine our
attention. Like the pancreas, the reproductive
organs have a double function. They not only
produce the actual germ cells — ova and spermatozoa
— but also internal secretions liberated directly into
the bloodstream. In order to understand the situa-
tion it is necessary to acquaint ourselves with the
structure of the organs. They differ in the two
sexes, those of the male being termed the testes,
those of the female, the ovaries. Both are usually
paired structures, but in birds the right ovary dis-
appears during development although it may occa-

THE LIVING BIRD

39

sionally persist (e.g. various pigeons and hawks)
and may actually be functional in some individuals.
Unlike most mammals, the testes are retained in the
body cavity in birds, lying in close juxtaposition to
the adrenals and kidneys. They are very small

Fig. 7. Part of Section of Testis and Ovary of Junco

A: part of transverse section of spring testis of Junco, Junco
hyemalis (diagrammatic). Int, connective and interstitial tissue;
Sem, seminiferous tubules, containing bundles of spermatozoa;
Tu, tunica albuginea. B: part of section of ovary of Junco
(semi-diagrammatic). Fo, follicle and theca; G. Ep, germinal epi-
thelium; Ov, Ova; St, stroma of comnective and interstitial tissue,
vessels, etc.

during the winter and relatively enormous in
spring. The greatest contrast in seasonal variation
occurs in the migratory species of the northern
hemisphere, in which the organs may weigh 1500
times as much during the breeding season as they

40 THE RIDDLE OF MIGRATION

do in midwinter. In tropical birds the difference
is slighter. In semitropical birds, such as the
canary, the winter testes are never as small as in
truly northern birds of similar size.

Obviously, with such marked seasonal disparity
in actual size we must look also for changes in
internal structure. The typical spring avian testis,
cut across the middle and examined under the
microscope presents the picture shown in Fig. 7A.
The larger part of it is made up of coiled tubules
{seminiferous tubules) bound together on the out-
side by a sheath, {tunica alhuginea) and communi-
cating with the sexual opening at the vent by means
of a duct termed the vas deferens. In these tubules
are produced the male sex cells, the spermatozoa,
during the breeding season. At other times the
tubules are quiescent and inactive and much dimin-
ished in size, the distention during the breeding
season being largely due to the enormous multiplica-
tion of the contained cells. Within the testis the
tubules are separated from each other by tissue
which is not concerned directly with reproduction
and has no duct communicating with the outside.
It consists chiefly of connective tissue cells in which
run blood and lymphatic vessels. In many wild,
particularly migratory, species large glandular look-
ing cells are to be found scattered through this con-

THE LIVING BIRD 41

nective tissue, but only at certain times of the year.
These are termed interstitial cells. They are known
certainly to be the source of the internal secretions
of the mammalian testis and there is little doubt
that this is their function in birds.

The ovary presents a somewhat different picture
(Fig. 7B). It lacks the tubular structure, the sex-
cell producing walls of the testicular tubules being
represented by the germinal epithelium, which, in
place of sperms, produces the eggs or ova. Whereas
millions of sperms may be demanded in a season,
the number of ova required is very small and many
start development only to be later resorbed. Dur-
ing the breeding season, such eggs as are destined
to mature, elaborate yolk material and become rela-
tively enormous. Each ovum is enclosed in a sac
of cells constituting the follicle, the various follicles
being bound together by connective tissue which,
with its blood vessels and lymphatics, forms the
ovarian stroma. Here develope glandular inter-
stitial cells at certain periods. As in the testis they
produce internal secretions.

At the age of puberty, when the internal sex
organs are rapidly maturing, the human boy and
girl undergo a change. They grow, almost over-
night, into man and woman. But physical changes
go hand in hand with change of temperament,

42 THE RIDDLE OF MIGRATION

change of outlook, of interests, of habits. Attain-
ment of maturity witnesses a metamorphosis of
mind and body. In animals changes may be even
more striking. In many species of those birds in
which sexual maturity is an annually recurring
event, a switching from one plumage to another or
from one mode of behaviour to another may be
observed every year. Thus the bobolink, in his
bright spring clothes is a combative individual for-
ever singing, ready to fight any other male, inter-
ested in the opposite sex, willing to build a nest, to
collect food for his offspring and so on, but when the
summer draws to a close and he dons his sober
winter apparel, all these activities slip from his
memory. Fighting and singing appeal to him no
more than does the opposite sex. He is quite con-
tent to be a silent and inconspicuous member of a
flock of males without individuality or personal
interests. For the time being he is sexless. When
at the end of April, he returns from the south to the
States, he arrives in the company of other males,
without the pugnacity which will develop later, but
already in song. His gonads are enlarging. They
have, however, not yet attained the maximum but
neither has he reached the zenith of his sexual
behaviour. Such behaviour depends directly on the
internal secretions of the testes. This assertion can

THE LIVING BIRD 43

be quite simply verified by castration, i.e. by surgical
removal of the testes. Provided the operation is
successful and complete, he never again exhibits
these characteristic male traits. He is permanently
sexless. In similar manner, after removal of the
ovary {ovariectomy) a female bird will drop its
normal female behaviour and (in poultry) will don
plumage resembling that of the male. When this
operation is performed, it frequently happens that
the ovary of the right side, hitherto vestigial, begins
to develope, but it frequently developes, not as an
ovary, but as a testis. The bird now comes under
the influence of testicular hormones. Theoretically
it should begin to exhibit male behaviour and this is
actually the case. The bird at its next moult
assumes male plumage, developes the head furnish-
ings of the male (in poultry), crows, and may actu-
ally attempt to mate with other hens. In one
extreme case such a sexually reversed bird, a good
egg producer till three years old and the mother of
chickens, began to crow at 3 J, took on all the male
characters (except that she retained her female
stance) and at 4J, on being mated to a virgin hen,
became the father of two chicks.

In gonadectomized birds, originally of either sex,
male sexual behaviour can be called forth either by
grafting another testis in some part of the body, or

44 THE RIDDLE OF MIGRATION

by the administration of testicular extracts. The
behaviour ceases if the extracts are discontinued.
These experiments, while they demonstrate the
dependence of behaviour on the gonad, or part of
the gonad, do not necessarily substantiate the claim
that the interstitial cells are responsible. The point
has, however, been demonstrated in mammals where
the sex cells have been destroyed by means of
X-rays leaving the interstitial tissue unimpaired.
Under such conditions the animal retains all its
male characters and behaviour, although it is now
sterile and no longer a functional male.

CHAPTER II

Environment, Past and Present

In the popular mind one of the chief distinctions
between plants and animals is that the former are
fixed, the latter capable of free movement. And
this is roughly true in spite of exceptions both ways.
It is natural that among the smaller forms of animal
life movements should be more or less curtailed for
limitations of size may be as effective as limitations
of organization. The microscopic Paramecium
which flashes across the field of the microscope
apparently at the speed of a greyhound, actually
travels only a few feet an hour but could it be en-
larged to the size of a horse and retain its own speed
in proportion, it would travel as fast as a horse in
full gallop. Its tie is diminutive size, not lack of
ability. There are, on the other hand, such beasts
as the giant cuttlefishes that may weigh nearly three
tons but yet ''get nowhere." They have size, not
speed; organization is as important as bulk.
Given both size and equipment, other limits may yet
be imposed on distribution. Thus a terrestrial ani-
mal, no matter how large or how speedy, constantly
confronted with barriers, cannot hope to compete

45

46 THE RIDDLE OF MIGRATION

with an aquatic animal of comparable attainments
while the latter, in its relatively dense medium, is
easily outdistanced by travellers in the air. A
winged insect may cover distances that the most
highly organised terrestrial vertebrate could not
contemplate. We may therefore reasonably expect
to find among birds the greatest and most spectacular
of migrations and our expectations are duly fulfilled.
But the ability to travel is not synonymous with
migration which is a particular type of travel with
quite distinctive features. We can formulate a
definition that clearly delimits the term. None
better than Gadow's exists and this we propose to
adopt. Migration is "the wandering of living crea-
tures into another, usually distant, locality in order
to breed there; this implies a return, and the double
phenomenon is annual. All other changes of the
abode are either sporadic, epidemic or fluctuating
within lesser limits. "^ This can be applied to all
animal migrations, even to those of fish which breed
but once in a lifetime. When they have reached a
certain age, the adults may travel far up rivers, or
across the ocean to breed and then die. The young,
however, come down the rivers in their youth to

1 H. F. Gadow, Migration, in zoology. Encyclopaedia Bri-
tannica, 11th ed., vol. 18, p. 433-437.

ENVIRONMENT, PAST AND PRESENT 47

return again at maturity, maybe years later, but
the phenomenon is double and annual as applied
to the species. The individual returns but once
after the lapse of years, but every year there is a
procession of individuals.

Such a definition is by no means an arbitrary one,
for one leg of typical migrations has reproduction
as its sole, or at all events, its most obvious goal.
Thus oceanic birds, which have never become wholly
emancipated from the land habits of their ancestors,
must return to land in order to breed, even though
they still have to depend on the ocean for their food.
Conversely most amphibia must breed in water
despite the fact that they spend the rest of their
lives on land. The land-crab of the West Indies,
Geocarcinas, comes down to the ocean to breed.
Its movements to the sea are annual and a return
inland follows upon breeding. Numerous sea-
snakes and marine turtles find themselves compelled
to come ashore to deposit eggs. Seals, sea-lions,
walruses and other aquatic mammals have never
solved the problem of giving birth to their young at
sea and, like any other mammals (except whales)
must be ashore to do so. On land they cannot feed
and reproduction is undoubtedly their driving force.
Various species of salmon and eels, some of which
undertake stupendous migrations, although spawn-

48 THE RIDDLE OF MIGRATION

ing in their own element, may forego feeding alto-
gether on the breeding run.

As far as the southward flight of many northern
species of birds is concerned it may, perhaps, be
classed as a feeding movement since continuance
in the north means inevitable starvation, but it is
only half the story. What of the return passage?
Food is neither obviously, nor certainly, in spite
of the assertions of many writers, a compelling
factor. If birds find ample sustenance on their
winter range they should find even more there in
the summer, the season of universal abundance.
The assumption that the bird can foresee a possible
forthcoming shortage of food as a result of impend-
ing spring increase, and therefore moves elsewhere,
is wholly gratuitous and unwarranted. Not even
man can certainly state that such would be the case.
There are good reasons for believing that it would
not. The same may, of course, be said of breeding,
i.e., that the birds could probably breed as well in
the south as in the north and therefore breeding is
not the urge, but at this juncture the point is es-
sentially immaterial. We shall return to it later.
Breeding is involved in the northern passage whether
constituting the inducing factor or not and this
is the element in true migratory movements that
makes them distinctive.

ENVIRONMENT, PAST AND PRESENT 49

Non-breeding birds may migrate, but typically
theirs is but a partial migration. Since several
species are known to breed in immature plumage
and to complete their migrations it may be difficult
to assess correctly the status of immature migrants.

Many writers include feeding movements, if of
sufficiently impressive range, in the term migration.
Such movements may vary from the few yards
covered by the limpet as it creeps from its perma-
nent seat to the nearest patch of seaweed and back,
to the hundreds of miles traversed by many shoals
of fish as they accompany and prey on swarms of
plankton drifting on some ocean current. Animals
in general must move to obtain their food ; move-
ment may, in fact, be considered an integral part of
the business of feeding. Whatever the distance
covered, the principle is the same and if such move-
ments are to be admitted into the category of migra-
tion any attempt at definition must ultimately
break down It is not always a simple matter to rec-
ognise purely feeding movements and this applies
particularly in the tropics. In severe winters in
the British Isles extensive movements may take
place, but they are carried out under stress of im-
mediate circumstances, during winter, not before
it, are quite sporadic and frequently entail a heavy
mortality in all of which they are in contrast to
true migrations.

50 THE RIDDLE OF MIGRATION

As good an example of confusing movements as
is to be found in the northern hemisphere, is pro-
vided by the white-winged cross-bill (Loxia leucop-
tera) of Canada and the States. These birds travel
mostly in bands and rove over enormous tracts of
territory. They feed on cones and change their
grounds whenever the spirit moves them. If they
strike a district in which the cone supply is ample
they apparently stay there as long as it lasts and
then again move on. They have no fixed breeding
areas and breed at the appropriate season wherever
the cone supply may have landed them. Thus it
happens that once in so many years the Edmonton
district receives an invasion of these birds. This
was the case in the winter of 1920-21 when a par-
ticularly heavy influx occurred, reaching far south
into the Province. The interesting point is that
the birds stayed to breed the following spring and,
in fact, did not finally depart till the end of the
next winter. These birds must actually have come
south from the north in the first place and thus in
this instance would actually have reversed our
entire conception of northern migrations, should we
admit such movements into our scheme.

This case appears to be analogous with the wan-
derings of most species of birds in the subtropics and
Australia, whose most characteristic movements,

ENVIRONMENT, PAST AND PRESENT 51

as far as they seem to be known, are wanderings
which keep them in touch with a changing food
supply that on the whole tends to vary with the
rains. Referring to these as they are seen in Aus-
tralia, Wetmore says (p. 33) "Though this seems
mere vagrancy, it is migration of a kind." And so
indeed it is but we know all too little about it. In
certain cases it may be true migration (p. 105). It
is on the border-line and it is with some reluctance
that one must exclude it by definition. But if a
discussion of migration is to serve a useful purpose,
it is imperative that a clear understanding of the
scope of the term be determined at the outset.

In no other part of the world has the custom of
migration been as extensively developed as in the
high latitudes of the northern hemisphere. A gen-
eral explanation of the situation is self-evident when
one scans the map of the world, particularly if one
takes a few liberties with it as we have done in
Fig. 8. The land masses of the southern hemi-
sphere have been superimposed, in their correspond-
ing latitudes, on those of the northern. They ap-
pear inverted and if the map is looked at upside
down the significance of the arrangement will be
better appreciated. Placed thus, the southernmost
tip of Australia is level with New York, while New
Zealand fails entirely to reach even the western

52

THE RIDDLE OF MIGRATION

W
Q

H
H

Q W
H

Q
W

O

w w

D PLh

CO ;>2
i w

> o

00

ENVIRONMENT, PAST AND PRESENT 53

Canada-United States boundary. The extreme tip
of Patagonia would fall short of Edmonton, while
Africa would have to be lengthened by nearly 200
miles to reach the latitude of Washington, D. C.
A majority of the birds of the northern hemisphere
thus come to summer and breed in localities much
further removed from the equator than birds of
the southern, while dozens of species breed more
than double the distance from the equator than do
even the southernmost breeders of Australia. They
come under the influence of conditions, to be con-
sidered in detail below, that are entirely unknown
to all but a few birds of the southern hemisphere,
an extremely important fact but seldom appreciated.

We propose then, to pay particular attention to
the well-known migrations of the northern hemi-
sphere. Nowhere else can migrations be more favor-
ably observed or more critically studied. If we
can analyse them here we can apply our findings to
migrations elsewhere after making due allowance for
other factors.

In order to have something specific in our minds,
let us review meteorological figures for the Edmonton
district. In general one may say that the further
north the more severe are the winter conditions,
though it must be remembered that exceptional
areas, such as the coast of Alaska, exist. But what-

54 THE RIDDLE OF MIGRATION

ever the climate, there is no exception to the varia-
tions in day-length. The further north one goes,
the shorter become the days in winter and the longer
in summer.

The mean annual temperature at Edmonton,
Alberta, is 36.8°F. The winter minimum is about
— 52°F. (though it may drop below — 60°F. in the
river valley) and the summer maximum (shade)
just over 100°F. Rapid changes of temperature
are characteristic through most of the year. The
summer nights are nearly always cool, often cold.
Annual average precipitation is in the neighborhood
of 12 inches: snowfall by itself, 45 inches. The last
sounds a good deal, but owing to the dryness of the
climate and periodic chinooks, it is but rarely that
a foot of snow is to be seen on the ground at any
time. Average monthly sunshine, 191.2 hours. On
June 21 the sun rises at 4.07 a.m. and sets at 9.04
p.m., i.e. the day is some 19 hours long, actually
somewhat more since both dawn and twilight are ex-
tremely protracted. On December 21 the sun rises
at 8.40 a.m. and sets at 4.24 p.m., giving a day-
length of less than 9 hours.

On the face of it, it is obvious that many species
of birds could not under any circumstances survive
a northern winter. With many of them the food
question alone would settle that. None of the

ENVIRONMENT, PAST AND PRESENT 55

warblers, thrushes and insectivorous birds generally,
or those depending on open water for sustenance,
could last through even a week of it. They must
depart or perish. But the food situation is not
quite so patent in other cases. There is, for instance,
the large group of seed-eating birds, which leaves us
almost to the last species in spite of the fact that the
average snowfall throughout most of the Northwest
is very light and there is probably no year in which
a virtually unlimited supply of food is not available.
Seed-eaters will be further discussed below.

But there are several sides to the food question.
The colder the weather gets, the more food does a
bird demand and if it needs more food, it requires
more time to procure it. The midwinter day in the
Edmonton latitudes is only 9 hours. Further north
it is still shorter. There must be a point somewhere
at which a junco, for instance, could no longer collect
sufficient food to meet its requirements in the hours
available even were the supply unlimited. The
colder the weather the further south would that
point lie. Moreover, when the temperatures be-
come extreme (about -30°F. and below) the birds
become markedly lethargic. But this is just the
time when extra energy is demanded for the col-
lection of extra food. At these temperatures it is
generally easy to catch any birds in my aviaries by

56 THE RIDDLE OF MIGRATION

hand since they become exhausted in a few minutes
and can be picked up. The same bird, released in a
heated room immediately after, shows no signs of
undue fatigue when being re-caught and it certainly
cannot be taken by hand. Many of my canaries
(which winter out) continue to sing to about 30°
below zero but at lower temperatures they are
silent. Food consumption goes up enormously at
these times.

In regions of really heavy snowfall the supply
may become entirely inaccessible.

The food problem thus comes to be not merely a
question of supply, but of availability, temperatures
and day-length.

Low temperatures in themselves are probably of
little moment. No native birds that I have kept
in the aviaries have shown any particular disability
apart from the lethargy already commented on.
The lowest temperature my captive birds have
actually experienced was — 52°F., i.e. 84 degrees of
frost. The canaries are quite frequently exposed to
temperatures ranging from —30° to — 45°F. yet I
have never had a winter death among these sub-
tropical birds. The feet and legs of Australian
budgerigars freeze at — 12°F.

There is yet another factor in the northern
winter environment to be considered but it seems

ENVIRONMENT, PAST AND PRESENT 57

to have been totally ignored by students of migra-
tion. It has been the subject of investigation at
Edmonton for some years. The further from the
equator one goes in either hemisphere, the less ultra-
violet radiation reaches the earth's surface. This
applies mainly to the winter but not entirely so, for
owing to the angle of the sun in the extreme north,
even at midsummer there is material reduction, by
atmospheric absorption, of the available ultra-
violet. In the latitude of Edmonton there is prac-
tically no effective ultra-violet radiation for at least
the four winter months when the sun is at its lowest.
Further north still this deficient period is greatly
extended. Even at the United States boundary the
winter months can provide but inadequate radia-
tion. These short-wave rays belong to the non-
visible series, i.e. although they can penetrate the
lens and cornea and so actually reach the retina,
they do not give rise in the human eye to the sensa-
tion of light. Their particular interest lies in their
power to produce the substance known as vitamin D
by direct action on certain other substances and
vitamin D is essential to the welfare of the animal
organisation, whether young or adult. This vita-
min occurs in various foods such as cod-liver oil and
a sufficiency may be obtained from sources of this
kind to meet all requirement in regions where there

58 THE RIDDLE OF MIGRATION

is normally but little sunlight as, for instance, the
islands of Behring Sea, notoriously sunless, which
have been inhabited for centuries by tribes of
Eskimos. The sebaceous glands of mammalian
skin, on the other hand, contain a chemical com-
pound known as ergosterol. When this is exposed
to ultra-violet rays, whether artificial or natural,
vitamin D is elaborated. It is resorbed by the
skin and the animal is thus able to obtain a supply
regardless of the kind of food it eats. Birds poSvSess
no sebaceous glands but in the single preen-gland,
situated above the tail and present in most birds,
they have a close equivalent. Analyses of preen-
glands of various species in the Department of Bio-
chemistry in the University of Alberta have demon-
strated the presence of ergosterol, and it is present
also in the fats extracted from the feathers. When
a bird preens, and therefore, as long as it is exposed
to sunlight, it will be absorbing small doses of vita-
min D as it passes the feathers through its bill.
Much more detailed studies carried out more re-
cently at McGill University have proved similar
in their findings and demonstrated a very close
relationship between the preen-gland and avian
nutrition.

Vitamin D deficiency in young animals produces
rickets, a common disease of children in the northern

ENVIRONMENT, PAST AND PRESENT 59

hemisphere when they are reared on a deficient diet.
Seed-eating birds avoid rickets in their young by
becoming insectivorous entirely, or in part, during
the rearing period. Cereals are notoriously defi-
cient in vitamin D; insects form a rich source.
Merlins and other hawks, which normally feed their
young on plucked birds, a diet that induces rickets,
meet the situation by periodically administering
feathers to them, an effective cure for rickets also
in meat-raised, hand-reared hawks. In adults the
symptoms are not so obvious. An interesting dis-
covery, made at the University of Toronto, is that
adult rats kept on a diet deficient in vitamin D and
sheltered from sunlight speedily succumb to infec-
tion from specific organisms, the resistance of rats on
a similar diet but exposed to sunshine (or artificial
radiation) being 50% to 70% greater. Lack of
vitality in embryos of the fowl, when the hens have
been laying while on a deficient diet, has also been
demonstrated. Exposure of the laying birds to
ultra-violet radiation cures the situation. In vari-
ous other ways the essential nature of vitamin D
to adult animals has been shown.

At Edmonton we have kept tree sparrows and
j uncos for a period of years to ascertain the effects
of compulsory residence in the north with its ultra-
violet deficient winter. Individual tree sparrows

60 THE RIDDLE OF MIGRATION

may survive for as long as three years but every
March sees some of them developing fits and suc-
cumbing. Nothing of the kind has been noted with
juncos but those that have attempted to breed in
the aviaries have never yet done so successfully.
They are close sitters but their eggs nevertheless
have always failed to hatch out. Generally the
chicks develop almost to the point of hatching but
they never leave the shell. The eggs have been
transferred to canaries, but the outcome is the
same. This certainly suggests vitamin D deficiency .^

Quite apart from their connection with vitamin
D, the ultraviolet rays are in themselves of direct
therapeutic value.

There is another phenomenon as characteristic of
the northern sections of the globe as the migration
of birds. This is the periodic fluctuations in the
numbers of resident animals. The ten-year rabbit
cycle of Canada is universally familiar. It occurs
also in the case of numerous other mammals and
of many resident birds. The animals concerned
slowly increase in numbers till they reach a maximum.

2 The anti-infection, or xerophthalmic, vitamin A is particular!}-
associated with resistance to disease but it is no doubt true of all
the vitamins that deficiency results not only in. specifically asso-
ciated ailments but in a lowering of general health and vitality
and hence resistance to disease.

ENVIRONMENT, PAST AND PRESENT 61

This may involve actual crowding and keen compe-
tition for food as in rabbits, or it may not, as in
the fisher and marten. Then suddenly there comes
an epidemic of diseases resulting in almost complete
extermination. Again there comes a slow recovery,
the attainment of a peak, another break and so on.
Thus in the thirty years that Alberta has been a
province and has had game laws, there has been a
closed season on grouse in the years 1907, 1917,
1927 and 1928. (The closed seasons have followed
on the heels of years of maxima.) The same years
have seen minima of rabbits and many other ani-
mals. The cycles, in fact, with a few exceptions,
are synchronous. They appear to bear no direct
relation to precipitation; the periodicity does not
agree with that of sun-spot changes; yet the fact
that many species, unrelated and of dissimilar feed-
ing habits, subject to a variety of diseases, come and
go together indicates some fundamental underlying
influence. There is some reason for believing that
annual variations in the ultra-violet radiation of
the north may prove to be the key. Reduced radia-
tion would mean diminished resistance to disease
and one would get exactly the conditions that favor
epidemics and universal death but extremely little,
unfortunately, is known of radiation in the Cana-
dian north and no such ten-year period has been
demonstrated.

62 THE RIDDLE OF MIGRATION

The reason for this digression is evident. No con-
sideration of conditions prevailing in the north as
they affect bird Hfe can be complete without taking
note of this striking phenomenon of cycles. Whether
there is a connection or not between these and
migration remains to be discovered, but one thing
is certain. Migratory birds have solved the prob-
lem of periodic decimation that overtakes many of
their non-migratory brethren. As far as we know,
no migratory species show the ten-year cycle.^
They take an annual risk and undoubtedly suffer
some annual reduction in numbers, but they avoid
the danger of almost total extermination every ten
years. In going south they guarantee themselves
an all year contact with the health-giving ultra-vio-
let rays of the sun. This applies particularly to
seed-eaters to whose continued welfare radiation

3 Some owls (e.g., snowy and short-eared) and probably certain
hawks, show a cycle in numbers. Their rate of reproduction
apparently becomes heightened with augmented food supply and
we hence get larger numbers during years in which mice are super-
abundant on their breeding grounds in the north. Mice show a
four-year cycle throughout most of the northern hemisphere and
their peak seems to be reflected in the abundance of certain
predatory birds. This is entirely distinct from the ten-year rabbit
and grouse cycle and appears to be a direct question of food
supply.

ENVIRONMENT, PAST AND PRESENT 63

may be as essential as a steady supply of insects
is to the insectivorous species.

It is interesting to note in passing that juncos and
other migratory seed-eaters released from my avia-
ries during mid-winter, with the ground under snow,
have been out for as long as two weeks before being
retaken and have fared so well on the vacant lots
that they have not even bothered to go to the food-
box on their return. The last of our liberated crows
to be shot was killed on February 28, 1930, in the
northern wilderness, after having been free for four
months during a quite severe winter. Wild indi-
viduals of various seed-eating species that have
failed to migrate, frequently survive the entire
winter. Such isolated examples do not, of course,
prove that the race generally could find a sufficiency
of food, but they distinctly suggest that such might
be the case, and automatically eliminate dogmatic
assertions to the contrary.

This viewpoint may be summarised in the follow-
ing manner. Assuming that vitamin D (like
vitamin A) is indispensable to the health of adult
animal life then inhabitants of the north must get
their supply either from their diet or from exposure
to the sun or from a combination of both. As-
suming that their diet is such that they require a
certain amount of sunshine per annum then, to re-

64 THE RIDDLE OF MIGRATION

tain their health in years of reduced ultra-violet
radiation, (e.g., persistently cloudy summers) they
must (a) change their diet, (b) move further south
or (c) suffer the consequences. Adoption of the
first alternative is extremely unlikely. Birds ex-
cept when rearing young show remarkable fixity
in their diet, though it must be admitted that even
a dog may instinctively eat grass under certain
conditions. An overwhelming majority of northern
birds go south for the winter and so automatically
fulfill the second alternative. The last descends
like a guillotine on a small number every decade.

Having seen something of conditions obtaining
in the northern hemisphere as it exists today, we
may now turn to a consideration of certain aspects
of the past.

Undoubted bird-remains go back in the record of
the rocks to the epoch known to palaeontologists as
the Jurassic, a matter of some seventy million years.
Two examples of the first-known birds are in exist-
ence, the one (Archaeopteryx) preserved in London,
the other (Archaeornis) in Berlin. Both were ob-
tained from slate quarries in Bavaria. They differ
so radically from modern birds, that were it not
for the feathers, of some of which very perfect im-
pressions remain in the slate, one would probably not
suspect them of being birds at all. The tail is long

ENVIRONMENT, PAST AND PRESENT 65

like a lizard's, with feathers springing in pairs op-
posite each other at intervals down its full extent.
The wings have three fingers (with free meta-carpals)
each with a well developed claw. The skull, with a
full complement of teeth, is reptilian rather than
avian. The wing feathers were well-developed,
however, and there seems little reason to question
their ability to fly in the true sense. They were
virtually feathered reptiles.

Between these and the next known fossils there is
a long gap. The lower Cretaceous (some forty mil-
lion years back) produced what might be termed the
first pseudo-modern bird in Ichthyornis, a gull-like
species. Judged by its wing construction and the
presence of a well developed keel on the breast bone
for the attachment of large flight muscles, it was
already an expert on the wing. But it still retained
teeth although those of the premaxillae (approxi-
mately the anterior half of the upper mandible) were
already lost. The skull is now much more bird-like
with a convincing bill. One of its most interesting
contemporaries was the large flightless diver
(Hesperornis), reminiscent in many respects of
modern loons (excepting its flightless condition)
but still retaining various reptilian structures in-
cluding teeth (absent, as in Ichthyornis, on the pre-
maxillae).

I

Cool

Recurring periods of glaciation
with intervals warmer than
present climate

Much as today
^ Gradually cooling

Warm. Greenland heavily for-
ested

Warm. Cycads growing in Ant-
arctica

o

<
>

o

s

<

<

a

<

»

s

1

a
•2 ^

sl

W

Ratites, Albatross
First modern species: gulls, par-
rots, woodpeckers, etc.

Forerunners of penguins, rails,
cranes, herons, hawks and other
recent species

Hesperornis, Baptornis, Ichthyor-
nis, etc., a primitive duck and
gannet

Archaeopteryx. First fossil bird
known

n

H

Recent
Pleistocene

Pliocene
Miocene

Oligocene
Eocene

Cretaceous

Jurassic
Triassic

J

Q

<

•<
K
W

Quarternary

one

million

years

Tertiary

20

million

years

Secondary

55

million

years

66

ENVIRONMENT, PAST AND PRESENT 67

The first toothless birds make their appearance
in the upper Cretaceous and the first truly modern
birds in the Eocene. The fossil series of birds is,
unfortunately, very badly broken and there are
still enormous gaps in the record.

Before leaving the subject it might be stated that
the gauging of the ages of the epochs is not a matter
of guess-work, but is based on various reliable
methods such, for instance, as the rate of sedimenta-
tion on the ocean-floors and the thickness of the
various rock-strata, or the relative amounts of lead
and radio-active substances in various minerals, and
it seems quite certain that birds have a distinctly
lengthy pedigree.

While we propose to make a brief excursion into
the climates of the past, back to the days oi Archaeop-
teryx, such a digression is probably of no great sig-
nificance, for whatever may have been the state of
the evolution of migrations prior to the last ice-age —
and northern migrations almost certainly go back
to the early Pliocene, i.e. before the ice-ages — almost
the entire story must have been retold for most
species on this continent after the last retreat of the
northern ice-sheet. There is no reason to imagine
that the factors at work either before the ice-age or
immediately after it differ essentially from those in
operation today and it would seem possible to iden-

68 THE RIDDLE OF MIGRATION

tify these with a considerable show of probabiHty by
analysing them as they are making migrations be-
fore our very eyes at the present moment.

The Jurassic epoch, in which Archaeopteryx and
Archaeornis make their appearance, was character-
ised by the mildness of its climate. Even the ex-
treme north was genial and there was no — or at the
most but a slight — accumulation of ice even at the
poles. Antarctica, now a blizzard-swept waste of
snow and ice, produced cycads and other temperate
vegetation while Greenland and Alaska boasted
enormous forests of species of trees which today
characterize the temperate zones of the globe. It
is probable that Archaeopteryx and its relatives en-
joyed a sub-tropical climate in Bavaria. Condi-
tions during the Cretaceous and Eocene, in which
the earliest representatives of modern toothless
birds first appear, remained much the same but the
end of the Eocene saw the inception of a very slow
progressive change to cooler conditions which cul-
minated in the ice ages of the Pleistocene. The
fossil record has preserved the story of the gradual
southward retreat of such tropical reptiles as croco-
diles and such plants as palms, of the evolution of
arctic forms of mollusca and other animals and an
invasion, from the north, of yet other forms, com-
ing south before the advancing ice-sheets. The

ENVIRONMENT. PAST AND PRESENT 69

musk-ox was to be found as far south as Arkansas,
walruses besported themselves off the coast of
Georgia, while lemmings and mammoths were
among the arctic representatives of the fauna of
southern France.

The Pleistocene glaciations are represented in
north America by at least four and perhaps five
different periods, which take us back about a mil-
lion years, while some fifteen to twenty thousand
years have elapsed since the close of the last Pleisto-
cene ice-age known as the Wisconsin. The ac-
companying map shows the extent of the glaciation
in the northern hemisphere during the Wisconsin
period and during all the other periods combined.
The patchy nature of the glaciation in the old world,
due largely to the formation of glaciers in connection
with the mountain ranges will be immediately
noticed. Such glaciation as occurred in the southern
hemisphere (apart from Antarctica) during the
Pleistocene was of this restricted type. The inter-
glacial periods were comparatively mild, some of
them milder than our present climate. While not
a great deal is known about them it seems certain
from recent investigations, chiefly on fossil pollen
grains, that in northern Europe the present period
is already getting cooler than it was 2000 years ago,
quite possibly presaging the approach of yet another

ENVIRONMENT, PAST AND PRESENT 71

period of glaciatlon. The same is in all probability
true of north America.

A close examination of the map will reveal that
at no time did the ice-sheets cover the whole of
Alaska or the Canadian arctic islands. What con-
ditions may have been lijce here during the ice-ages
it is, perhaps, difficult to surmise with any assur-
ance ; but we know that even at the height of glacia-
tion, plants of various kinds existed on these areas.
That being the case, the presence of insect-life may
be taken for granted, the more so since insects were
known to be abundant during the interglacial
periods. In other words this region was probably
able to support at least certain forms of summer
bird-life right through the ice-ages. Yet even so
it would, of course, prove birdless if inaccessible.
There remained, however, the ocean border on the
west of the mountains, no doubt an open road
throughout the period of glaciation. There is also
considerable evidence that, during the Wisconsin
age at least, there remained a lane from Alberta
north, on the east side of the mountains, which was
only partly glaciated, with open lakes and marshes
in summer, which may have provided food and a
migratory path to the north. Geese, various ducks
and a number of shore-birds may thus very well
have continued to breed in the far north even at

72 THE RIDDLE OF MIGRATION

the height of glaciation. The number of birds
breeding in northern Alberta and beyond today,
that reach their breeding grounds via the mountains
from the shores of the Pacific, a curious and other-
wise inexpHcable migratory route, may conceivably
represent species that never ceased to breed in the
far north and that had a trans-Rocky route thrust
on them by the exigencies of the \Msconsin or a
previous ice-age. But for most species, particularly
the huge group of the Passeres, the northern two-
thirds of the American continent was probably
(though not inevitably) a closed book.

The southern third of north America must then
have presented conditions very similar to those now
obtaining over most of Canada except, of course, in
the matter of day-length, a crucial distinction.
The climate might have been that of the north but
the seasons must have been as they are now.

Whatever the details of climatic change during
one period or another, of one thing we can be sure.
At no time were the essential conditions on the globe
required by living organisms radically different
from what they are today. The composition of air
and water, for instance, can never have varied ma-
terially. Even the present order of terrestrial tem-
peratures can not have been greatly different. A
drop of but 6"" or 7°C. in the mean temperature of

ENVIRONMENT, PAST AND PRESENT 73

the northern hemisphere as we know it now would
mean a return to glaciation. A deviation of but
2°C. above or below the present mean would induce
marked climatic change. The fossil record, even
though it be one of constant evolution as far as
certain groups of animals are concerned presents
us with an unbroken thread of life. Here we note
the disappearance of species that have survived for
hundreds of thousands of years, there the first ap-
pearance of others and everywhere the inconceiv-
ably slow transformation of one form into another.
But at the same time there are species existing
today that have successfully survived unchanged
for millions of years. The creation of one, the
extinction of another, the constant change that we
term evolution, these things reflect a locally chang-
ing environment. The effective changes, great or
small, may have been of a thousand different kinds
but in the survival of numerous forms, unchanged
and resembling exactly their progenitors of millions
of years ago, we find evidence that bears but one
interpretation — the essential conditions required by
living organisms have remained unaltered through
the ages. They are undoubtedly the same today as
they were at the dawn of life. The fundamental
requirements of the birds of the present epoch were
also those of the earliest Eocene species. We are

74 THE RIDDLE OF MIGRATION

safe in assuming that the environmental influences
to which birds respond today, must have eHcited
response twenty milHon years ago. In analysing
the present-day factors that might have a bearing
on migration we are at the same time, with little
question, examining those of the distant past. The
ice-fields of today may be restricted to the vicinity
of the poles; yesterday they may have stretched
south to Kansas. This is merely a shift of particular
environmental conditions. The fundamentals re-
main constant. We may therefore proceed to ex-
amine more precisely the effects of the northern
environment on bird life as it now exists and apply
our findings to the past with some sense of confi-
dence. We may be, and no doubt shall be, wrong
in details but there seems no reason to make mys-
tery of the past. The essentials could not have
been radically different.

We have noted above that many species of birds
could not possibly survive a northern winter. They
meet the situation by going south in the fall. To
go north, or east or west at the same latitudes,
would be as fatal as remaining. Their road south
maybe quite indirect, even circuitous. Some species,
for example, go south a certain distance, then w^est
across the mountains and then further south; others
go mainly south-east, to winter in the eastern States;

ENVIRONMENT, PAST AND PRESENT 75

yet Others (e.g. adult golden plovers from the Barren
Lands) first fly due east to Labrador before starting
south. The yellow-billed loon actually appears to
north in the first place, then west (or east), then
south. But they all ultimately go south.

Before we consider the birds, let us picture our-
selves paying a visit to the barren lands as gentlemen
of leisure and means, with the intention of staying
there twelve months. Towards the end of the sum-
mer we take stock of our supplies and find that we
have neither a sufificiency of food or fuel to see us
through the winter. Shortening days and early
frosts warn us that it is time to act. We fully com-
prehend their significance . Thanks to other people's
knowledge of the north we can foresee threatening
blizzards, extreme temperatures and eternal night.
We would mentally scan the continent (thanks to
book-knowledge) and probably decide that Cali-
fornia would be a good refuge. We would forth-
with radiograph for an airplane, fiy south under the
care of a navigator, surrounded by instruments and
gadgets from a compass to tooth-picks, and there-
after probably never cease to bore our friends with
tales of our prowess. We could do it all without any
previous personal experience. We could form a
mental picture of the entire environment from the
observations of others. Consciously and inten-

76 THE RIDDLE OF MIGRATION

tionally we would arrange to flee from impending
cold and starvation and spend the winter where
winters are pleasant and food is plentiful.

But a bird goes neither to school nor college; it
has no libraries nor navigating instruments ; it knows
nothing about the experiences of others; yet it
reaches its predestined winter quarters with the
sureness that we would. Accidents might befall
in either case. It is obvious in fact that in trying
to explain the southward passage of birds we cannot
do so in terms of human behaviour. We must find
an explanation that involves neither human in-
telligence, deliberate intention nor the conscious
use of the experience of others.

It has been assumed times without number that
young birds find their road south by accompanying
their parents or even members of other species ; that
they are, in fact, guided south. In some cases the
young do stay with their parents and probably com-
plete their entire migration in company, but such
cases are no more frequent than those that undoubt-
edly do not. The assumption that some other
species provides the guidance is the merest assump-
tion. The young cowbirds of Alberta, hatched and
fed by foster-parents of some thirty different species,
do not spend the winter scattered all over the south
as would be the case if they accompanied the birds

ENVIRONMENT, PAST AND PRESENT 77

that reared them. They go neither with their true
parents, which depart long before they are ready to
travel, nor with their foster-parents. They find
their own road when the time comes. It is their
very first taste of migration but they nevertheless
take the correct road. The numerous waders that
breed on the barren lands to the north of us, prac-
tically all leave their young as soon as they can feed
themselves, and depart south. Later, but inde-
pendently, the young follow. When they reach us
in September far behind their parents, they are
hopelessly mixed up and seven or eight species are
frequently seen together in a flock, all birds of the
year. But winter finds them in their correct
quarters, be it the Argentine or Florida, Peru or
California. Our Franklin gulls, which breed locally
in colonies of many thousands, pay no attention to
their young after these can feed themselves. They
are left to find their own way to Texas and Peru.
The extreme case is undoubtedly the American
golden plover. The adults take the Atlantic route
south while the young travel by themselves, 2000
miles to the west, through the interior of Canada.
Yet they re-join their parents later in the Argentine.
It is their first migration. Intent must be entirely
ruled out. They cannot even be conscious of the
fact that they are travelling south or making for
the Argentine.

78 THE RIDDLE OF MIGRATION

This type of thing is matched in certain other
migrants. Thus the young of Geocarcinas, the
land-crab of the West Indies, are hatched in the sea
but when they reach a certain stage of development
they automatically take to shore. There is no
parental guidance, for the old crabs have returned
after spawning. The young desert the scores of
other species with which they are then associating
and travel inland by themselves to adopt the — for
Crustacea — unusual habitat of dry land. Whatever
it be, there is undoubtedly something in their
hereditary make-up that controls their behaviour.

In these and all similar cases, whatever the de-
tails, intent, and no doubt consciousness as well,
cannot be admitted into the argument. Many mi-
gration writers assume that birds are ''driven"
south by fall indications of coming winter. Such
an assumption credits birds with a knowledge of
affairs that they cannot possibly possess. It infers,
for instance, that they can discriminate between one
type of food shortage and another — the irremediable
shortage of fall and the temporary shortage oc-
casioned by such phenomena as summer snowstorms,
protracted droughts, fires, etc., some or all of which
must at one time or another have brought them to
temporarily straightened circumstances on their
breeding grounds but without eliciting a migratory

PAST AND PRESENT 79

response. It infers further that they know that
winter is coming, an inference equally unwarranted.
If they are birds of the year they have never ex-
perienced a winter. If they are migratory, their
parents, for countless generations before them have
left the north before winter has materialised. How
can they possibly arrive at the knowledge of the very
existence of such a thing as a northern winter? If
they do not know that it is coming, they can surely
not flee from it. It assumes still further that they
can intelligently distinguish north from south.
Why should they proceed south! Why not east or
west or north again? Whatever the solution, the
performance is not peculiar to birds. Sea-lions
annually make infallible return to their breeding
grounds from mid-ocean without any landmarks
to steer by. Like travelling penguins, their eyes
are at water level and their field of vision is negligi-
ble. Various fish find their way back to the same
spawning streams year after year. The European
eel, which breeds only once in its life, crosses the
vast Atlantic from Europe to the West Indies to
spawn there and then die. The larvae, minute
elvers, may take three years to make the return pas-
sage, but they spend their lives in European inland
waters after traversing 3000 miles of ocean that
they have never before seen. Their parents are
dead. There can be no question of guidance.

80 THE RIDDLE OF MIGRATION

When one attempts to invoke the magnetic field
one is immediately faced with serious difficulties.
The idea has many times proved attractive to in-
vestigators and experiments have been designed to
examine the concept but they have led to nothing.
Known facts of the anatomy and physiology of a
bird suggest no means by which terrestrial magnet-
ism could be perceived. The inner ear, with its
semi-circular canals might logically be surmised to
be the seat of such perception (if it exists) but evi-
dence is entirely wanting.

An attractive theory has been propounded at-
tempting to show, from the physicist's viewpoint,
that since the magnetic dip and declination vary
from place to place and offer unlimited variety of
combination, a bird could thus detect and recognise
different areas of the earth. This assumes that the
relationship between dip and declination is not
exactly similar at any two points of the globe but
even if this were not open to question, as it appears
to be, there still remains no known mode of percep-
tion by which a bird might become aware of the
situation.

Magnetic sensibility might be an integral com-
ponent of the homing sense as well as of true mi-
gration but the two things are essentially distinct.
Pigeons have been used many times for the investi-

ENVIRONMENT, PAST AND PRESENT 81

gatlon of problems of migration, but were a pigeon
not entirely non-migratory it could not be trained
to home. Only by virtue of its constant attach-
ment to one spot does it become the world's premier
homer. No less suitable subject could be selected.
When a young golden plover in its first fall leaves
the barrens it is doing exactly the opposite to homing
— it is deserting its home.

In the homing of pigeons it seems certain that
sight and topographical memory are the salient
factors. We cannot here discuss the mass of evi-
dence but when critically analysed this conclusion
is inevitable. There seems little question that most
birds have wonderfully good memories while the
occurrence of two, or in some species, three foveae in
the retina of the eye can only mean one thing — ex-
ceptionally perfect vision. It is more than probable
that many species make use of these two faculties in
migrating. Of the many interesting facts brought
out by bird banding, none is more striking than the
return of given individuals to the same nesting box
year after year, in some cases with thousands of
miles of travel to their credit during the intervening
nine or ten months. That topographical memory
is involved in a feat of this nature seems more than
likely. But the return of an adult bird to its
nesting ground is homing in essence. It has been

82 THE RIDDLE OF MIGRATION

there before and given a sufficiently good memory,
no other explanation is necessarily called for. The
classical experiments of Watson and Lashley, with
noddy and sooty terns nesting on the Tortugas Keyes
suggest that in this case homing may have involved
something more than memory but even here the
evidence is not entirely conclusive. The past his-
tory of the birds used was unknown and they may
conceivably in previous autumnal wanderings, have
covered the ground. Breeding birds were taken by
boat to Cape Hatteras (850 miles north) and liberated
here and at various points on the way, some of
them at sea and out of sight of land. By climbing
to the skies on liberation, however, land would soon
be observable. At 15,000 feet (the greatest height
at which migrating birds — cranes — have ever been
recorded) land would be visible at over 150 miles.
The sighting of landmarks by these terns can thus
not be excluded from the possibilities of the occasion.
Yet there is homing of a type from which sight
and topographical memory must apparently be
excluded. Penguins, for instance, returning to
their habitual breeding grounds cannot possibly be
making use of landmarks. Their life is spent chiefly
among drifting ice-floes. They are flightless and
cannot take observations from the air. But, like
so many winged migrants, they nevertheless make

ENVIRONMENT, PAST AND PRESENT 83

precise return to their accustomed stations. In
northern waters birds have frequently been noted
flying back to their breeding cliffs in thick fog, ap-
parently with as much certainty as though visibility
had been perfect. Such birds are homing; they are
returning to a known site.

In the juvenile migrations of many species we en-
counter something fundamentally different. The
individual traverses enormous tracts of country
that it has never before seen. Sun, stars and land-
marks can mean nothing to it. Memory, based on
personal experience, can be no part of the achieve-
ment. This is migration in its most striking form.
If we attempt to analyse it we run into one obstacle
after another.

If we term it instinctive behaviour, as we must,
we are assuming that the bird is profiting from the
experience of its ancestors, that it has what might
be termed ''inherited memory." This difficulty
applies to all examples of instinctive behaviour, not
solely migration. But of inherited memory, scienti-
fically speaking, we know nothing. We can define
instincts, (p. 87) but of the essential laws under-
lying them we are ignorant. If w^e adopt the
Lamarckian viewpoint and assume that by con-
stant repetition, generation after generation, a par-
ticular habit can ultimately become hereditary,

84 THE RIDDLE OF MIGRATION

we may be suggesting a possible mode of origin of
instinctive behaviour, but we are still left with
the much more difficult problem of how repeated
habits can impress themselves on the nervous system
and constitution and ultimately the chromosomes.
Moreover it is difficult to see how such an explana-
tion could apply to many curious instincts in the
insect world.

Subsidiary to this main problem are several minor
ones. Even if we are satisfied that topographical
memory is the mainspring of homing ability, it can-
not apply here. Again we are tempted to substitute
magnetic sensibility — the only possible proposition
in our present state of knowledge — but on top of the
difficulties already considered, we must now add an
inheritance factor. How a bird could distinguish a
particular part of the magnetic field at all, is un-
known. How it could recognise it, or find its way
to it, on its first sortie into the wide world, is even
more inexplicable. Yet it does go south rather than
north or east or west, even if its flights are nocturnal
anditcannot possibly be "following the sun." Mag-
netic sensibility is an intriguing hypothesis but the
best we can say for it is that it has never been dis-
proved. There is no evidence in its favor.

Instinctive behaviour can be modified by experi-
ence (p. 87). As far as adult birds are concerned

ENVIRONMENT, PAST AND PRESENT 85

there is no reason to exclude individual memory
from the migratory performance. There is every
reason to believe that it plays an important part
with many species, and may account not only for
preci^on in the return to a specific locality but, by
being superimposed on the inherent "sense of di-
rection," for the ultimate modification of migration
routes. Young American golden plovers on leaving
the barren lands drift south through central Canada.
They occur in large numbers all over the plains.
Their migration — an initial and instinctive perform-
ance— is a south-eastward drift on an enormous
front rather than a flight over a well defined route.
Adults turn up merely as stragglers. Old-time
hunters of the golden plover, such as G. H. Mackay,
stress the fact that the huge flocks of birds driven
inland by storms from the Atlantic in Massachusetts
in September were composed entirely of adults.
This species thus presents us with two completely
different fall routes, 2000 miles apart. The adults
traverse the ocean, the young cross the Canadian
plains. We may assume that the latter route is
instinctively adopted. Its diffuseness, the fact
that the young take it on their first migration in the
fall and all American golden plovers use it in the
spring, all suggest this. The fall route of the adults
might be explicable if we infer that the rich berry

86 THE RIDDLE OF MIGIL\TION

harvest of eastern Canada was first discovered by
accident, through wandering, that the discoverers
came to make annual use of their knowledge and in
time all adults, after deserting their young, took
the east-south road to the Argentine. This route
would thus represent a comparatively recent in-
novation, a modification of instinctive behaviour
based on experience, and on memory perpetuated by
precept from generation to generation. Underlying
it is the inherent urge to go south to the Argentine.
To summarize: Homing is a return to a place
already familiar. In a majority of cases sight and
topographical memory alone suffice to account for
the performance. But there are cases, like that of
the penguin, to which such a simple explanation
could not apply. That these birds can detect and
make use of the variations in the magnetic field is
a possible suggestion but one that must, in the
present state of knowledge, be considered entirely
hypothetical. In the case of many young birds,
migrating for the first time and alone, topographical
memory can play no part. If we invoke magnetic
sensibility, itself a mere postulate, we must add one
unknown to another, and assume that such birds
can inherit sensitivity to and recognition (not
necessarily mental) of particular components of the
magnetic field. This leaves us with a somewhat

ENVIRONMENT, PAST AND PRESENT 87

nebulous hypothesis but even if unsatisfactory it
might yet serve a useful purpose. Migrating birds
and a suitably controlled magnetic field brought
together would provide a combination with rather
interesting experimental possibilities.

If the fully established migrations with which we
are so familiar today in the northern hemisphere
depend neither on a bird's personal experience nor
on a conscious knowledge of the experience of its
ancestors or the factors of its environment, their
seasonableness, precision and accuracy, on the other
hand, can leave little doubt that they hinge in some
other way on the experience of past generations.
Such experience has not been perpetuated by word
of mouth or in writing and we must therefore as-
sume that it has been handed on by another method,
— genetically, by inheritance. We must assume
that the habit has been acquired by individuals of
the past, that it has somehow become inherent and
that it has survived because it remains of value or
is even essential, today. In short, it is now instinc-
tive.

Instinctive behaviour may be defined thus in the
words of Lloyd Morgan^ — "those complex groups of
coordinated acts which are, on their first occurrence,

^ Animal Behaviour, London.

8S THE RIDDLE OF MIGRATION

independent of experience ; which tend to the well-
being of the individual and the preservation of the
race; which are due to the cooperation of external
and internal stimuli ; which are similarly performed
by all the members of the same more or less re-
stricted group of animals; but which are subject to
variation and to subsequent modification under the
guidance of experience."

If we have satisfied ourselves that this is the cor-
rect attitude to adopt towards migration we can
proceed to analyse the implications involved. Two
readily separable aspects are concerned and for the
sake of clearness we can consider them independently.

When birds go south they do so at a specific time
of year which varies from one species to another but
is more or less constant for individuals of a given
species in a given locality. There must evidently
be one or more stimuli — external or internal or both
— that set the wheels in motion, so to speak, and
start the southward flight at the appropriate time.
Whatever the stimuli, they recur year after year
and invariably elicit a similar response. They are
annual, something of the moment and therefore
something tangible that should, theoretically at
least, be amenable to scientific analysis.

On the other hand, the series of coordinated reflex
actions called forth by the stimuli, resulting in the

ENVIRONMENT, PAST AND PRESENT 89

southward flight, evidently embodies events of
centuries and the experience of hundreds of genera-
tions of birds. Although it happens now, it is really
a summary of past experience.

We must thus enquire into two quite distinct
things (a) Factors of the past that have induced and
built up the migratory custom; (b) Factors of the
present that annually set the migratory machinery
in motion.

CHAPTER III

The Evolution of Migrations

It has been pointed out above that migrations are
being elaborated today and that we can see some-
thing of the process. Further, there is no reason to
beheve that the forces now at work are different
from those in operation in the early days of bird
evolution when the necessity for migration must
first have arisen. We may therefore commence by
examining, once again, the present state of certain
affairs in the northern section of the globe.

There are some few species of northern birds that
we call migratory to which the term is not as un-
reservedly applicable as it is to the great majority.
The most outstanding of these is the mallard.
Although many migrants annually leave a few
individuals behind them in the fall there is reason
to believe that in most cases the laggards are
suffering from some anatomical defect that either
makes them heedless of the calls of migration or
unable to migrate. But a percentage of mallards,
under certain conditions quite a large one, appears
to be actually loath to leave the north. The species
is invariably the last to vacate the Province of

90

THE EVOLUTION OF MIGRATIONS 91

Alberta, and a certain number of birds always stay
where open water and food remain available. In
years in which the fall is late and open, a far larger
number stay behind. Alberta mallards are mainly
grain eaters in the fall and, regardless of the type of
water they may be frequenting (i.e. it may, or
may not, provide food) they have our expansive
wheat fields to afford them unlimited sustenance.
They appear to be unaffected by severe cold but
should a heavy snowfall obliterate the fallen grain
in the stubble, they speedily starve to death unless
artificially fed. In this particular instance, failure
of their food supply is the lethal factor. Central
Alberta alone may be snow covered, or the entire
Province, or even the States south of the Canadian
boundary. Every normal winter sees the exter-
mination of a certain number of mallards but every
winter, on the other hand, also sees the survival of
others, birds, for instance, that frequent certain
hot-springs that never freeze over and from which
they derive an adequate food supply. These
birds do not go south and as long as they survive
and reproduce they will add to the number of mal-
lards that may be termed resident. The number
can never increase beyond certain limits because
the favorable waters are restricted and those win-
tering elsewhere are more than likely to be wiped

92 THE RIDDLE OF MIGRATION

out. The ones that go somewhat further south face
a diminished risk; those that go still further may run
no risk at all apart from exceptional winters.
There is thus set up at the hands of the winter fates
a system of selection. Resident mallards can never
become numerous. Partial migrants stand a better
chance while complete migrants get the best chance
of all. They will always be the dominating class as
long as the Alberta climate remains what it now is.
In particular sections of the north the case is
otherwise. Mallards are resident in Alaska and
Iceland, while Greenland actually boasts a distinc-
tive resident sub-species, quite within the realm of
legitimate expectation if the area involved (as is the
case in Greenland) supports a sufficiently large popu-
lation. Breeding isolation will probably result and
sub-specific differentiation become likely. The
mallard, like several other ducks, can feed at night
(and does so habitually in Alberta) and as long as
open water providing enough food for continued
sustenance, containing a sufficiency of vitamins,
is available and if the species can sustain the pre-
vailing temperatures, there would be nothing to
force migration on the race. They would survive
as residents. These conditions are but sparsely and
very locally fulfilled in Alberta. Numerical re-
strictions entirely preclude the development of a

THE EVOLUTION OF MIGRATIONS 93

resident Alberta race. The small numbers are
annually swamped by the southern migratory
hordes coming up each spring from the south.

The mallard thus comes to be represented by
races — not necessarily distinguishable morphologi-
cally— which differ in their migration dates and dis-
tances and perhaps routes. For reasons considered
in detail below, the same applies to all species of
wide range. Each community has worked out its
own salvation at the dictates of local conditions
and the migrations of any given species as a whole
thus come to show infinite local variation in detail.
Migrations of a single species of wide range cannot
be accurately described in a single comprehensive
statement.

The failure of some mallards to go south and their
subsequent extinction affords an admirable example
of the working of natural selection, a basic principle
in the evolution of migrations. With reference to
normal winter conditions in Alberta the mallard is
unfit; it fails to survive. The principle applies to
all birds in the northern hemisphere (as elsewhere)
though the food supply is not inevitably the de-
termining factor. It may be low temperatures,
water supply, ultra-violet radiation, shortage of
daylight or something else. It may be obvious or
it may be apparently insignificant, difftcult to detect

94 THE RIDDLE OF MIGRATION

and perhaps depending, like Darwin's classical ex-
ample of the clover crop, on a network of other
factors. Each species has to face the selection
committee of the northern elements to be judged
on its own merits. If it fails to meet the require-
ments it has no place in the north during the winter
season. Every year sees a few individuals of vari-
ious species failing, for one reason or another, to go
south. Every normal year witnesses their exter-
mination. During the exceptional winters they
survive for varying periods and their presence is
duly commented on. The mild winter of 1930-31,
unprecedented within living memory, provided a
particularly good example. Several thousand mal-
lards survived. Even the odd robin and meadow-
lark seem to have got through successfully. But
this happens only once in a long run of years and
the survivors, if any, count for nothing. They are
not even a drop in the bucket of competition.

As has been mentioned, the selective factor is
not necessarily obvious. This remark would apply
to ultra-violet radiation. Deficiency would not
affect the individual in the same way as food short-
age for it would not prove immediately fatal. The
bird, other things being favorable, would still be
there in the spring. But its general vitality might
conceivably be affected seriously enough to produce

THE EVOLUTION OF MIGRATIONS 95

unfitness in its progeny, or like my captive juncos,
it might fail to produce young. One or more genera-
tions might thus result in a race quite unequal to
the stringencies of a northern winter and so the
species would again, in final analysis, be represented
only by individuals that went far enough south to
meet their ultra-violet requirements. Ultimate
elimination from the north during winter would
be as certain as though immediate starvation of the
individual were its fate.

Winter life in the north is impossible for most
birds. If they stay, their end is death. But this
statement offers no explanation as to why or how
they have ever found their way south . They cannot
sum up their environment and decide to leave after
the manner of men. They cannot tell that the
south, and the south alone, spells salvation. The
d'iscovery must have been entirely fortuitous.

Before attempting to investigate this phase of the
question, however, we may profitably turn to a con-
sideration of southern conditions to see if there is
any incentive for birds of the south to vacate this
region in the spring. To take a concrete example,
let us imagine that the present wintering head-
quarters of the Lapland longspur (Calcarius lap-
ponicus), the middle States and Texas, represented
the original breeding area of the species in the

96 THE RIDDLE OF MIGRATION

middle of the Pliocene. The climate was then
cooling and was probably not very different from
that of today. Here, let us suppose, the birds bred
in the spring and remained in the winter. If con-
ditions were favorable, their numbers must have
shown a steady increase and their breeding area must
have slowly extended in all directions, north, south,
east, and west, but later only north and east, the
mountains in the west and the Gulf of Mexico to the
south providing barriers in these directions. That
such spreading would actually have occurred we
can hardly question, for this is exactly what is
happening today in other cases. Introduced birds in
all parts of the globe have illustrated the principle
many times over. The European starling (Sternus
vulgaris) in the eastern States and the Hungarian
partridge (Perdix perdix) in western Canada are
two cases in point at the moment. The starling has
found an effective barrier in the Atlantic on its
eastern front; the partridge has met the mountains
on the west. Spread has therefore been in the other
directions only. The partridge was introduced at
Calgary, Alberta in 1908 and 1909. By 1930 it had
gone north to Fort McMurray (450 miles) and east
to central Saskatchewan (300 miles) and south at
least 200 miles. Every suitable locality traversed
in its spread now boasts a heavy population of

THE EVOLUTION OF MIGRATIONS 97

partridges. The starling was released in New York
city in 1890 and 1891. It has now reached north
to Canada, south to South Carolina and west almost
to the Mississippi. It has as a rule appeared spo-
radically some years ahead of permanent establish-
ment in the various places it has progressively
reached. Such spreading is assured if a bird is once
successfully established and has an adequate rate
of reproduction and survival. The demands of
territory, competition for nesting sites and food and
other factors inevitably bring it about.

The Hungarian partridge is non -migratory. As
yet it has encountered no obstacles in its rapid
northward spread. How far north winter conditions
will continue tolerable remains to be seen. In vari-
ous parts of the west the mourning dove (Zenaidura
macroura) and black-billed cuckoo (Coccyzus eryth-
rophthalmus) are at present extending their range
northwards. These birds are migratory. New ac-
cessions to their breeding range must be vacated for
the winter. Their slow rate of progress is in great
contrast to that of the partridge. As illustrations
to the argument outlined below they are the more
fitting. But they are migrants of long standing
and their present rate of spread must be infinitely
faster than it was in the bygone centuries when
migration, as a custom, was first being established.

98 THE RIDDLE OF MIGRATION

Let US assume, then, that the Pliocene buntings
were forced to spread. As the centuries rolled by
the birds reached the Canadian plains and began to
feel the selective effects of a winter that they could
not tolerate. The birds that remained were wiped
out. Others, again under pressure, would replace
them the following spring and breed. These birds,
as they spread north, encountered summer con-
ditions progressively more favorable than the con-
ditions under which the southern contingent were
rearing their broods. They had little competition,
food must have been all but unlimited and they had
in addition an appreciably longer period of day-
length in which to gather it for their growing young.
Under these circumstances their numbers must
almost unquestionably have increased at a greater
rate than those of their southern allies. At all events
among the passerine species of today, the northern
representatives of a majority of genera either lay
more eggs to a clutch than the southern, or they
rear two or more broods in place of one. Such a
state of affairs can hardly be coincidence and we can
quite safely attribute it to a particularly favorable
breeding environment. The net result of the situa-
tion would be that the northern birds were increas-
ing at a greater rate than the southern but they
could not survive the winters unless they happened

THE EVOLUTION OF MIGRATIONS 99

to go far enough south in the fall to escape the
selective elements. We are familiar today with
the fact that many species of birds wander in the
fall to all points of the compass. Gulls and herons
provide examples on the most striking scale though
such everyday occurrences as the crowding of birds
to a suddenly available source of food supply indicate
extensive random wandering on the part of many
species and at almost all seasons. Every winter a
certain percentage of longspurs must have wandered
south and survived while others moved north, east
and west and perished. Winter would find the
southern suvivors in a region more or less crowded.
With the reawakening of sexual impulses in the
spring, spreading and dispersal would again be im-
posed and some individuals would inevitably return
to the north. From this fortuitous battledore and
shuttlecock state of affairs a north-and-south swing,
synchronous with the seasons, must sooner or later
have established itself. That it involved the loss
of incredible numbers of birds or took hundreds or
even thousands of years to set up is immaterial.
Millions of birds and millions of years have been
available.

The argument assumes that neither intelligence,
intention nor conscious understanding of the factors
of the environment were involved on the part of the

100 THE RIDDLE OF MIGRATION

longspurs. Yet this may be an extreme attitude
to adopt, for the return to the north with the re-
estabHshment of spring conditions and sex fever may
conceivably have been achieved, in part at least,
with the aid of memory and a rudimentary intelli-
gence. Similarly the presence of the sun, always to
the south during the major portion of the day, may
have been a sort of general drawing card to that
point of the compass during the shortening days of
autumn. Such factors might materially have has-
tened the process, but the assumption is superfluous.
But the present distribution of the Lapland long-
spur includes only northern breeders wintering in
the southern and central States. The explanation
of such a distribution can, of course, never be
satisfactorily demonstrated; but with a migratory
swing once established, each spring would see a
spreading further and further north as long as sum-
mer conditions remained favorable in all respects.
Natural selection however is constantly and every-
where in force and selects as certainly in the sum-
mer as in the winter. The selective forces, even
today, are in many cases extremely difficult or even
impossible to detect. The crow (Corvus brachyhryn-
chos) is just now getting consistently more abundant
in western Canada in spite of the heaviest persecu-
tion that man can devise. Some altered factor in

THE EVOLUTION OF MIGRATIONS 101

the western environment has greatly increased its
survival rate and the extermination of many thou-
sands annually at the hand of man has failed to prove
an effective check. Whether it is increased culti-
vation, as has frequently been suggested, is proble-
matical. There is evidence that 35 years ago, be-
fore general cultivation had begun, crows were more
abundant than 20 years ago. The magpie, we know,
has in recent years returned to territory from which
it had wholly disappeared some 30 years previously.
The Canadian West, incidentally, is now nearly as
dry and waterless as it was in the nineties. Ducks,
comparatively scarce then, are again greatly re-
duced. The environmental factor, even though in
our limited state of knowledge we may fail to detect
it, that proves beneficial to one species may prove
detrimental to another. With ducks getting scarcer,
crows and magpies are increasing. The cessation
of breeding in the southern range of the longspur
may have been due to one or several of many fac-
tors but even though it would be the merest guess-
work to attempt to consign them, we can rest as-
sured that selection has, in one way or another,
decreed the situation. Changes on the face of the
North American continent have been considerable
at different periods. What was luxuriant forest
once is now grassy prairie or even desert. What is

102 THE RIDDLE OF MIGRATION

at present a heavily forested area was, only a few
thousand years ago, a continuous sheet of ice.
With each local shift of environment the factors of
selection must have changed for that district. On
such changes probably hinge the details of the mi-
gratory movements of the Lapland longspur and the
same may be said of many another northern
migration.

In this hypothetical reconstruction of the origin
of the migratory habits of the longspur, a number
of assumptions have been made. They are not of
equal value and we may briefly review them.

The operation of the laws of natural selection
have been taken for granted. They represent a
biological principle the workings of which can be
seen on all sides and at all times. In this assump-
tion we are assuredly justified.

We have assumed that the rate of reproduction
of the longspur must have been such as to force it
to spread from an original center. This assumption
is subject to no serious objection for we know that
exactly this happens in the case of many other birds
today.

We have assumed that longspurs have wandered
in every direction at the beginning and close of the
breeding season. We know that such wandering
is characteristic of most — possibly all — species
today.

THE EVOLUTION OF MIGRATIONS 103

Finally, we have assumed that the constant repe-
tition of a north and south oscillation has finally
established the tendency as an inherited instinct.
In the light of present biological knowledge such
an assumption is almost wholly unwarranted. It
supposes that the Lamarckian hypothesis — that
acquired characteristics can be inherited — is accep-
table. Experiments of great variety and ingenuity
have been devised to put this conception to the test
but none has been a convincing success. Yet there
is this to be said. Failure to prove a given hypothesis
is an entirely different thing from disproving it and
if we may not accept the Lamarckain view as es-
tablished we are still fully entitled to consider it an
open question. It has admittedly never been
proved, but neither has it been disproved. The
answer is in the lap of the gods; the solution lies in
the future. But migration must inevitably be an
acquired characteristic even if it is not inherited.
The ancestors of birds — reptiles, sluggish and earth-
bound — could hardly have been migratory although
there are migrants among modern reptiles. Archae-
opteryx and Archaeornis vj^re dXmost certainly sede-
tary. Ichthyornis must have been strong on the wing
but the environment of the Cretaceous probably did
not demand migration, a necessity imposed by local
circumstances. With a few exceptions recent birds

104 THE RIDDLE OF MIGRATION

possess an inherent ability to migrate; by far the
greater number of those that have tapped the re-
sources of the north have been compelled to make
use of that ability. Some time during past ages the
custom has been acquired. Whether or not it is
now hereditary is a topic of paramount importance
to which we shall return later.

Migrations of the northern hemisphere only have
been thus far dealt with. Given an animal like a
bird with its great powers of flight and a persistent
tendency to wander, natural selection alone can
completely account for the facts, provided that an
acquired habit ca7i become inherited custom. As has
already been remarked, the same laws, although
conditions may be entirely different, must apply
elsewhere, in which case the results will also differ.

Let us examine the circumstances of a subtropical
species that winters on the plains and migrates into
the mountains to breed. Starting with random
wandering as before, if selection establishes a higher
rate of winter survival on the plains and a more
successful summer rate of reproduction in the
mountains, providing there is a periodic wandering
from the one zone into the other, migrations from
the plains to the mountains and back again will in
time become established. Such migrations will
have no particular relationship to the four points of

THE EVOLUTION OF MIGRATIONS 105

the compass since none of them is involved as one
of the selective factors but the fundamental prin-
ciple of natural selection is still the organizing force.
Migrations within the tropics are equally subject
to the sifting of selection, but the factors involved
are again different. Here, where the environment
is favorable the year round, wandering is both fea-
sible and likely on a magnificent scale impossible
elsewhere. Movements may depend on the rainy
season, or the dry, the windy or the calm, but they
need not necessarily amount to anything more
than wandering. There are probably few eliminat-
ing factors climatic in nature and in many cases
it will make no difference to the welfare of the race
if a species breeds in a given locality a few hun-
dred miles north, south, east, or west. Hence a
random following of the food supply may become
the rule. The struggle for existence may be keen
and competition particularly severe in country
climatically equable the year round. Both would
be factors in reducing the rate of increase and de-
laying, perhaps even eliminating, the expansion
that might ultimately push the species far enough
north or south to induce true migration. Omnivo-
rous feeders might well, under such circumstances,
provide entirely sedentary races, while specialised
feeders would produce wandering forms. Such

106 THE RIDDLE OF MIGRATION

vagrants, In regions such as those of the trade winds
which blow with marked regularity and produce
seasonal rains, might well convey the impression of
being true migrants. Their movements would take
on a suggestive periodicity that might, on final
analysis, even prove to be true migration.

A non-competitive environment, with all require-
ments found in a circumscribed area ultimately
leads in birds, to the condition furthest removed
from migration — loss of the power of flight.

The great chasm between conditions in the
tropics and in the far north is in no respect more
emphasised than in breeding dates. Many, perhaps
all, tropical species show a breeding rhythm but
there is no agreement in dates. A given species
may breed from January to July, another from July
to December, but birds' eggs may be found at all
times of the year. In the far north the breeding
season of hundreds of species is synchronous and
crowded into a few short weeks. There is no sea-
sonal selection in the tropics to compare with that of
the north.

The operation of selection is in no case better in-
stanced than among the crepuscular nightjars. The
most diurnal of these on the north American conti-
nent, the common nighthawk {Chordeiles virginianus)
ranges further north than any of the other species.

THE EVOLUTION OF MIGRATIONS 107

Unduly lengthened days would have as detrimental
an effect on crepuscular species as protracted nights
on the diurnal. As summer birds they have no
place in the far north.

Assuming again that the migratory rhythm can
become inherent, the enormous migration of some
of the far northern breeders — golden plover, Arctic
tern, etc. — become comprehensible. They are
merely an exaggerated form of lesser migrations and
any able flyer overshooting the mark of the original
wintering ground on its road south would tend to-
ward such exaggeratipn. If repeated sufficiently
often such an expanded migration might become the
custom. Its success would depend on the nature of
the territory added. If no antagonistic factor were
encountered, the addition would have no detrimen-
tal effects on the numerical status of the species and
would not be eliminated by selection. Among the
most powerful flyers we might thus expect to find
what we actually get, an annual trip from virtually
one end of the globe to the other and back again.
As was pointed out earlier, many species are in-
cessantly on the wing and whether the daily mileage
is done more or less in one direction or in circles has
no bearing on the amount of energy expended.

Finally, an exceptional type of migration must be
noted. The flight of the Pacific golden plover from

108 THE RIDDLE OF MIGRATION

Alaska to Hawaii, 2000 miles across the Pacific
Ocean, can obviously not have developed by a proc-
ess of gradual spreading. It suggests a route of
sudden origin which is actually what it may have
been, possibly an incidental outcome of extensive
glaciation. But we must of necessity make the as-
sumption that this plover, before its present distri-
bution was adopted, was already a long-established
migrant. Its venture would surely have ended in
complete catastrophe had the case been otherwise.
The same may be said of the turnstone and other
shore-birds reaching Hawaii regularly. Sudden
enormous movements of birds over great distances
are not unknown tocJay. The periodic invasions of
France and Britain by large numbers of Pallas' sand-
grouse {Syrrhaptes paradoxus) from central Asia
afford one example; the precipitate arrival of hun-
dreds of European lapwings (Vanellus vanellus) on
the shores of Canada (Newfoundland) in 1927 offers
another. Neither the sand-grouse nor the peewit
are, however, great migrants and their sudden ex-
cursions have merely led to failure and death. But
these cases raise the possibility of an entirely dif-
ferent spontaneous origin for certain migrations, or,
more correctly, for certain migratory routes and
present distribution. But they can hardly be of
universal or even of wide application.

THE EVOLUTION OF MIGRATIONS 109

It will be seen, then, that the necessity to migrate
must have existed before the ice-age if birds had at
that time reached the northern sections of the north-
ern hemisphere. Towards the end of the Pliocene,
when modern birds were fully established and the
climate was much as it is now, a majority of species
must already have been fully migratory. The
southward march of the ice-fields may have crowded
most of them into the southern section of the North
American continent but the subsequent retreat of
the ice would have enabled the entire story to repeat
itself — the spreading north, the winter extermina-
tions and the final establishment of the migratory
swing. Whatever the climates at different periods,
day-lengths have undergone no change and the
species that now require a certain minimum amount
of daylight for feeding or of ultra-violet radiation
must have needed it then. Various species may
have continued to use the far North as summer
quarters even at the height of the various glaciations
and there is, indeed, indirect evidence of this in their
present migratory routes. But in either case the
ice-age has had no miraculous effect on birds and
there is not the least necessity to postulate that
birds have returned north after the various periods
of glaciation because they had for hundreds of
generations retained a memory of pre-glacial homes.

110 THE RIDDLE OF MIGRATION

There is not the sHghtest evidence that any such
thing is possible in any group of animals known and
no occasion to assume it.

The same general principles that may be held to
account for the evolution of northern migrations
can be applied to migrations elsewhere, but a wholly
different environment will produce wholly different
results.

CHAPTER IV

Annual Migrations

We can now pay some attention to the possible
stimuli that call forth the migratory impulse twice
annually with special reference to conditions as we
see them in the North. In other parts of the globe
where migrations occur the effective stimuli need
not necessarily be the same. It is, indeed, more
than likely that they will differ but the general
principle will be similar.

All told there are about 30 species of birds that
winter in the Edmonton district and to the north,
out of a total list of some 300 — approximately 10
per cent ; or to put it another way, nearly 90 per cent
of the birds occurring in Alberta and northwards
are migratory. This includes Alberta passage
migrants, species breeding in the Arctic. Although
the southward passage is not as precisely according
to schedule as the northern it is, nevertheless,
strikingly accurate with many species. That it
should be more protracted and spread out is to be
expected for young, which with many species travel
later than their parents, are involved and in other
cases the adults themselves may be seriously held

111

112 THE RIDDLE OF MIGRATION

back by the young. Thus crows, for instance,
destroying eggs of ducks, grebes, and other birds,
force repeated laying and incubation on many indi-
viduals of these species and so delay their normal
southward passage. The whole migration thus
becomes lengthened. Wetmore points out (p. 178)
that on September 6, 1920, when he noted golden
plovers in Paraguay, the same species was on the
same date recorded from Lake Athabasca (Alberta) ,
6,000 miles farther north. The latter were un-
doubtedly young, the former probably adults.

Breeding birds of Alberta, whose departure can be
accurately noted, are on the whole astonishingly
precise in their departure dates. Crows are a good
example. As a general thing one may be quite
certain that the main exodus will commence during
the last half of August and will be virtually over
by the 10th of September. The birds begin to con-
gregate in common roosts in July, the numbers get-
ting greater as the later families are added. About
the 20th of August the first bands pull out, the roost
continuing to be occupied by ever-decreasing num-
bers till the last contingent leaves. As the birds
forgather at sundown there may be, on any given
night, hundreds less than there were the previous
night. This happens simultaneously throughout
central Alberta. In the last week of March of the

ANNUAL MIGRATIONS 113

year following the first stragglers are back; ten days
later the country is full of them. When one con-
siders the vagaries of the climate, their precision is
the more astonishing. Yet crows are typical of an
overwhelming majority of northern migrants. Ac-
curacy is particularly marked in many species of
shore-birds returning north from South America and
in the stronger flyers such as ducks, gulls, geese, and
hawks. Factors that may interfere with the prog-
ress of migration are well known and need hardly
be detailed. Here it is only wished to stress one of
the few facts on which there appears to be universal
agreement among migration writers, the remarkable
observance of dates shown by northern birds return-
ing to and departing from their breeding grounds.
With this fact in mind the hunt for a possible
stimulus involves a narrowed field. Any of the
factors already considered that make it impossible
for one species or another to survive the winter in
the North, might prove to be the key for the south-
w^ard migration but, with two exceptions, they are
all notably unstable, varying greatly from year to
year. That they have all been instrumental in
bringing migrations about there can be no doubt.
But there is no particular reason why any of them
should now be the stimulating factor. Let us con-
sider them individually.

114 THE RIDDLE OF MIGRATION

Failure of the food supply has been perhaps most
frequently assumed to provide the stimulus. That
this cannot be the case with many species is patent
since they leave in July before the supply has at-
tained its peak. Food supplies are irregular, vary-
ing greatly in their dates of maturing and there are
years when they may be an almost complete failure.
It is extremely unlikely that they provide the stimu-
lus for a phenomenon that is outstandingly regular.

Temperatures probably come next in popularity.
Our early migrants must again be excluded. As to
the later ones, we may get zero temperatures before
September is out or we may not get them until
January. Some years not even the smallest sloughs
may be frozen over by the beginning of November.
Periodically we get a killing frost in June or August
or on rare occasions even in July. Temperatures
can hardly be considered fundamental.

Barometric pressure has been assumed to be the
stimulus by a number of writers. It has been
shown by various observers that high pressures may
precipitate both the northern and the southern
migrations (particularly the latter) but neither high
nor low pressures instigate migration during the
sedentary periods of summer and winter. We can
therefore consider pressure effects as incidental at
best. They may, and sometimes do, speed up

ANNUAL MIGRATIONS 115

migration, but they cannot be the fundamental
stimulus. Migrations would be occurring at all
times of the year were this the case.

Change of color in the leaves of trees has been
suggested. It has been experimentally shown that
birds can appreciate the yellow end of the spectrum
and from this viewpoint there is no difficulty. But
birds that should leave us in September, let us say,
would find themselves in a serious quandary in the
years that have experienced early frosts and have
witnessed the falling of the leaves in August. July
migrants have probably never in their lives seen the
autumnal tints of the northern woodlands. This
suggestion seems to be wholly untenable.

Ultra-violet radiation offers a factor of the envi-
ronment that is presumably, though by no means
certainly, stable but we know of no mechanism
whereby birds could appreciate its seasonal varia-
tions with sufficient promptness or intensity to
render them effective as stimuli.

The one factor of the northern environment that
is thoroughly dependable and certainly without
fluctuation is the seasonal variation in day-length.
Daylight (with its sunshine) is of vital importance
to most members of the animal kingdom and exerts
a profound effect on life generally. It has induced
curious rhythms in living protoplasm from bacteria

116 THE RIDDLE OF MIGRATION

to man. Nearly all animal activities are related in
one way or another, directly or indirectly, to the
influence of the sun. If a species leaves Alberta
habitually in the first week of September it may
leave (in different years) with the barometer either
high or low; in depressingly warm weather or with
the smaller lakes frozen over; the earth may be sun-
baked or the very gopher holes may be spouting
water; the leaves may be golden or they may have
fallen weeks before ; the food supply may be abun-
dant or it may have failed or be completely covered
by a pall of snow. Only one factor of the environ-
ment would be certainly constant — the length of
day. Its dependability suggests it as the inaugurat-
ing principle.

That many species of migratory birds when in
captivity exhibit unusual excitement and restless-
ness during their normal migratory period is well
known to all aviculturists. Only two factors of the
outside environment can effect them indoors — baro-
metric pressure and day-length. The former alone
cannot be held to elicit the symptoms. Day-length
again suggests itself as the probable key. More-
over, recent investigations by different workers on
the roosting, waking and singing periods of various
species have demonstrated remarkable sensitivity
to variations in light intensity.

ANNUAL MIGRATIONS 117

Supposing then, for the sake of formulating a
working hypothesis, that we adopt the view that
day-length provides the environmental stimulus
that arouses the autumnal migratory impulse.
This in itself is not enough for we know that a bird
cannot consciously evaluate day-length. We must
therefore leave intellect and deliberation out of the
argument. As a substitute for mental we may
legitimately postulate physiological control. We
have already seen that the reproductive hormones
can evoke particular modes of instinctive behaviour
and since, according to our definition (p. 46)
migration involves reproduction, we are justified in
considering it to be a particular phase of sexual
behaviour. A gonadal hormone, elaborated at a
specific season — the time of migration — might thus
provide the stipulated physiological stimulus.

The suggestion becomes more attractive when we
recall that the reproductive organs of birds exhibit
a very marked rhythm, synchronous with the migra-
tions themselves. The northward journey coin-
cides with enlargement: the southward with dimi-
nution. The noticeable anatomical changes might
well be accompanied by physiological. Appear-
ances, at least, support our hypothesis.

To complete the chain, there would still remain to
determine the actual mechanism by which an altera-

118 THE RIDDLE OF MIGRATION

tion in day-length could regulate the production
of a hormone, a distinctly complex problem. For
the moment we can do no better than postpone its
consideration.

Before proceeding, let us recapitulate the argu-
ment. Northern migrations were evolved and
became established as respective species independ-
ently invaded the north. The process may have
taken many centuries, but as we see them today
migrations are thousands, perhaps millions of years
old and fall into the category of instinctive be-
haviour. Just as other instincts lie dormant till
some external stimulus elicits response, so with the
migratory passage. By elimination, on various
grounds, we have selected day-length as the most
plausible environmental stimulus and have assumed
that it controls the developmental condition of the
gonads. We have further assumed that a hormone,
which provides the physiological stimulus to migra-
tion, is elaborated when the gonads are in a par-
ticular phase of their cycle, i.e. either increasing
or decreasing.

It was in the hope of ultimately finding some way
of applying the experimental method to the subject
of migration, that a study of the field aspects, par-
ticularly those obtaining in the northern hemisphere,
was begun a good many years ago. The conception

ANNUAL MIGRATIONS 119

of migration outlined above has been the outcome.
It may be right or wrong but has the merit of being
amenable to experimental analysis. It would be
a quite simple matter, for instance, to control day-
lengths artificially, to observe if such manipulation
has any effect on the development of the gonads, to
examine these under the microscope for changes in
the interstitial tissue and, if changes were noted, to
liberate the birds and observe possible modifications
of behaviour that might be correlated with histo-
logical changes. It might even be possible to
induce reversed migration experimentally and
thereby procure tangible evidence that migration is
an inherited custom.

In 1924 our first aviaries were erected. The bird
used was the junco {Junco hyemalis connectens) a
regular migrant, readily trapped; not pugnacious,
hardy and thriving in captivity. The last, and
most successful, series of experiments with juncos
was conducted in the fall and winter of 1927-8.
The birds were housed in a large and well designed
aviary the erection of which was made possible
through research grants from the Royal Society of
London. The winter turned out to be particularly
severe, thereby adding considerably to the value of
the results obtained.

The general plan of the experiments was as fol-

120 THE RIDDLE OF MIGRATION

lows : Two aviaries were used , the one provided with
electric light, the other not. Birds housed in the
former will be referred to below as the experimen-
tals, those in the other (unlit) as the controls. The
lights consisted of ordinary frosted electric globes
totalling 1050 watts, distributed in such a way as to
ensure uniform lighting throughout the aviary.
The illumination was not intense but sufficiently
strong to enable minute details to be readily made
out on the ground without stooping. Lighting was
started at the beginning of November (1927)run-
ning, on the initial day, from sunset, 5.05 p.m., till
6 p.m. The following day the lights went on again
at sunset (as daily thereafter) but ran till 7 J minutes
after six; the next day till 6.15 and so on — an increase
of 7| minutes each night. Sunset was chosen as
lighting-up time as the birds were then still fully
active even on cloudy days. The interval of 7J
minutes, after deducting the 2 or 2| minutes of later
sunrise each morning, gave an effective daily in-
crease of about 5 minutes, roughly equal to the
spring increase of day-length experienced by north-
bound juncos in southern and central Alberta.

From December 3 onwards the interval was
reduced to 5 minutes daily till December 15 when
the lights were burning from sunset till 11 p.m.
Thereafter increases were discontinued, the lights

ANNUAL MIGRATIONS 121

going out nightly at 11 till January 9 (1928) when
the experiment terminated.

The experimental were thus artificially provided
with approximately spring conditions as far as
illumination was concerned. In the matter of heat-
ing they received what the fates might contribute.
The aviaries were unheated, removed from all extra-
neous sources of warmth and wide open on two sides
to the weather. Eleven days of November regis-
tered temperatures below zero (with a minimum of
-23°F.); December, the severest recorded in 32
years, produced 23 days with the thermometer
below zero (minimum — 44°F.).

Keeping the birds in unheated cages in the open
served two purposes. It demonstrated their re-
sistance to extreme cold and disproved the generally
accepted view that the spring recrudescence of the
gonads is attributable to rising temperatures. This
opinion is universially adopted in popular literature
and has been expressed by several scientific investi-
gators. In the hypothesis outlined above it was
assumed that light, not temperature, controlled the
changes. By exposing the birds to the extreme tem-
peratures of an Alberta winter it was obviously put-
ting this viewpoint to the severest possible test. In
this regard the results of the 1927 experiment were
very striking. By January 9, when the experiment

122 THE RIDDLE OF MIGRATION

was over, the gonads of the experimentals had
attained the spring maximum. They increased in
volume particularly rapidly towards the end (as is
also the case with wild juncos in the spring) the
minimum temperatures during the 7 days of most
rapid growth being respectively 21, 31, 44, 37, 36,
20 and 4 degrees helow zero Fahrenheit. A more
convincing refutation of the increasing temperature
conception could hardly be wished for.

The controls, in the meantime, were receiving no
artificial light. They were subjected to the nor-
mally decreasing days of an Alberta autumn. In
all other respects, unlimited food supply, tempera-
tures, etc., their environment was identical with
that of the experimentals. Samples were taken at
regular intervals from both aviaries and the gonads
preserved, measured and sectioned for the micro-
scope. Gonads of the controls, already quite small
when the birds were trapped at the end of Sep-
tember, continued to diminish till they reached the
winter minimum in November where they remained
till the end of February.

Various modifications of this experiment were
conducted. The remarkable dependence of the
state of development of the gonads of the junco on
lighting conditions was strikingly illustrated by
some of the birds that had been brought to spring

ANNUAL MIGRATIONS 123

condition as recited above, and thereafter (from
January 9) kept in an aviary in which it was possible
to reduce the lighting in stages. By February 13
their gonads had returned to the winter condition.
They were then turned into another aviary where
they received normal daylight then increasing at
about 4 minutes per day. The last samples, killed
May 30, were again in full breeding condition.
Assuming that these birds had bred in the summer
of 1927 (they were adults) before being trapped in
September, their gonads thus dropped from the
maximum in June to the minimum at the beginning
of November, were brought back to the maximum
by the beginning of January, again reduced to the
minimum by mid-February, returning once more to
the maximum in May.

Ordinary electric light bulbs emit no ultra-violet
rays. The results obtained could therefore not be
attributed to ultra-violet radiation. The question
as to exactly what might account for them was
therefore of particular interest. All the evidence
suggested that the solution might be found in the
amount of exercise the birds were getting. By
curtailing the day-length, exercise would be re-
duced; by extending it, it would be increased, pro-
viding the birds kept awake during the available
period. This idea could obviously be put to a

124

THE RIDDLE OF MIGRATION

simple test. Two cages were built of identical
dimensions and placed in an empty room with a
single window that could be completely shuttered if
desired. Both were similarly fitted up with perches,
food, water troughs, etc. Juncos were put into

c A

Fig. 11. The Compulsory Exercise Cage
yl, perch, running the length of cage; B, food and water troughs;
C, CS travelling bars sweeping the perch and floor at regular
intervals. Reprinted from Nature ^ July 7, 1928.

each. The only difference between the cages was
this, that one of them was provided with a travelling
bar (painted white) which moved along the floor,
leaving the cage at the side and travelling up the
outside, then re-entering it and sweeping the perches,

ANNUAL MIGRATIONS 125

returning to the floor and thence along it and so on.
Nowhere in the cage could a bird sit still for more
than 20 seconds at a time as it was moved on by the
travelling bar. When the juncos had been trained
to the movement in full daylight, the room was
shuttered from 6 in the evening till 9 the following
morning. The room was then completely dark but
for a small 5 watt electric-light bulb suspended from
the ceiling. This provided just enough glow for
the birds to see the advancing bar. It was so feeble
that the control birds in the other cage, getting the
same illumination fell asleep in spite of it. The first
night the mechanism ran for 7| minutes, the next
for 15 and so on, the 7^-minute increase being
adhered to till the termination of the experiment.
In brief, the rate of development of the gonads
exactly paralleled that of the birds getting similar
periods of extended illumination outside, while the
gonads of the caged controls remained stationary.
It was impossible to run the machinery in complete
darkness as it would have meant certain death for
the birds. The illumination, however, was such as
to preclude any question of radiation. The length
of time spent in activity, rather than the amount of
exercise obtained (which must have been slight
owing to space limitations) seems to be the crucial
factor. Here then, apparently, is the missing link

126 THE RIDDLE OF MIGRATION

in our chain that connects gonad changes with varia-
tions in day-lengths.i

Microscopic examination was made of the repro-

^ Trans-equatorial migrants appear to present an obstacle to
the theory here outlined. When they cross the equator in Sep-
tember or October they are encountering day-lengths that are
increasing. The effect of this on j uncos and similar northern
birds would be induced recrudescence of the gonads but there is
no such response in trans-equatorial migrants. Their organs
reach the minimum and remain there through the northern winter
months in spite of the fact that the birds are then actually enjoy-
ing the long days of the southern summer. If one supposes that
in these species the annual rhythm has become stabilized and is
not susceptible to interruption by exposure to adverse light con-
ditions for a single winter, then sojourn in the south would have
no effect. Yet compulsory retention in the south for a period of
years might prove a very different thing. Deciduous trees, for
instance, taken from the northern hemisphere to the tropics,
retain the habit of shedding their leaves during the northern
autumn months, but only for a few years. Thereafter the pe-
riodicity ceases. The leaves are cast at any time of the year.
The timing elements of the north are wanting in the tropics.
The annual rhythm, although persistent for a few years, finally
breaks down. Trans-equatorial migrants are exposed for part
of each year to the northern environment. This may be quite
sufficient to perpetuate the annual rhythm throughout their
life-time. There is some evidence (storks) that permanent
removal from the north may ultimately end in the adoption of a
rhythm in harmony with southern conditions, the exact opposite
to the rhythm of the north.

ANNUAL MIGRATIONS 127

ductive organs of the many samples taken. At the
minimum phase all the tissues incorporated appear
to be in a state of rest. After recrudescence has
commenced, interstitial tissue begins to appear,
reaches a maximal development and then, as the
organs get markedly larger, again disappears almost
entirely, a few scattered cells remaining here and
there. The sex elements are now getting active and
the eggs in the ovary and the tubules in the testes
attaining their greatest size. Regression from the
maximum is at first extremely rapid but the rate
soon diminishes and when the organs are approach-
ing their minimal condition, interstitial tissue again
develops, gets quite abundant, subsequently to dis-
appear again as the gonads finally reach their winter
condition. Interstitial tissue is thus present and
at its maximum just when the organs are in the
phase found in birds at the height of their
migrations.

Having, then, discovered a method of manipulat-
ing the annual rhythm of the reproductive organs
and having procured details of their histology
through the entire cycle, it became possible to liber-
ate birds at known stages of development and to
ascertain something of their behaviour. According
to our hypothesis it should be possible to retain
juncos at Edmonton, hundreds of miles north of

128 THE RIDDLE OF MIGRATION

their normal wintering grounds, until the gonads
have reached their minimum and then to liberate
them and expect them to stay, i.e. to prove as
sedentary as though they had completed their
migrations to the middle States. The hormone
producing tissue, of interstitial cells, is then want-
ing. If this tissue provides the stimulus to the
migratory impulse, the impulse should be entirely
absent at the minimal stage. The experiments
were run for four winters. During that time nearly
100 control juncos were liberated between Novem-
ber and February. Except for half a dozen that
were almost certainly killed by cats and northern
shrikes, every one was retaken in traps kept per-
manently set and baited outside the aviaries. In
spite of winter conditions, sometimes mild, at others
extreme, and the fact that the birds were far to the
north of their true wintering grounds, they showed
not the slightest interest in the possibilities of
escaping to southern latitudes. Some of them were
out and around for over two weeks before finally
being retaken in the traps.

This is paralleled among wild birds. Thus mal-
lards that have stayed in various parts of the Prov-
ince till the end of November or December never go
south thereafter, no matter what weather may over-
take them. They will stay and starve to death

ANNUAL MIGRATIONS 129

after heavy snow although strong, well fed and per-
fectly equal to taking an immediate southward
flight of a few hundred miles. Physically they are
evidently fit to migrate; physiologically they are
incapable.

Analogous, though differing in detail, is the case
of birds whose gonads are diseased and fail to func-
tion normally. These individuals will probably
also fail to migrate. In the Province-wide hunt for
crows after liberation of our experimental birds,
half a dozen wild crows that had failed to go south
were shot and sent in for examination. All were
males and every one showed a pathological condi-
tion of the testes.

The behaviour of the experimentals was entirely
different. No control was ever heard to sing during
the winter months. The experimentals, on the
other hand, were in full song at Christmas even at
zero. In all, 92 experimentals were liberated (in
batches under varying weather conditions) during
the four winters. Of these, 2>^ individuals (over 40
per cent) were never again seen or retaken. The
percentage of departures varied appreciably with
circumstances. Summarising the results it may be
stated that there appeared to be no discernible cor-
relation with barometric pressure, but that ex-
tremely low temperatures and conditions that pre-

130 THE RIDDLE OF MIGRATION

eluded ready access to food (deep snow and heavy
rime) were both effective in inhibiting departure.
From what has already been said about the effects
of low temperatures on juncos and other birds in
captivity (p. 55) the former case is probably self-
explanatory. That birds that have been kept in
captivity for two or three months with an unlimited
quantity of food supplied them, should promptly
return to this known source on discovering nothing
available outside, is also comprehensible. All birds
liberated have shown themselves to be somewhat
bewildered upon first regaining their freedom and
for that reason the traps were always closed for
several hours after releases, just long enough to
force the birds to take time to orient themselves and
regain their confidence in the outside world. Weeks
of captivity must surely dull a bird's ability to
forage for itself. Under the extreme conditions
prevailing at the time of some of the releases,
inability to find food immediately would no doubt
exercise a restraining influence and induce a speedy
return to the one known source of supply.

Under favorable weather conditions over 80 per
cent of the individuals of a single release have dis-
appeared. They have included birds with gonads
in a state of recrudescence or in regression. The
controls invariably stayed. The experimentals

ANNUAL MIGRATIONS 131

whose gonads had reached the maximum also
proved to be stay-at-homes. The junco is unknown
to most people and is, in Alberta, protected by law.
It would thus have been both useless and illegal to
attempt to get Albertans to watch for the birds,
secure them and return them. And not a single
return has been accidentally obtained. But it was
obviously most desirable to discover what became
of these departures. Did the birds with the increas-
ing gonads actually go north ? Did those with their
organs in regression go south? If the first were the
case, the assumption so freely made by writers on
the subject, that migration is inherent, instinctive
behaviour, would be actually demonstrated. Un-
der no other circumstances would it be possible to
evoke the northern migration in the fall. Were it
not inherited behaviour, dependent on the past
history of the race, not on the experiences of the
present generation, it would be quite impossible to
induce the individual to go north when the entire
environment dictated migration in the opposite
direction. It would only be possible were the indi-
vidual entirely ignorant of the significance of the
act of migrating northwards.

Support for the argument is provided by the
persistent northward flight of bluebirds (Sialia
currucoides) and other species in belated springs

132 THE RIDDLE OF MIGRATION

when they arrive here merely to starve to death
because winter still holds the country in its grip.
Could they exercise discretion they would not pro-
ceed. They might even turn back for a spell. But
in this they fail. Their activities are evidently
under physiological, not mental, control.

Various species of seed-eaters other than the
junco have been used in small numbers in these
experiments. All have been species that do not
cross the equator and the results have been entirely
uniform. Each case, however, suffered from the
same objections as that of the juncos when it came
to getting information about them after liberation.
If the point was to be satisfactorily settled it was
imperative that some other species be used, one
familiar to the man in the street, not protected by
law, and able to fend for itself under winter condi-
tions in Alberta. An obvious solution could be
found in the crow, a good migrant, large and con-
spicuous, an omnivorous feeder, universally known.
The only objection, but rather a serious one, was
the supposed impossibility of trapping the species
during the summer months. In 1929, however,
thanks to the generosity of the directors of the
Bache and the Elizabeth Thompson Funds, money
was available for repeating the experiments on a
much bigger scale and the crow was decided on.

ANNUAL MIGRATIONS 133

In August a trapping camp was accordingly estab-
lished in suitable territory and every conceivable
means of getting crows alive was attempted. We
learnt a great deal that was interesting and much
that was amusing about the mentality of crows but
the middle of September when the last bird had gone
south, saw us with only 140 individuals caged at
Edmonton whither they had been shipped. Not all
of these sur\aved and after the necessary samples
had been taken for blood analyses, examination of
the gonads, etc., only 83 remained at the time of
liberation, 69 experimentals and 14 controls. All
of them were in perfect health and first-class
condition.

They received treatment on exactly the same lines
as that given the juncos. The lighting, totalling
12,500 watts, was based on the ratio (of watt per
cubic feet of cage space) that we had found most
effective in the case of the juncos. The crows
remained fully active while the lights were on. The
experiment commenced on September 28, the daily
extension of illumination being exactly that of the
1927 junco undertaking. On Saturday, November
9, the birds were turned out during the morning.
The fact that they had been released was broadcast
from various radio stations on the Saturday night
and the universe in general invited to shoot them

134 THE RIDDLE OF MIGRATION

and return them to the University for examination.
Every bird was banded. Since Sunday shooting is
prohibited in Alberta, the crows had nearly 48 hours
in Avhich to get accustomed to their regained free-
dom and to choose their roads before being hunted.
The controls were turned out together with the
experimentals. The following Monday being
Thanksgiving Day it was hoped that crow shooting
would be widely indulged in and our returns duly
benefited. The birds remained wild and wary,
however, and although hundreds of gunners turned
out, only a small percentage of birds was retaken.
This was disappointing for had it not been for the
incidence of this national holiday on the 11th, the
birds would have been liberated two weeks later.
Interstitial tissue was just appearing in the testes of
the experimental samples but two more weeks of
lighting would probably have produced greater uni-
formity. As far as the controls were concerned we
felt that we were perhaps liberating them pre-
maturely and that some of them might still proceed
south, but the advantages of a large turn-out on
Thanksgiving Day promised to outweigh the dis-
advantages of a somewhat early liberation.

Of the 69 experimentals, all but 28 left for good.
These were retaken within three days, given two
additional weeks of lighting and then again turned

ANNUAL MIGRATIONS 135

adrift under less favorable weather conditions.
This time 15 remained.

Of the 14 controls, 6 remained, 8 departed.
Those that stayed were again turned out together
with the experimental on the 24th, when none of
them left.

The returns — apart from those that remained at
home — were distributed as follows. Of the 54
experimental departures, 28 were ultimately re-
ported killed, 12 locally (within 10 miles of Edmon-
ton). Eight were returned from the north and
northwest, (the farthest, 2, at Whitecourt, 100 miles
in a direct line northwest of Edmonton). Eight
were reported from the south and southeast. South
Dakota being the remotest point. The remaining
26 are unaccounted for.

Of the 8 control departures, 2 were locally killed,
4 to the southeast (the farthest at about 200 miles) ,
none north or northwest while 2 are unaccounted for.

Scores of sight records of crows were sent in from
various parts of the Province, nearly all of single
birds or of twos and occasionally threes, but from
various points along the south shores of Lesser Slave
Lake and beyond to Peace River Town, 300 miles
northwest of Edmonton, persistent reports of groups
of crows (up to 15 together) were received till about
the beginning of January. A number of attempts

136 THE RIDDLE OF MIGRATION

were made to collect some of these birds but none
was successful. This is good country for ravens but
it seems quite certain that the birds reported were
actually crows. Both species are well known to
local residents, both in appearance and in voice, the
latter entirely distinctive even to a novice. No
reports whatever were received from other equally
likely places where ravens abound, e.g. Lac la Biche.
The reports came in quite independently, none of
the senders being aware that others had made
similar observations. Moreover, the reports orig-
inated in several cases with people who were entirely
ignorant of the experiments and were forwarded to
me through other channels. The second of the two
Whitecourt birds, for instance, was shot on Febru-
ary 28 by a trapper in the wilderness on the far side
of the Athabasca River who had no idea that crows
had been liberated in the Province that winter. He
first saw the bird at a distance and came to the
conclusion that it was a crow. Ravens had period-
ically been around his camp. The sight of a crow
so astonished him that to make sure he was not
suffering from delusions he got his rifle and shot it.
On examining it he noted the band, which was duly
preserved. In May he returned to Edmonton,
exhibited the band and recounted the episode at a

ANNUAL MIGRATIONS 137

local hotel where he learned about the experiments
and later brought the ring in to the University. In
this instance, at least, there was no error in identifi-
cation and any one who knows both birds can hardly
doubt that any observant individual living in raven
country where crows also occur in summer could
readily distinguish the two species.

The first sight record from the Lesser Slave Lake
country was obtained on November 18, nine days
after our releases and two months after the last
crow had previously been noticed in the district.
Without exception, not one of the old-time residents
consulted on the shores of Lesser Slave Lake can
recall even a single crow wintering there in previous
years. It was most unfortunate that Edmonton
should have been situated so as to have large
tracts of muskeg and sparsely settled wilderness
within 100 miles to the north and northwest from
which returns could hardly have been expected,
but circumstances made it impossible to carry out
the original decision to conduct the experiment a
hundred miles further south. Of the experimental
birds that left us, 48 per cent have thus never been
accounted for, while 15 per cent have been returned
from points south of Edmonton, and another 15
per cent of the north. Of the eight controls that

138 THE RIDDLE OF MIGRATION

left, 25 per cent are not accounted for, 50 per cent
have been returned from the south, while nothing
has come in from the north. The country to the
south and east of Edmonton is well populated and
was excellently patrolled. The districts north and
west are less heavily settled and soon give place to
wilderness. One can hardly believe that 50 per cent
of the controls should have been recovered to the
south and only 15 per cent of the experimentals if the
latter had also mostly gone south. Had there been a
general dispersal in all directions from Edmonton
returns from the south and east should greatly have
outnumbered those from the north and west for
there must have been a dozen watchers in the south
to every one in the north. Yet the number of ex-
perimental returns was the same in each case (15
per cent) again suggesting that the Lesser Slave
Lake crows must have represented some, at least,
of our lost experimentals.

While circumstantial evidence thus strongly sug-
gests that a majority of our experimental birds
travelled northwest, the final verdict must remain
open since not even one of the Lesser Slave Lake
crows was secured. Although quite improbable,
they may actually have been wild birds that win-
tered there for reasons unknown. For the sakeof

ANNUAL MIGRATIONS 139

argument, let us suppose that they were our birds.
We should still have no proof that the northward
migration depended on the state of the reproductive
organs even though they were in a physiologically
acitve condition. A bird possesses various ductless
glands that might respond to variations in day-
length. Examination of all the endocrine organs of
our juncos and crows lends no direct support to this
assumption, but an obvious test can be applied to
the gonad idea. The world over it is common
practice to caponize domestic poultry, i.e. to remove
the testes by a simple operation. It converts the
rooster into a better table product. If the gonads
are in truth the physiological seat of the recurring
stimulus to the migratory impulse, a castrated bird
should show no migratory inclination, no matter to
what experimental conditions it may have been
subjected. It should remain sedentary when liber-
ated, or merely w^ander.

This and other modifications, were planned for
the 1929 experiments but shortage of birds neces-
sitated all-round curtailment and precluded the
opportunity. It is hoped, however, to be able to
repeat the undertaking on a yet larger scale in the
future, when the hypothesis will be more precisely
tested.

CHAPTER V
Summary

In conclusion we may briefly review the line of
thought that has been adopted in the foregoing
pages.

The fact that birds are intimately adapted to
journeying in the air forms the fundamental basis
for the stupendous migrations that are so charac-
teristic of many species. Air surrounds the entire
globe and, apart from storms which are temporary,
is without barriers. It is the ideal medium for
continuous travel. Ability to use it, however, does
not constitute migration. It merely makes exten-
sive migrations possible. A bird living in an en-
vironment congenial throughout the year will re-
main non-migratory. It may even forfeit its ability
to use the air and become flightless in non-com-
petitive surroundings.

The environment must, therefore, be looked upon
as an integral factor in the evolution of migrations.
Particular attention has been paid to conditions in
northern regions of the northern hemisphere and
an attempt has been made to ascertain how they
could have induced the custom of migration. The

140

THE RIDDLE OF MIGRATION 141

general principles that have suggested themselves
can be applied to migrations in other parts of the
world.

It has been seen that the avian brain is of com-
paratively lowly organisation, showing many rep-
tilian affinities and that intelligence of mammalian
level cannot be expected in birds. It was therefore
found necessary to assume that natural selection
has worked on a passive organism that has re-
sponded more or less without being aware of its
response. Migrations would as certainly have been
evolved by such a method — given sufficient time
and material — as by an intelligent understanding
of the environment on the part of the bird.

The custom of migration having been established
at some time during the history of any given migra-
tory species, it is now assumed to be inherent and
to be evoked at the appropriate seasons annually
by certain stimuli, external and internal. For
reasons given in considerable detail, variations in
day-length are assumed to be the primary external
stimulus. Experiments in which juncos have been
principally used have largely substantiated this
viewpoint.

The internal stimulus has been assumed to be a
hormone produced by the interstitial tissue of the
reproductive organs. It has been experimentally

142 THE RIDDLE OF MIGRATION

shown that this tissue (in the junco and several
other passerine species) can be brought to known
stages of development by manipulating the external
stimulus — duration of day-length. It has further
been shown that the behaviour of birds so treated
actually varies upon liberation with the varying
state of the reproductive organs.

The grounds for adopting the latter assumption
are partly based on the knowledge that the hormones
of the gonads control sex-behaviour in vertebrates
and since, in accordance with our definition, migra-
tion may be legitimately looked upon as a particular
phase of breeding behaviour it, too, should come
under control of this regulatory mechanism. Be-
haviour induced by internal secretions is instinctive.
All that we know of typical migration lends support
to the view that it falls into this category.

In considering the evolution of migrations it has
been pointed out that we are discussing events of
the distant past and are left to draw inferences to a
considerable extent. We know, however, that
migrations are in some cases being built up at the
present time and by analysing the factors involved
in such instances, we can identify at least some of
them and be reasonably sure that similar factors
must have been operative in the past. Living
organisms as we know them today existed millions

SUMMARY 143

of years ago. We cannot conceive of the essential
requirements of birds of today being different in
any fundamental respect from those of their an-
cestors of byegone ages.

The experiments in migration, of which some
account has been given in the pages above, are the
outcome of a critical examination, in light of modern
biological knowledge, of the available field facts
and existing theories of migration. The arguments
on which they are based apparently fit the facts,
but it must be borne in mind that any theory that
is new must remain subject to correction until
fully substantiated by repeated effort and the
critical investigations of fellow workers looking at
the problem from some other angle. The attitude
of the scientist must be entirely impartial and
without bias. He sets out to test a given hy-
pothesis, not to prove it or to disprove it. It is
not his business to make the pendulum swing in
any particular direction, but to watch and measure
its swing and to record its observed amplitude. His
mind must ever be open.

Experimental biology, however, is peculiar in
this respect, that the investigator must of necessity
have a preconceived theory upon which to work.
He must have something concrete in his mind in
order to organise his line of attack. But it should

144 THE RIDDLE OF MIGRATION

matter nothing to him whether the results are
positive or negative. Even the latter are valuable
in the controlled elimination of observable factors.
Repeated failure may lead to ultimate success. A
problem as wide in its scope as the migration of
birds involving, as it does, many branches of
biological science, will inevitably take many years
of patient effort to analyse on an experimental basis.
But the mere fact that one is dealing with animal
behaviour is an incentive with a deep and lasting
appeal. One feels continually that one may some
day encounter something that will not only provide
a key to the migrations of birds, wonderful per-
formances, in many cases (e.g., young birds)
achieved with neither thought nor forethought, but
also to those curious streaks of thoughtless be-
haviour that so frequently reveal themselves in
man himself.

EPILOGUE 145

EPILOGUE

"To study the migration of birds is to investigate
the nature of animal behaviour, and to do this is
to probe the inmost mysteries and to ask the very
meaning of Life itself."

— A. Landsborough Thomson,

INDEX

{All numbers refer to pages. Numbers in heavy type indicate pages
on which figures occur)

Activity, 6, 7.

Adaptations to flight, 6-12.

Air-sacs, 11.

Alaska, 68, 92.

Alberta, 7, 50, 61, 76, 92, 93, 96,

111, 116, 128, 129, 135, 136
Altitude and land marks, 82.
Amphibia, 9, 47.
Anatomy of birds, 5-44.
Antarctica, 68.
Apes, 16.

Archaeopteryx, 64, 67, 68, 103
Archaeornis, 64, 68, 103.
Arctic tern, 107.

Balance, organs of, 29.

Barometric pressure, 114, 129.

Bats, 9, 10.

Bavaria, 64, 68.

Black-billed cuckoo, 97.

Bluebird, 131.

Bobolink, 42.

Bone, 11.

Brain, 13, 16-19, 20, 21, 22.

Brain-stem, 17.

Budgerigar, 56.

Calcarius lapponicus, 95.

Canary, 23, 24, 56.

Captivity, behavior of birds in,
7, 55, 116, 130.

Castration, effects of, 43, 139.

Cere, 29.

Cerebellum, 18.

Cerebral cortex, 19.

hemisphere, 19.

Chordeiles virginianus, 106.

Cinclus cinclus, 23.

Climate, northern, 53 et seq.

, of the past, 67-74.

Coccyzus erythropthalmus, 97.

Cochlea, 33.

Collicuh, 33

Coloeus monedula, 23.

Color of leaves as stimulus to mi-
gration, 115.

Color-vision, 35.

Columella, 33.

Cones (of retina), 34, 35.

Cornea, 34.

Corpus callosum, 20.

striatum, 21.

Correlation centre, 18.

147

148

INDEX

Corviis hrachyhrynchos, 100.
Cowbird, 76.

Cretaceous, 65, 67, 68, 103.
Crow, 63, 96, 112, 129, 132-139.
Cycles of animal abundance,
60-64.

Day-length, 54, 72, 115 et seq.

Diabetes, 38.

Dipper, 23.

Direction, sense of, 80-85.

Ductless glands, 37-44, 139.

Ear, 29, 30, 31, 32-34.
Edmonton, (Alberta), 50, 53, 54,

55, 57, 59, 111, 133-139.

, meteorological statistics, 54.

Eel, 47, 79.

Endocrine organs, see ductless

glands.
Endolymph, 32.
Environment, 41-85.
Eocene, 67, 68, 73.
Equilibration, see balance.
Ergosterol, 58.
Experiments with crows, 132-139.

juncos, 118-132.

Eye, 34^36.

Feathers, 10, 11.

Feeding movements, see move-
ments, feeding.
Fibre, see nerves.
Fish, 9, 17, 18, 46, 47, 48, 49, 79.
Flight, 9.

Follicle, ovarian, 41.

Food problem in winter, 55, 56,

114.
Fossil birds, 64, 65, 66, 67.

pollen grains, 69.

Fovea, 35, 36, 81.
Franklin gull, 77.
Frog, 19.
Furnarius cunicularius, 24.

Gadow, Hans, 46.

Gannet, 28.

Geocarcinas, 47, 78.

Geological epochs, methods of

estimating, 67.
Germinal epithelium, 41.
Gonads, 38-^4.
, internal secretions of, 42-

44, 117.
Gonad changes and behavior,

130, 131.
in relation to forced

exercise, 124, 125.
light,

122, 123.
tem-
peratures, 121, 122.
Golden plover, American, 75, 77,

85, 107, 112.

, Pacific, 107, 108.

Greenland, 68, 88.

Hawks, 29, 36, 59, 62.

Heat, conservation of, 7 et seq.

Hesperornis, 65.

INDEX

149

Hoatzin, 27.

Homing, 80-87.

Hormone, see internal secretion.

Howard, H. Eliot, 24.

Iceland, 92.
Ichthyornis, 65, 103.
Instinctive behavior, 22-24, 84-86,
131.

, definition of, 87, 88.

Insulin, 38.
Intelligence, 17, 22.
Internal secretion, 36-44, 128.
Interstitial cells, 41, 127, 128.
Iris, 34.

Jackdaw, 23.

Junco, 59, 63, 119 et seq.

J unco hyemalis connectens, 119.

Jurassic, 64, 68.

Kiwi, 28.

Lagena, 23.

Lamarckian hypothesis, 83, 84,

103, 104.
Lapland longspur, 95-102.
Lapwing, 106.
Lashley, K. S., 82.
Lemming, 69.
Lens (of eye), 34.
Limpet, 49.
Loxia leucoptera, 50.
Lungs, 11.

Magnetic field, 80.

sensibility, 80-87.

Magpie, 101.

Mallard, 90-94, 128.

Mammals, 9, 13, 16, 27, 28, 34,

41, 47, 58.
Mammoth, 69.
Man, 16, 19.

Membranous labyrinth, 32.
Memory, 24-26, 81, 83, 86.

, inherited, 83, 84.

Migration, definition of, 46.

, evolution of, 90-110.

of amphibia, 47.

fish, 46, 47, 48, 49.

immature birds, 49, 78,

79,83.

mammals, 47.

oceanic birds, 47.

reptiles, 47.

■ in northern hemisphere, 53-

60.

tropics, 51, 105, 106.

experiments, see experiments.

stimuli, 113-118.

, trans-equatorial, 116.

and magnetic field, 80 et seq.

during ice-age, 71, 72.

Mind, 19.

Morgan, Lloyd, 87.
Mourning dove, 97.
Movement, 45.
Movements, feeding, 49-51.
Musk-ox 69.

150

INDEX

Nasal organ, 27.

Natural selection, 93-110.

Nerve fibre, 14.

Nerves, 13-16.

Nervous system, 13-26.

Nest building, 22-24, 25.

Night hawk, 106.

Northern hemisphere, fluctuations

in numbers of animals, 60-64.

, land masses, 51, 52.

, winter conditions, 53-

64.
and seed-eating birds,

62, 63.
Nostril, 28.

Oidemia deglandi, 7.
Olfactory lobe, 18.
Ova, 41.

Ovariectomy, 43.
Ovary, 38, 39, 41.
Owls, 36, 62.

Palinurus, 30.
Pallas' sandgrouse, 108.
Pancreas, 38.
Para-thyroid, 38.
Parrots, 8, 29.
Partridge, 96.
Pecten, 35.
Peewit, 108.
Pelican, 28.
Penguin, 79.
Perdix perdix, 96.
Perilymph, 32.

Pia mater, 19.
Pigeon, 24, 81.
Pineal, 38.
Pituitary, 38.
Pleistocene; 68.

glaciations, 69, 70, 71, 72.

Pliocene, 96, 100, 109.
Pons varolii, 21.
Posterior colliculus, ZZ.
Preen-gland and nutrition, 58.
Pyramids, 21.

Raven, 136.

Reflex arc, 14, 15, 16.

Reproductive organs, see gonads.

Rhythm, 126, 127.

Reptiles, 6, 7, 9, 13, 21, 35, 36,

103.
Respiration, 11, 12.
Retina, 34, 35.
Rods (of retina), 34, 35.

Salmon, 47.

Scoter, white-winged, 7.

Sea-Uon, 47, 79.

Sea-snake, 47.

Seal, 47.

Seed-eating birds, 63.

Semi-circular canals, 32, 3>3>.

Seminiferous tubules, 40.

Sense organs, 13, 26-36.

Sex-reversal, 43.

Sialia currucoides, 131.

Sight, 35.

Skull, 26, 27.

Smell, 27, 28.

INDEX

151

Speech, 33.
Spermatozoa, 40.
Starling, 96.
Sternus vulgaris, 96.
Stimuli to migration, see migra-
tion.
Supra-renal, Z^.
Syrrhaptes paradoxus, 108.

Ultra-violet radiation, 57-64, 94,
115, 123.

Vanellus vanellus, 108.
Vas deferens, 40.
Vestibule, 32.
Vitamin A, 60.
D, 57-60.

Tactile organs, see touch.
Taste, 28, 29.
Temperature of birds, 8.
Temperatures, effects of low, on

birds, 55, 56, 114.
, terrestrial, past and present,

72, 73.
Testis, 38, 39, 40.
Thyroid, 38.
Tongue, 29.
Tortugas Keys, 82.
Touch, 29.
Tree sparrow, 59.
Tunica albuginea, 40.
Turbinal bones, 27.
Turnstone, 108.
Turtle, 47.
Tympanum, 32,

Waders (shore birds), 77.

Walrus, 47, 69.

Watson, J. B., 82.

Weather, see climate.

Wetmore, A., 51.

Whale, 27, 47.

White-winged crossbill, 50.

Wings, 9, 10.

Wisconsin ice-age, 69, 70, 71.

, effect on migration, 72.

X-ray, effects on testis, 44.

Yellow spot, see fovea.
Yellow-billed loon, 75.

Zenaidura macroura, 97.

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