The Cambridge Manuals of Science and
Literature
THE MIGRATION OF BIRDS
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CAMBRIDGE UNIVERSITY PRESS
London: FETTER LANE, E.C.
C. F. CLAY, MANAGER
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Edinburgh: 100 PRINCES STREET
Berlin: A. ASHER AND CO.
Leipzig: F. A. BROCKHAUS
New York: G. P. PUTNAM'S SONS
Bombay and Calcutta: MACMILLAN AND CO., LTD.
_All rights reserved_
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[Illustration]
THE
MIGRATION
OF BIRDS
BY
T. A. COWARD
Cambridge:
at the University Press
New York:
G. P. Putnam's Sons
1912
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_With the exception of the coat of arms at
the foot, the design on the title page is a
reproduction of one used by the earliest known
Cambridge printer, John Siberch, 1521_
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PREFACE
Any attempt to elucidate the problems connected with the
Migration of Birds must, in the present state of knowledge, contain
some theory and speculation, but the diligent observations of an
army of careful workers yearly add facts, which though they may
appear insignificant when considered alone, tend in the aggregate
to confirm or repudiate the conclusions of past workers. I have
endeavoured to bring together some of the more important theories,
and to give prominence to ascertained facts; I have also striven
to check desire on my own part to wander into realms of pure
speculation, though conscious that I have not always evidence to
support my suggestions.
The numbers in brackets ( ) in the text refer to the books or
papers mentioned in the list at the end of the volume, which is in
no ways an attempt at a full bibliography. I have quoted freely
from the works of past and living ornithologists. To these I offer
apologies if I have misconstrued their arguments, and acknowledge
my indebtedness to those whose observations or writing have given
me light. In particular I tender thanks to Mr Wells W. Cooke for
his permission to reproduce the maps facing pp. 76, 78, 80. I have
found his writings and those of Herr Otto Herman and Mr W. Eagle
Clarke especially valuable. Mr Eagle Clarke's long looked-for book
on Migration is, as I write, still in the press; had mine been more
than a manual I should have hesitated to publish until his had
appeared.
T. A. COWARD.
BOWDON, CHESHIRE,
_4 November 1911_.
CONTENTS
CHAP. PAGE
I. MIGRATION OF BIRDS 1
Definition--Variation of migration.
II. CAUSE AND ORIGIN OF MIGRATION 13
Direction of passage--The potentiality of flight--Habit
of wandering--Memory--Extension of range--Influence
of Temperature--Desire for Light--Glacial Epoch--Food
Basis--Sexual Impulses--Competition.
III. ROUTES 33
Route or Broad
Front--Coasting--Fly-lines--Isepipteses
--Land-bridges--Coast Lights.
IV. THE HEIGHT AND SPEED OF MIGRATION FLIGHT 47
Altitude of Normal Migration--Variation in
Speed--Effect of Wind.
V. ORIENTATION AND ROUTE FINDING 56
Route Finding--Use of Memory--Eyesight--Errors--Guidance
of Young--Beam Winds--Homing of Terns.
VI. THE DISTANCES TRAVELLED BY BIRDS 65
The Swallow--Variation in Distances--Marking
Birds--Results--Routes of the Golden Plover--Evolution
of the Routes.
VII. MIGRATION AND WEATHER 83
Knowledge of Approaching Weather--Favourable and
Unfavourable Conditions--Importance of Winds--Cyclonic
and Anticyclonic Winds--Continental Migration.
VIII. THE PERILS OF MIGRATION 104
Contrary Winds--Lighthouses and Lightships--Leeward
Drift--Catastrophes.
IX. EARLY IDEAS OF MIGRATION 114
Literature--Hibernation--Carriage of Small by Large
Birds.
X. SUGGESTIONS AND GUESSES 119
Trans-Atlantic Migration--Ship-borne Wanderers
--Storm-blown Birds--Casual Wanderers--Swimming and
Walking.
XI. SUMMARY 126
BIBLIOGRAPHY 131
INDEX 135
LIST OF MAPS
FACING PAGE
Map showing the range of the American Golden Plover,
with its known migration route 76
(From _The National Geographic Magazine_.)
Map showing the evolution of the migration route of the
American Golden Plover 78
(From _The National Geographic Magazine_.)
Map showing the evolution of the migration route of the
Eastern or Pacific Golden Plover 80
(From _The National Geographic Magazine_.)
Map to show that a bird leaving Norway, near Aalsund,
might be carried round the British Islands in twenty-four
hours. The arrows indicate the actual directions
and force of wind at the times marked during a
slow-travelling circular storm in autumn 1901.
Speed of bird about twenty-five miles per hour 98
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THE MIGRATION OF BIRDS
CHAPTER I
MIGRATION OF BIRDS
Migration is the act of changing an abode or resting place, the
wandering or movement from one place to another, but technically the
word is applied to the passage or movement of birds, fishes, insects
and a few mammals between the localities inhabited at different periods
of the year. The wandering of a nomadic tribe of men is migration;
the mollusc, wandering from feeding ground to feeding ground in the
bed of the ocean, migrates; the caterpillar migrates from branch to
branch, even from leaf to leaf; the rat leaves the ship in which it
has travelled and migrates to the granary; we pack our goods, hire a
removing van and migrate to a new abode. The word migration thus applied
may be literally correct but it fails to convey the generally accepted
meaning, and the expression Bird Migration suggests periodical and
regular movement, the passage as a rule between one country and another.
The popular application of a term does not do away with the need
of definition, especially as there are many complicated phases of
migration. The migration of birds is as a rule between the breeding
area or home and the winter quarters, but there are many migrants which
never reach breeding quarters in spring, and many others which leave the
regular breeding quarters or the place of residence in winter to perform
a very real migration under peculiar stress of circumstances. Again the
spasmodic movements of certain gregarious species, which at irregular
intervals change their location in large numbers to take up their abode
in another part of the range, is really migration, though it is now
usually described as irruption, incursion or invasion.
Newton says (38) that bird migration is "most strangely and
unaccountably confounded by many writers with the subject of
Distribution," but the very act of the bird which extends its range, the
first step in distribution, is migration. The histories of present-day
distribution and migration are irrevocably interwoven; as Mr P.A.
Taverner remarks (51), "migration is a dispersal, and conversely, this
dispersal, as it manifests itself, is migration," whilst distribution is
the outcome of dispersal.
Broadly speaking, all birds migrate, though the length of the journey
varies in different species, and in some cases in individuals of the
same or closely allied species, from the merest change of elevation to
a voyage almost as wide as the world itself. The sedentary red grouse
nests on the moors, often less than 1000 feet above the sea, but "when
snow-bright the moor expands" it feeds and resides in the cultivated
valley, and as shown by the committee appointed to study grouse disease,
not infrequently migrates from range to range across wide valleys. Many
tropical birds, usually considered non-migratory, are subject to short
movements, the origin and purpose of which is search for food and safe
nesting places.
The knot breeds in countless numbers in Arctic Greenland and America, so
far north that only a handful of ornithologists have traced its home;
it travels south in summer so far as Damara Land. The Arctic tern has
a northern breeding range extending perhaps as far north as that of
any bird, and it has been taken far to the south of South America in
the Antarctic regions; if the thesis that the further north the bird
goes in summer the further south it travels in winter is correct, as
it can be proved to be with some species, some of these terns must
annually travel about 22,000 miles (21). Between these extremes are an
endless variety of distances travelled and methods of migration, with
striking differences in the performances of individuals of the same
species. Take one instance, a song thrush reared in a nest in our own
garden. We may see and recognise this bird up to the middle of July,
but what trained ornithologist can, yet, say with certainty where that
bird will be by the end of the month or in three to four months time?
We know that all through the winter there are some song thrushes near
the house, and that they are the birds which not only begin to sing
early but actually nest with us; we know too that before there is any
marked immigration of northern thrushes there is a recorded emigration
from our southern coasts, presumably of thrushes which have nested with
us, beginning towards the end of July; further we know that there is
an autumn immigration of Scandinavian or other northern song thrushes,
sub-specifically distinct to the expert eye, and some, small and dark,
whose origin is by no means proved, as well as later emigrations of
birds to the Continent or Ireland, both regular and occasioned by
exceptional weather. Will our young July thrush remain in England or
will it join one of these streams, and if so which? We do not know yet.
I repeat "yet," for the study of races, sub-species or local variations
is commanding more and more attention; the patient work of the
"splitters," scorned by the old school of "lumpers," will eventually
solve many of the problems of to-day.
The ancients--a usefully ambiguous term--realised that birds migrated;
our immediate forefathers of two or three centuries ago realised that
certain birds vanished in winter and wondered how; and within modern
times the phenomena of migration, the "mystery of mysteries," has been
the subject of much study, speculation, and literary exposition. Indeed
a full bibliography of migration would be a considerable volume. Even
workers within the last few years have declared that certain phenomena
were beyond human understanding, only to be explained by instinct, a
word capable of most varied interpretation. In truth there is much to
learn, much to which we must still answer--we do not know; but the
speculative theory of yesterday is now either myth or fact, and the
theory of to-day may be proved true and add something to the data of
which knowledge is built. The wildest speculations, based on slender
locally ascertained facts or on no foundation whatever except the
fertility of the brain, have been offered as solutions of the mysteries;
the literature of migration is a jumble of contradictions. John Legg, in
1780, said "In relating so many instances of unparalleled credulity, I
confess I cannot suppress the irascible passion" (33), and Herr Otto
Herman, only a few years ago, pointing out the ingenious dogmas "void
of every firm foundation," says that "really it is a field in which
every thinking ornithologist may create new theses to any extent and
more or less incredible" (31).
Herr Herman's system of "ornithophænology," the accumulation of
substantiated observations and facts, will not prove everything, but
his work in Hungary, that of Dr Merriam and Mr Cooke in America, and
of Mr W. Eagle Clarke in Britain, each aided by a numerous band of
careful workers, are striking examples of what can be accomplished.
Whatever errors future enlightenment may show in their conclusions their
ascertained facts will remain positive knowledge; theirs is not what
Herr Herman himself described as "pretended authority."
In order to grasp the problems of migration it is necessary to get rid
of the puerile and insular aspect of the subject, namely that migrants
are merely those birds which come to us, like the swallow and cuckoo in
the spring, and those, like the fieldfare and brambling, which visit us
in winter but are not with us in summer. The complication of the subject
may be demonstrated by a rough classification of the migrants to be
observed in the British Islands.
Arbitrary grouping of the members of an avifauna is only for general
convenience; many species are represented in more than one group.
1. Permanent Residents: birds which remain in Britain all the year
round. These are comparatively few in number, and largely consist of
insular races of birds which perform regular and often long migration
journeys in other parts of their range. Most, if not all, perform short
migrations, in some cases only seasonal changes of altitude, spending
summer on the hills and winter in the lowlands; examples, the red grouse
and dipper. Others, like the tits and creepers are nomadic and more or
less gregarious in the colder months. Few appear to remain in the same
locality at all seasons, but possibly some of our British robins and
song thrushes, both sub-species of migratory Continental forms, may be
non-migratory.
2. Summer Residents: birds which nest in our islands, leaving in
autumn for countries to the south, and return in spring. In addition
to the regular summer visitors, which all leave in autumn, this group
includes a number of wagtails, pipits, finches and other birds which are
represented in winter in our islands by a proportion which remain.
3. Winter Residents: birds which nest to the north or east of our
islands and arrive in Britain in autumn, leaving in spring for their
breeding area. With birds like the fieldfare, brambling and jack snipe,
which do not nest in Britain, must be included many (for example the
robin, rook, song thrush and common snipe) which are also permanent
residents.
4. Birds of Passage or Spring and Autumn Migrants: birds which neither
nest with us nor normally remain for the winter, but merely use the
British Islands as feeding and resting places on their journey between
the northern breeding area and the southern or eastern winter quarters.
This group is an especially difficult one, for in it must be included
such birds as dunlins and curlews, which are represented as breeding
species in Britain, and also a number of birds which apparently go no
further south than our islands in winter, and others which, though not
breeding, go no further north in summer. The actual status of these
individual birds is uncertain. In this group too we have the Greenland
wheatear, so closely allied to our familiar early migrant that, unless
the bird can be measured, its identification is uncertain.
5. Irregular Migrants: birds which may be classed in other groups.
Some of these are really winter residents, but their visits are so
irregular that they may for convenience be classed with spasmodic or
occasional invaders, such as Pallas's sand-grouse, which arrive at
uncertain intervals in large numbers. Some of their number, during these
irruptions, usually breed and thus the bird becomes an irregular summer
resident or even, for the time, a permanent resident.
6. Stragglers or Wanderers: birds whose occurrence in our islands is
more or less accidental, due apparently to their having lost their way
or to their ordinary wandering habits having taken them far from the
normal range of their species. Some of the rarer petrels and other
oceanic birds certainly pertain to this group, but our knowledge of
the migration routes of others is still so slender that it is unwise
to declare dogmatically that they are lost. Some too of the so-called
stragglers may have been artificially or accidentally introduced; many
"records" prove on investigation to be the aimless wandering of escaped
captive birds, whilst others are known to have been aided in their
journey and carried out of their usual course when resting on shipboard.
When Mr Eagle Clarke was on the Kentish Knock Lightship, off the mouth
of the Thames, he found that in autumn there were continuing practically
simultaneously the following streams of migration. Immigration from
the Continent to England from east to west, and from south-east to
north-west, and passage along both lines; emigration from north to
south-south-west, and from north-west to south-east, with passage from
north to south-south-west. Birds of the same species actually crossed
paths, travelling in contrary directions (16).
The above grouping applies to the British avifauna, but a somewhat
similar arrangement might be made of the birds of any particular area,
large or small. The grouping of birds for the study of Geographical
Distribution is of little consequence in connection with migration,
but the mapping of the world into various ornithological rather than
zoogeographical regions is of considerable importance, both for
convenience in tracing the ranges of migrants, and in the discussion
of the history of migration, which almost certainly began in the
form of short wanderings from the centres of distribution. It is of
comparatively small importance what boundaries we take for the various
regions; these depend largely upon the view of certain ornithologists as
to which groups of birds shall be considered as typical of the regions
in question. Sclater's six regions are perhaps the most universally
used. They are as follows:--
1. Palæarctic, embracing the whole of Europe and northern Asia.
2. Ethiopian--Africa, Arabia, Madagascar and roughly half of the
Atlantic and Indian Oceans.
3. Indian, including India, Further India, Southern China, the western
portion of the Malay Archipelago and the Chinese Seas.
4. Australian, embracing Australia, New Guinea, New Zealand and the
southern Pacific.
5. Nearctic, roughly America north of the Gulf of Mexico.
6. Neotropical, America south of the Gulf.
Newton suggested an alteration, a continuous northern region to be
called the Holarctic Region, which embraces almost the whole of
the Northern Hemisphere, and the division of the Australian into
Australian and New Zealand Regions. Each of these southern regions is
the winter home of some of the Holarctic birds, and it is a matter of
dispute whether many of these originated in the northern or southern
hemispheres. The value of these artificial divisions of the world is
rather in the consideration of the conditions their varied climates and
physical features present as attractions to birds in search of suitable
nesting places and food supplies.
The study of Migration involves reference to the work of ornithologists
of the past and present, the mass of contradictory literature already
referred to, and we are repeatedly faced with the difficulty that some
particular theory about the vexed questions of the cause or origin of
migration, the height and speed at which birds travel, whether they
do or do not follow routes, how they find their way, in what order
they migrate, how and why they do or do not avoid dangers, or any
similar problem, which seems to give finality so far as certain cases
are concerned, is met by an absolute negation in other instances. The
truth seems clear; more than one factor has influence on most birds,
and different species in different places are influenced by different
factors. Elliott Coues' sweeping statement, though I strongly disagree
with the article in which it occurs, expresses much that is true.
"Isepipteses and magnetic meridians, coast-lines and river channels,
food-supply and sex-impulses, hunger and love, homing instincts and
inherited or acquired memory, thermometer, barometer and hygrometer, may
all be factors in the problem, good as far as they function; but none of
them, and not all such together, can satisfy the whole equation."
Some of the theses may be laws or rules, but there are no rules without
exceptions, and these exceptions may become local rules. Laws regulating
migration in one area, whether it be the great continent of America, the
British Islands or the islet of Heligoland, may have little application
in other parts of the world: local evidence alone can never solve the
great problems.