CAUSE AND ORIGIN OF MIGRATION
The question--What makes Birds Migrate? or what causes them to remove
from one zone to another at certain seasons, has been answered, no doubt
to the satisfaction of the respondents, in many varied ways. Closely
connected with the question of immediate impulse is the deeper, and less
easy to prove problem as to how migration originated.
It has been dogmatically asserted repeatedly that birds invariably
breed in the most northerly part of their range, and winter in the
most southerly. Winter, when speaking of Holarctic birds, only applies
to the season in the northern hemisphere; the birds which pass south
of the equator winter in summer. Whilst accepting this as a rule, two
reservations must be made. First, that it only applies to birds of the
northern hemisphere, and secondly that it is a rule with exceptions. It
seems probable that the breeding area of some of the birds which reach
the British Islands in autumn by the so-called east and west route is
in more southerly latitudes than our islands, and certainly it seems
evident that the temperature of the winter refuge has more effect upon
the birds than its geographical position. Perhaps the statement that a
bird always nests in the coldest part of its range is more universally
correct. Even this may not be invariably the habit, but in acknowledging
it as a rule we must clearly understand that this cold district is
resorted to at the period of the year when its temperature is at its
highest. There are certain birds which breed in Australia and winter in
Oceanic islands where the temperature is cooler than in their breeding
area.
When considering the migration of birds which summer in the extreme
north or breed in the extreme south--alas, but little is known about
the migratory habits of many southern breeders--it is comparatively
simple to offer an explanation; in the long winter months this home,
so desirable in the short weeks of daylight, is dark, ice-bound, and
foodless; it is wholly unsuited to the requirements of birds, which,
in spite of many assertions to the contrary, have never been proved to
hibernate, the only way in which animals can survive for any lengthened
period when food supply is entirely cut off.
Birds are structurally provided with the means of escaping from the
disastrous effects of adverse circumstances; the power of flight, though
not the only way in which animals can migrate, is at the root of the
migration of birds. The advantages of the power of flight, to which also
it owes its development, include the ability to avoid active and passive
enemies, and to remove from one feeding ground to another undeterred by
the barriers which restrict the terrestrial animal. A natural sequence
of this ability to take advantage of aerial locomotion is the habit of
wandering in search of food, more or less noticeable in all birds. The
habit of wandering led to the discovery of feeding grounds and suitable
nesting places; where these nesting places, probably at first, only
removed a short distance from the parents' nesting site, were suitable,
dispersal and an extension of the distributional area or range of the
species followed; but where the feeding area was unsuited or not so
well suited to the needs of the species, hereditary attachment to the
original home and memory of the direction of this home, or even in some
cases accidental wandering back to the more suitable locality, would
originate a migration. Coupled with this are two important factors which
would tend to make the habit periodical and regular both as regards
time and locality. The memory of the bird, call it instinctive memory
if we like, would limit the wanderings in search of food to a certain
number of places where food was most abundantly found, and the passage
between feeding area and breeding area become regular journeys, at the
seasons of the year when an increasing number of young birds in the
breeding area drove the overgrown population to seek food further from
the base, and again when the sexual impulses urged the birds to seek
secure nesting sites. The other factor is the weeding-out influence of
mistaken effort, the natural selection which leads to the survival of
the fittest. The young wanderer which reached unsuitable lands must
either wander further or perish. Judging by the juvenile mortality
amongst young birds the failures would be many, and only the successful
competitors would return to leave progeny.
Great stress has been laid on the attachment of birds to certain nesting
sites, an undoubted fact, and it has been argued that because, in some
cases, for hundreds of years certain sites have been occupied by the
same species, it is evident that after the death of parents the young
will return to and occupy the home. This has even been put forward as
evidence that birds do not wander in search of fresh nesting sites. The
argument is not sound. It is improbable that in most cases both parents
perish in the same year. Birds of prey, and many of the cited instances
of long tenancy refer to raptorial birds, have a wonderful power of
finding a mate, male or female, to complete the hatching and rearing of
the young, when one of a pair has been destroyed. The survivor of any
pair might have the home attachment and by bringing a fresh mate create
an attachment which would be passed on from mate to mate indefinitely.
Again it must not be overlooked that certain sites present advantages
to particular species which must be evident to all in search of those
advantages; it by no means follows that the occupiers of a nesting site
are in any way related, except specifically, to those which occupied it
in previous years.
The answer to the argument that birds do not seek fresh nesting places
and thus extend their distributional area, is evident when we consider
those species which, at the present time, are extending their range.
Within the last few years, for instance, the turtle dove and tufted duck
have begun to nest regularly in many parts of England in which they were
entirely unknown twenty or thirty years ago. The starling has spread and
in some parts is spreading still, and many other similar cases might be
cited.
In this manner migration, as we know it to-day, may have originated, and
as Mr P. A. Taverner expressed it, "however instinctive their habit may
now be, there must have been a time when migrations were intelligent
movements, intended to escape some danger or secure some advantage"
(51). Granting this, however, as the first cause, we are only on
the threshold; the question still remains unanswered, what actually
impels the birds to seek fresh food supplies or to look for safe nesting
places? The natural answer, the cravings of nature and sexual impulses
fails to give satisfaction in every case. Wanderings in search of food
might lead in any direction, and probably did in the first place, but
now birds in the main travel south in search of food and north in search
of home, and many of them perform immense journeys, passing over or
through lands which are capable of supporting a wealth of bird-life even
in the winter months.
The majority of Arctic birds or those nesting in high latitudes leave
before the great harvest of autumn fruits, and even our common swift
begins to depart--for all do not go at once--towards the end of July,
when insects are more abundant than at any other time of the year. Food
supply has not failed when most birds start their journey in search
of food! Again in spring, when it is claimed that the powerful sexual
impulses are sufficient reason to account for the northward journey,
hosts of sexually immature birds and of others which are apparently
mature but do not breed that spring, migrate northwards, some even
arriving before the mature birds of their own species.
The earlier students of migration insisted that temperature was the sole
cause of change of abode; that the northern lands became unsuitable
through their falling temperature, and that the birds deserted them for
warmer climes, returning when the lands they wintered in became too hot.
As a variant of this notion, which cannot be lightly cast aside, the
suggestion was mooted that it was not cold but the lack of food during
the cold months which drove them south, and that in the Tropics, where
at one time it was thought that all migratory birds wintered, food was
scarce during the months of extreme heat. Dr. Wallace went further and
stated that the incentive to northern migration was the inability to
find sufficient soft bodied insects suitable for the nestlings in the
Tropics during summer (54). Yet there are birds which do find food
enough for their young, and some of them are insect eaters.
Seebohm, arguing with reason that the first home of the _Charadriidae_,
was the Polar Basin (44), suggests that the desire for light
originated the idea or the action, and though this was only applied
by him to Arctic birds, others have striven to show that the longer
hours of daylight would be an advantage to all birds, even though the
difference of dark and light in the zone retired from and in that
arrived at might be inconsiderable (41). Against this must be taken
into consideration the fact that many waders and ducks, northern
breeders, feed by night or day, according to the state of the tide.
Light is not an absolute necessity to them.
The suggestion that migration owes its origin to the Glacial Epoch,
"that supposed solution of so many difficulties," to quote Mr Gadow
(28), has had many exponents. Some take for granted that the Polar
Regions were the original home, the centre of dispersal, of all northern
birds, and consequently that migration originated in the gradual pushing
back of avian life as the ice gained more and more land each year.
During the summer, the birds, urged by an irresistible love of home,
travelled as far north as the ice allowed them, but gradually they were
driven to nest further and further south until they found refuge in the
unglaciated parts of the earth. The individuals and the species, if not
the whole families of birds, which failed to retreat, went the way of
the "thousand types." On the retreat of the ice, the birds, impelled by
a mysterious hereditary memory of home and of the good times enjoyed by
their remote ancestors, for very very many generations must have been
born under more or less sedentary conditions during the Ice Age, began
the same pushing forward each year to the limits allowed them. In this
case they travelled nearer and nearer to the original home instead of
constantly being driven further from it.
Surely the question of original home, at any rate of the home in
pre-Glacial days, may be entirely left out of the question. No one can
ever prove that this wonderful memory did or could exist. Post-Glacial
dispersal northwards, and the foundation of migratory habits of
advancing to the new food-producing areas, suitable also for the rearing
of young, was doubtless a fact, but would have taken place in any case.
The congestion due to the increased numbers driven to a restricted area,
would involve a rebound outwards, and the uninhabited areas northward
of the refuge would be the natural bourn towards which the birds would
travel. The seasonal return of cold would drive them southwards in
winter, and the periodical migration habit would thus be originated.
The intense love of home during the spread of glacial conditions would
tend rather towards extinction than the formation of any new habits.
The birds which possessed the greatest attachment to the particular
district would be less likely to fly from adverse conditions, and the
reduction of their numbers through the ordinary physiological changes
in habit--reduction of the number of young produced, and possibly
disinclination to pair--would inevitably end in extinction. The stronger
the attachment to home the more likely the bird to remain to the bitter
end, and if driven away by increasingly severe winters, to return
and attempt to nest in the locality which had become unsuitable for
nesting. The spread of glaciation would be gradual and so would be the
annihilation of the species, but the end would be sure.
Birds which are cited as species which have shown this remarkable
attachment to home, have disappeared before adverse circumstances--the
great auk and the Labrador duck.
From what little we do know about the behaviour of our summer birds in
their winter home, we may safely conclude that their habits are similar
to those of winter visitors to Britain. Only in a few species are there
two restricted areas, two abiding places or homes. The necessity of
retaining a secure home for the young and the care of these young during
their more helpless age keeps the individual birds within a certain
area during the breeding season, but at all other times the bird is
more or less of a wanderer. The variation, however, of the wanderings
is remarkable. For instance the flocks of fieldfares, redwings, and
some of the finches which come to winter in the British Islands wander
continually from feeding ground to feeding ground, remaining in one
place only so long as the food supply is plentiful. When there is a
plentiful harvest of beech-mast, chaffinches and bramblings will linger
near one clump or avenue of beeches for many weeks, but when, as often
happens, the mast crop fails, they become nomadic, and pass from place
to place in their hunt for food. They visit fields top-dressed with
manure, glean the refuse of the harvest, frequent the farm-yards, and
in early spring, visit the budding larches to prey upon their insect
pests. On the other hand golden plovers and lapwings are remarkably
local in their winter habits, and so long as the weather remains open
will frequent the same fields throughout the winter. Severe weather,
especially snow, which effectually closes their chance of obtaining
food, at once drives them away. They will migrate to the unfrozen
mud-flats of the coast, or to those parts of England, generally the
south-west, and Ireland, where the climate is normally milder, or they
will even leave our islands altogether under great stress.
The wandering habit, except during the breeding season, is confirmed in
most birds, and experience shows that the same species of birds visit
the same districts again and again when there is some particular food
supply to attract them. Memory and experience guide them from place
to place. This regular visitation of certain food bases, being of the
greatest importance to birds which have a long period of travel or
wandering before them, tends to originate the so-called route by which
they travel. The fact that as a rule these stages are in consecutive
steps southward is surely due to the fact that the temperature is
falling in the north more rapidly than in the south. That they are not
always due south is certain. The American golden plover, as Mr Wells
W. Cooke so lucidly demonstrates, at first travels eastwards from its
home in western Arctic America to the fruit-laden lands of Labrador
and Nova Scotia, where it feeds for some time, stoking up for its long
oversea journey due south. Mr Cooke says, "It can also be said that food
supplies _en route_ have been the determining factor in the choice of
one course in preference to another, and not the distance from one food
base to the next. The location of plenty of suitable provender having
been ascertained, the birds pay no attention to the length of the single
flight required to reach it" (21). During the evolution of the route
many bases would be found which were superior to others, and skipping
and the gradual shortening of the journey from one to another would
result. The final goal, the food base which in any weather or season
provides the safe sufficiency of food, having been reached by the birds,
this becomes the winter quarters. They return to this secure retreat
each winter, instead of aimlessly wandering in search of a better, and
thus the long-distance migratory habit is formed. Heredity tends to
confirm this and it becomes an instinct.
Any observer may verify the assertion that birds regularly visit certain
favourable food-bases by paying attention to the occurrences of birds of
passage. The study of a county, for instance, shows that certain species
show partiality for particular localities. Thus in Cheshire goldeneyes
pass through every spring and autumn, and may be met with occasionally
on any of the meres; but at Oakmere, in the Delamere district, one may
be almost certain of seeing parties of this species any time during the
periods of passage. The curlew may be heard or seen passing over any
part of the county, but only in the Delamere fields do we frequently
meet with flocks feeding in inland Cheshire. Before the winter resident
golden plovers have arrived in autumn and after they have departed in
spring, the favourite fields are regularly visited by passing flocks,
and the lower reaches of the Mersey, where the common sandpiper is rare
as a summer resident, are visited every autumn by parties of birds on
passage. Chance may lead a casual wanderer to a good food-supplying
spot, but the regularity of appearance suggests habit and memory.
A fact which supports the theory that birds ramble far in search of
food in their winter quarters, is that in many species the winter range
is more extensive than the breeding area. Thus Mr Cooke shows that the
known breeding area of the Pacific golden plover has an east and west
extension of some 1700 miles, but in winter it ranges over an area with
an east and west extension of about 10,000 miles. The scarlet tanager,
however, has a breeding range extending for some 1900 miles across
eastern Canada and a winter home in north-western South America of only
some 700 miles in extent.
The winter quarters, or the outermost limits of the individual but not
necessarily the specific range, having been reached, the bird spends
its time in seeking food, remaining in one place if food is plentiful,
or wandering, according to necessity or the habit of the species.
The assertion that some birds have a second breeding season in their
southern home is either unsupported by any direct evidence or is the
result of a mistake in identification; the bird which has been found
breeding has in several instances been shown to be a southern form or a
related species of the one it was thought to be.
As the northern spring approaches, the strongest of all animal
instincts, on which reproduction and the very existence of the species
depend, overcomes all other desires, and the bird grows restless. The
hereditary instinct, the origin of which we have endeavoured to show,
urges the bird to seek the breeding area which has by degrees become so
far removed from the winter quarters. The bird returns home.
But here is a serious difficulty urged by some writers as a powerful
argument against the sexual impulse as the great factor in the return
journey. Many of the birds which migrate northwards or homewards are
sexually immature, and others of them are undoubtedly to be classed as
"non-breeders," which means that during that particular summer they will
not be engaged in the work of reproduction; why, then, should young
birds or non-breeders migrate from the winter base. Possibly in the
early days of migration only the mature birds did return; that we cannot
state one way or the other. But it is reasonable to argue that once a
regular migration habit has become not only confirmed by heredity but a
very true advantage to the species, its influence will be felt by each
and every individual. Again it is clear that the sexual impulses, in an
undeveloped form, are appreciated by the adolescent, and in many animals
by even the most juvenile. The play of all young animals is either an
imitation or reflection of the search for food--the hunting instinct--or
the love-making and sexual quarrels pertaining to reproduction, the
pretended competition by the young for the favours of the opposite sex.
They may play at and actually perform a migration which is so closely
bound up with the life of the species. That this impulse has not always
sufficient strength to force them to perform the whole journey is
apparent from the fact that many non-breeders, young or sexually mature,
on their northward journey through our islands or along our coasts,
never reach the breeding area; the food supply on the way attracts them
more than the memory of home; they linger with us until the breeding
season is over and the return journey has begun. Knots, sanderlings,
turnstones and many other waders may be seen on passage late in June,
and some remain on our mud-flats throughout the summer; in July the tide
of migration has turned.
It has been suggested that some of the sexually mature non-breeders may
be actually enjoying their winter during our summer; in other words that
they have bred in southern breeding-stations whilst their congeners
wintered in the same zone. This means a double breeding-area for certain
species--a possible explanation, but one hardly supported by known
facts. When a bird had so cosmopolitan a range that in the course of
its dispersal its breeding areas were separated, we almost invariably
find that the birds inhabiting these two areas are distinguishable
geographical forms or sub-species. Mr W. H. Hudson, in his "Naturalist
in La Plata" refers to the godwit, _Limosa haemastica_, which spends
the southern summer in La Plata and breeds in the north, and to birds
of the same species which winter in La Plata, arriving from supposed
breeding places to the south when the northern birds leave. Captain R.
Crawshay, author of "The Birds of Tierra del Fuego," found it in this
little known land, but speaks somewhat doubtfully of its identity; we
shall probably learn that the southern form is sub-specifically distinct
from the northern. There are other wide-ranging waders which are
suspected of having a southern nesting area, but we still await proof.
The lack of sufficient or suitable food in the winter home during our
northern summer may also cause the exodus, but this is a difficult point
to prove when it is remembered that the winter home of every bird is
not the parched tropical land or the waterless desert. From some zones
removal must be a necessity, but in others there is food for all, so far
as man can tell.
Dr J. A. Allen, a severe but discriminating critic of migration
theorists, says--"Migration is the only manner in which a zoological
vacuum in a country whose life-supporting capacity is a regular
fluctuating quantity, can be filled by non-hibernating animals" (51).
When in the early days of migration this periodically-supplied northern
zoological vacuum was filled to overflowing by the increased numbers of
avian inhabitants at the close of the breeding season, the natural food
supply would be taxed to its limits; the falling temperature drove some
and finally all to seek food further south, and their short migration
to lands already filled with old and young birds, caused pressure and
overcrowding further south. Further outward and usually southward
movement was necessary and the zone of stress was gradually extended,
though probably in those early days no particular species took long
passages. The winter passed and the vacuum was again provided, and the
rebound to fill it would create a slackening force all along the line;
birds would spread from congested districts so soon as food supplying
areas opened to receive them.
Mr Taverner, arguing on these lines (51), shows that competition
would be originated in areas containing the earliest breeders, and be
severest in the most productive districts. Weaker and later breeders
would be driven out or prevented from colonizing by the stronger and
earlier species, and the evicted ones would encroach on others, forcing
them in turn to trespass on a wider circle of species. He then argues
how the gradual recession of the glacial ice would increase the possible
northward breeding area, and cause longer migration, and that this
migration would delay breeding and conversely delayed breeding would
assist the evolution of migration.
But the lengthening of the journey might surely be occasioned in
another way, and the evolution of migration assisted apart from any
glacial influences. Each successive increase of the length of the
journey taken by the stronger and more go-ahead individuals, leading
them in advance of the bulk of southward moving and competing birds,
would be a distinct advantage to the individual and consequently to the
species. The pioneer would arrive, like the slower movers, in a land
already peopled with an avian population, but it would not have its own
fellows to add to the stress of competition; it would be ahead of the
greatest struggle. So the fittest would mould for the species the most
suitable journey both in distance and route, and the laggards would
gradually fall out of the competition.
Dr Wallace, without destroying these arguments, has shown that the
survival of the fittest has a powerful influence. Those birds which
do not leave the breeding area at the proper season will suffer and
ultimately become extinct, and the same will happen to those which fail
to leave the winter quarters when it would be a distinct advantage to
the species to move into lands better suited for reproduction.
It has been put forward as a serious objection to many arguments that
migration, instead of being advantageous to birds, is a danger to the
race; that the perils of the journey are greater than those occasioned
by more sedentary habits. It has even been suggested that migration is
a habit specially created to thin down the surplus bird population. Dr
W. K. Brooks, however, puts this idea, which is not entirely devoid of
truths, in rather a different way. "Adaptations of nature are primarily
for the good of the species--beneficial to individuals only so far as
these individuals are essential to the welfare of the species" (9).
The destruction of overabundant young, the thinning down of superfluous
numbers, may be an economic advantage. It is one thing to say that
migration has been caused to kill off a surplus, and another to show
that, once a habit has been originated and become an advantage, it will
be conducive to a greater prolificness, and that the natural sequence
of an increased birthrate, when food supply and other conditions remain
unchanged, must be an increased mortality. Thus the perils of migration
may become a boon to the species.
The theories of C. L. Brehm (7) and Marek that birds are living
barometers, foretelling by intuition the changes of barometric pressure,
may be dismissed as purely speculative. That birds begin their journeys
during particular barometric conditions is certain, but what they know
of forth-coming weather conditions is guess-work.
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