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.