BIRDS, MIGRATION OF. The broad fact of the seasonal journeys of birds is a matter of common knowledge, and has been recognized from very ancient times. The author of the Book of Job wrote of the hawk that stretches her wings toward the south ; Homer wrote of the flight of the crane and Anacreon of the return of the swallow; primitive peoples have used the names of migratory birds for the months in their calendar. Latterly, also, the theoretical problems presented by migration have attracted the interest of biologists, and a knowledge of the facts has become of real practical importance in economic ornithology. Yet to day, after untold centuries of popular knowledge and many years of more deliberate investigation, there still remains much about migration which is unknown.
Some part of this ignorance is due to the difficulty of observing migration. In most regions very little is to be seen of migration actually in progress. The observer may note the annual reappear ances and disappearances of many species, and in autumn the "flocking" which often precedes departure. He may also, but less readily, note the fluctuations in the numbers of other species which are never wholly absent. Only occasionally, as a rule, will he see birds arriving on the coast from over the sea.
Much migration takes place by night, and this even in the case of species which are wholly diurnal in their ordinary habits. Noc turnal migrants may sometimes be heard calling overhead, but they are, naturally, rarely to be seen except when the lantern of a lighthouse exercises that fatal attraction, usually in misty weather, which lures so many to destruction. Otherwise, there is merely the sudden presence one morning of species not previously seen, or the disappearance of others. Mere fluctuations of numbers cannot easily be gauged, except at some favoured island or other isolated station where the resident bird population is negligible and large numbers of resting migrants are concentrated within a small area. On occasion, nevertheless, great diurnal movements are visible, and the steady procession of vast flocks of birds forms an extraordinarily impressive sight.
With this initial difficulty of observation, scientific investigation of migration is seriously handicapped. Even where opportunities for observation at a particular place happen to be good, the results are difficult to interpret : mere local ob servation can obviously never give a satisfactory picture where distance and movement are of the essence of the matter. Much can be done, and has been well done in both Europe and America, by concerted schemes of observation in which simultaneous rec ords from many localities over a wide area are collected and cor related. Although this method has been fruitful, and has pro vided the main basis of present knowledge, it still leaves some aspects of the question almost untouched.
Of late years a supplementary method has been greatly devel oped which approaches the matter from a different angle, that of the individual bird. This consists of marking birds in large numbers for the sake of the information obtainable from such of them, necessarily only a small percentage as a rule, as may subsequently come to notice. Birds are marked either as nestlings still unable to fly, or as adults caught for the purpose in some non-hurtful way. The usual mark is a small aluminium ring or band resting lightly on the foot. The ring bears an address to which the finder can report, and an identification number which enables the date and place of marking to be traced from the original record. Marking is especially valuable in, so to speak, disentangling the movements of individual birds, typical of those native to a par ticular region, from the vast and complex movements of the species as a whole which can otherwise be observed only in the mass. The method has already yielded much interesting informa tion of an accurate and definite kind not obtainable by other means, and its further employment should result in important additions to knowledge.
Migration consists essentially of an an nual cycle of movements between two alternative habitats. Of these, the summer habitat provides the special opportunities which are necessary for the breeding season, but becomes too inhospi table in winter. The other habitat suffices for the purposes of ordinary life in winter, but is presumably inadequate for the more exacting requirements of the breeding season. Alternation be tween the two, made possible to birds by their great mobility, is thus related both to the seasonal changes in the respective environ ments and to the different needs of the bird in the reproductive and non-reproductive phases of its life.
It is a general rule that a bird breeds in the coldest part of its range. The breeding area of a species is usually clearly defined, and individual birds, as marking results have amply shown, com monly return to their native localities in successive years. In some cases the winter area may be almost or quite as definite : in other cases it may be more vaguely circumscribed, and in some instances the annual movements seem to be mere wanderings in which the breeding place is the only fixed point.
In the comparatively constant condi tions of the tropics many species are wholly stationary. In high latitudes the winter conditions are so severe as to make migration a necessity for almost all birds. Intermediately in temperate climates, migration in some degree is the rule and wholly station ary species are in a minority.
Migration is in some cases no more than a descent from high ground to the neighbouring valleys, or a movement from an inland area to the adjacent coast. In other cases an entire region may be deserted in favour of another at any distance up to several thousand miles and possibly separated by wide seas. Migration from high latitudes, furthermore, may carry the birds across the equator into the temperate regions of the other hemisphere. Many birds from the northern hemisphere thus winter in the summer of high southern latitudes, leading their non-reproductive life during what is the breeding season for birds native to these parts: it so happens that there is scarcely any counterpart to this in the case of birds breeding in the far south.
Different species of birds have different needs, and what serves as a winter area for one may be suitable only as a summer area for another. Thus in a temperate region there are so-called sum mer visitors and winter visitors. There is no difference in princi ple, but the observer is so placed as to see the two migrations from different angles. Other species he sees only as birds-of passage, twice a year, on their journey between widely separated summer and winter areas. In still other cases the two areas of a species are adjacent or partly overlapping, and the winter area may even lie wholly within the wider summer range. When there is overlapping, the species is present throughout the year, although some or all of the individuals are migratory. Even individuals of the same species and native to the same locality may differ, some being stationary and others migratory. Furthermore, im mature or other non-breeding individuals may sometimes linger in the winter area during the summer.
Although the movements of migration are described as taking place in autumn and in spring, they are by no means restricted to two short periods. Some species move sooner than others, and individuals from different parts of the range may also differ widely in the times of their migration. The arrival even of a single species in a particular area is usually spread over a considerable period, and the performance of the journey itself also takes time. With all these differences, migra tion as a whole is spread over the greater part of the year. The latest arrivals in the highest latitudes complete their northward journey only a little before midsummer, by which time some young birds of earlier species further south are almost ready to leave. Autumn migration is on a grander scale than spring migra tion, with the number of birds at its maximum after the breeding season. The autumn movements also tend to be more leisurely and protracted, and they are often supplemented by weather movements which accompany specially severe conditions during the winter : these may almost overlap the first spring movements in the opposite direction.
Passing reference may be made to certain erratic migration phenomena which are exceptional in that they do not occur annually. This is apart from the question of late weather movements which may be regarded as irregular extensions of the ordinary autumn migration, and from the vari able wanderings of some species in which migration is not devel oped to a pitch of regularity. The most remarkable case is that of Pallas's sandgrouse (Syrrhaptes paradoxus), a species normally native to central Asia and the extreme south-east of European Russia. It is not ordinarily a great migrant, but at intervals of sev eral years an "irruption" takes place in spring in a northwest erly direction. Numbers are then recorded throughout Europe, and the species may attempt to breed in the new countries which it thus reaches. Most of the birds perish, however, and the evidence of a definite return movement is slight. Other examples are known both in Europe and in North America : most of these consist of autumn movements of unusual magnitude and extent which are performed by certain species at irregular intervals of several years.
Contrary to former be lief, migrants do not fly at great altitudes, but generally keep within 3,000f t. of the ground, and often close to the surface of the land or water. Great heights above sea-level are of course reached when high mountain-ranges are crossed, but otherwise only exceptionally.
On migration an unhurried, steady speed is usually maintained, the ordinary flight of the species and not the accelerated velocity of which it is capable over short distances in emergency. The total journey performed by an individual bird may be anything up to several thousand miles : marked swallows and storks from northern Europe, for instance, have been recovered in South Africa, a distance of some 6,000m., and this must be exceeded in some cases. How long a flight is performed at a single stage is difficult to determine. Probably about Zoo miles, in a day of from six to eight hours' actual flying, is not unusual. Some sea-cross ings, however, involve more sustained efforts: the Pacific golden plover (Charadrius dominicus fulvus) is credited with an unin terrupted flight of some 2,000m. from Alaska to Hawaii, without possibility of rest near the direct route.
Migration does not necessarily take place in a north and south direction, although that is the general tendency. In Europe, for instance, much migration follows a diagonal course ; and some even proceeds on an east and west line, a more tem perate winter being found on the Atlantic seaboard than in equal latitudes further east. The general directions of the main streams of migration in different parts of the world remain the same from year to year. These directions may nevertheless be widely diver gent, in any region, for different species or even for different individuals of the same species.
In spite of the constancy of the general directions of move ment, it is probable that too much has been made in the past of the idea that migration is restricted to definite narrow routes. Coastlines are certainly followed to a large extent, and much migration may be observed in the valleys of great rivers, but it is equally true that stretches of land or sea are often traversed on a wide frontage. Whether the fly-line of the individual migrant is more precisely determined in the one case than in the other, or whether there is merely a great concentration of fly-lines along certain geographical features, is not known.
The males of many species tend to be rather earlier than the females in reaching the summer quarters in spring. In the autumn the migration of young birds is in many cases earlier than that of the adults, although this is by no means the rule for all species. In the special case of the common cuckoo (Cuculus canorus) of Europe, the reverse order is notably exhibited : the adults take their departure very early, while the young are reared by foster-parents of other (and often non migratory) species and do not leave until later.
While a general relationship obviously exists between the events of bird-migration and the annual cycle of the seasons, it has been disputed whether there is any close correspondence between particular migration events and day-to day weather changes. There is, however, much good evidence, both European and American, that the weather conditions prevailing in the area of origin of a movement have an important influence. If favourable conditions exist at the starting-point at the proper season, a movement will take place therefrom irrespective of the conditions prevailing at the goal or in the area in which the latter part of the journey must lie. In north-western Europe, for in stance, emigration is favoured by anti-cyclonic pressure condi tions, with settled fine weather and light winds : this applies both to the autumn and to the spring movements, and the mere direc tion of the wind is unimportant. Temperature, also, has an in fluence, a falling temperature in autumn and a rising temperature in spring being favourable to the inception of a movement. The maximum effect is obtained when both the barometric pressure and the temperature are favourable, and when this follows a period of adverse conditions during which migration has been retarded. Movements thus tend to begin when the weather is good at the starting-point, but storms may nevertheless be encountered on the way, leading to delay and loss of life; and it sometimes happens that very unpropitious conditions are found at the end of the journey, as when summer visitors arrive in a spring snow storm. Fog is a serious hindrance to migration and leads to loss of direction and to delay.
From a general consideration of the known facts of bird-migration, one receives an impression of vastness of scale. Thousands of species and -millions of individuals take part in migration. The movements in total are almost world-wide, and even the journeys of individual birds are often of great length. There is almost no time of year when some kind of movement is not in progress. Added to this is a great complexity. Species differ from one another in the times, directions and extent of their migrations, and even within a single species there may be much difference between individuals. The directions of flight are so various that simultaneous streams of migration may cross each other at right angles or even follow opposite directions over the same course. Yet these vast and complex phenomena show a truly wonderful degree of regularity. The same arrivals and departures are annually recorded at the same times. Individual birds accurately find their former homes on returning from half-way across the world. The same general directions of flight are maintained year after year. All the events of migration, in fact, show a regular recurrence on the selfsame plan.
Bird-migration cannot be ex plained on a basis of intelligent action, on the analogy of a human being who seeks a milder climate for the winter, without assum ing an analytical appreciation of the recurrence of seasonal events and a power of rational action altogether beyond anything which there is ground for considering as characteristic of the avian mind. To birds which are only summer visitors to high latitudes winter is a thing unknown for countless generations ; there can be no conscious knowledge of it, and therefore no reasoned intention in the act of avoidance. The possible traditional element, the imi tation of each generation by the next, is also excluded in many cases by the fact that in these the young of the year perform their first migration apart from their parents.
Nor, at the other extreme, can it be said that migration is due to the sheer compulsion of physical conditions. Although com pulsion by external forces may partly explain the "weather move ments" which occur late in the season and vary with the severity of the winter, true migration is much too complex and much too regular to be created anew each year by mere pressure of circum stances. The conditions are in any event quite inadequate as causes. A bird migrating southwards in July is not driven from its homeland by the advent of winter, and a bird migrating right across the tropics does much more than is necessary merely to avoid the winter rigours of its native area.
The performance of migration must be regarded as the expres sion of an inborn instinct, a racial custom transmitted by inheri tance, and only on this basis can any approach be made to a satisfactory understanding of the facts. (The nature of instinct is. of course. itself a riddle, but it is a well known factor in animal life and in no way peculiar to this special question.) And if it be admitted that migration behaviour is essentially instinctive, several problems naturally present themselves.
The instinct must, in the first place, serve useful ends : the custom is an expensive one and would not persist if it did not bring advantage to the race. At least some part of this advantage is obvious. Migration enables a bird to inhabit two different areas at the times of year most favourable in each and suited to the needs of life at these seasons. The advantage of winter absence from high latitudes lies in avoidance of the cold and stormy weather, of the scarcity of food, and of the short hours of daylight. The advantage of return to these latitudes lies in the availability of suitable breeding places, in abundance of food at the critical period of the year, and in the long hours of daylight for collecting it.
The fact that the instinct is useful does not explain its existence, although it may ac count for its survival. A thing does not happen merely because it is advantageous that it should happen : some effective cause must operate so that the end may be attained. The instinct must have been implanted in the race in some way, and the custom prob ably originated in the far past as the result of some circumstance or development in the history of birds and of their distribution over the face of the earth. Possibly some great climatic change, such as a glacial epoch, acted as a compelling cause and, at the time by sheer necessity, established the custom which still per sists. Possibly migration gradually evolved during the natural spread of various species from an original area of resident habi tation. Or, again, it is even possible that the custom originated suddenly in one case and then another, as the result of abrupt efforts of range expansion. The resulting custom, whatever its origin, may be regarded as to some extent an annual repetition by the individual of some process or event in the past history of the race.
There must be a further element in the causation of migration. In addition to an origin or ultimate cause there must be an immediate stimulus or occa sional cause, on the analogy of the hand which packs the explosive charge in a cartridge and the hand which pulls the trigger and so releases the pent-up force. Given the instinct lying dormant in the race, something must arouse it to activity with the recur rence of the appropriate seasons. The annually recurring stimuli which evoke the instinctive response may either be climatic or other external factors arising from the cycle of seasonal changes in the physical world, or be physiological factors arising in the cycle of alternation between the reproductive and non-reproduc tive phases in the bird's life. The stimuli may be a combination of both factors, if indeed these could in any event be wholly sep arate, with perhaps a predominance of the extrinsic element in autumn and of the intrinsic in spring. There are probably pri mary or preparatory stimuli of these kinds which produce the un rest which precedes migration, and secondary or effective stimuli in the form of favourable meteorological conditions which set migration in actual motion, not only at the outset but also at the beginning of each successive stage of the journey. The whole answer obviously does not lie in weather stimuli, as is shown by the fact that these are effective only when they occur at the appropriate seasons, occurring at other times without this result.
Finally, granted a useful pur pose in migration, granted an origin for the instinct, and granted recurring stimuli at the appropriate seasons, there remains the great problem as to how migration accomplishes its object once the instinct has been brought into play. How do migrants find their way, and what determines the way to be found? No ex ternal physical forces exist which seem adequate as factors which could determine the path and goal of migration flight in its more highly developed forms : the bird in its journeyings is no mere plaything of chance or of the winds. The paths of migration are both constant and complex, and it would seem that a capacity for following more or less definite paths must form part of the inherited instinct. It is indeed difficult to avoid the conclusion that there is some inherited memory of the path and goal, as the knowledge cannot be traditional or the action imitative where young birds migrate for the first time unguided. As to how the path is followed, it seems likely that vision is the sense principally concerned, even in nocturnal migration. The adverse effect of fog is an important point in favour of this conclusion, although vision may seem inadequate in the case of oversea paths. Other ordinary senses do not seem likely to be important. There is no evidence of any "magnetic sense"; and although very highly de veloped powers of orientation undoubtedly exist, the idea of a special "sense of direction" seems devoid of exact meaning.
A clear appreciation of these several problems is necessary, because too often there has been a failure to distinguish the different issues involved. It is said, for instance, that birds migrate to avoid the cold of winter, and this is then illogically taken to mean that the cold causes migra tion. There has also been confusion between the originating cause and the immediate stimulus, as in the fallacious argument that if food scarcity can be shown not to be the annual stimulus to autumn migration it can likewise have played no part in the origin of the instinct.
To restate the position, the main theoretical problems of bird migration are concerned with the nature of the series of factors which must be assumed to exist, as follows:—factors which make migration advantageous and thus give the instinct a survival value; factors which may in the past have helped to originate and develop the instinct in the race; factors which periodically stim ulate the instinct to active expression in the individual at the proper seasons ; and factors which determine the manner in which migration is actually performed. The nature of some of these hypothetical factors must perhaps always remain a matter for speculation, but progress towards the better understanding of others should become possible as further knowledge of the con crete facts of migration is obtained.
See W. Eagle Clarke, Studies in Bird Migration (1912) ; F. von I.ucanus, Die Ratsel des V ogelzuges (1923, bibl.) ; A. Landsborough Thomson, Problems of Bird-Migration (1926, bibl.) ; Alex. Wetmore, The Migrations of Birds (1926). (A. L. T.)
