Another superstition centring around migration has been the supposed hibernation on the part of some birds, a belief that dates back to early times and has been prevalent both in Europe and America. Aristotle attributed hibernation to the swallow and vari ous other birds, saying that some individuals became torpid, and so passed the winter in the shelter of caves or hollow trees in a state of suspended animation. In later years hibernation was used mainly to explain the disappearance of swifts, swallows and the sora rail. Many and detailed have been the arguments over this matter, and over 200 papers have been written dealing with supposed cases of hibernation. It was related that the sora rail abounded in its favourite marshes, until, overnight, the birds turned into frogs, or sank in the mud, to remain until the follow ing spring. Naturalists with great detail described how swallows gathered on reeds growing in water until their combined weights bent down these slender supports and the birds were submerged in the water. Those to whom this theory did not appeal stated that swallows and swifts hibernated in hollow trees and clefts in rocks, and the finding of birds in such situations in winter was described in great detail in a number of instances, all, however, under some misunderstanding of the circumstances. In short, though hibernation, or its correlate aestivation, is common among mammals, reptiles, amphibians, and even in fishes, it has never been proved among birds. The frequent coupling of this super stition with swallows may be explained, perhaps, by the fact that these birds in autumn regularly roost at night in marshes, and that during storms many perish to fall into the water and sink into the mud, never, however, to revive.
Among other superstitious beliefs mention may be made also of a transmutation theory prevalent among early writers, who sup posed that at the approach of winter a bird might be transformed into another species, to remain thus until spring, when it resumed its proper form. Apparently this was first suggested by Aristotle, who held that the redbreast or robin (of Europe) changed to the redstart. The confusion resulted, apparently, from similarity of form and difference of plumage in two related species, where one disappeared in southward migration about the time that another arrived from northern regions, thus lending colour to a transmuta tion argument.
Theories of Origin.—An explanation of migrational move ment has been that of seasonal change in food supply. According to this belief, with the approach of winter in areas remote from the Equator, there is failure of the food supply that causes birds to travel, and as the food supplies remain more constant toward the Tropics, birds move in that direction. When, with the approach of spring, the instinct for reproduction becomes paramount, as the food supplies of the broad equatorial area are not sufficient to sup port the great host of young birds that will appear, the adults move out again to their summer homes in temperate or boreal regions, where they rear their families. It may be argued against this that the tropical belt supports the migratory hosts when they have be come grown, so that it would seem that these same individuals might obtain sustenance there while in younger stages.
An allied theory contends that migration is controlled by cold, from which birds retreat in autumn. Under this hypothesis some have held that birds originated in the north, were driven south by the advance of ice in the Pleistocene, and have returned to the north with the coming of milder climate. Each year now they retreat before the breath of approaching winter, while each spring a love of birthplace calls them to their natal homes. It must be noted that many birds retreat south early in the season, long before there is climatic necessity for their movement. This, with similar facts, has given rise to the theory that all species of birds have had their origin in the south, and through a natural struggle among individuals have spread to the north, especially for the period when each pair must be bound to a restricted territory for its breeding ground.
A somewhat different theory is that of phototropism, which holds that birds move toward the region of greatest light, this bringing a natural ebb and flow of bird life with the changing seasons as the sun moves north and south across the Equator. It is true that the course of migration, in general, follows the ad vance and retreat of the sun, but it would seem that it is the chang ing season and not the change in intensity in light itself that affects our bird-life. Through phototropism we may not, for example, ex plain the migrations of nightjars, which are nocturnal, and which, therefore, find in darkness the period of their activity. Such birds should find their optimum conditions of life in the equatorial regions, where the hours of the day are divided between daylight and darkness. The nightjar and whippoorwill, to give two well known examples, travel north to breed, and in June in their north ern ranges have their hours of activity greatly curtailed through the lengthened period of daylight. A recent theory, supported by some experimental evidence, asserts that the onset of migration is physiologically controlled by the relative length of day and night, as is known to be the case with the flowering of plants.
The main difficulty with these and a number of other theories that have been promulgated in connection with migration is that they attempt to explain this great and impelling semi-annual move ment by some single factor. When we consider that the known history of the bird in its evolutionary development, from present knowledge goes back through an enormous stretch of time, so vast that it may be noted in figures but is beyond human comprehen sion, to the fossil species known as Archaeopteryx and Archaeornis of the Jurassic period, it must be recognized that our present day species have had their instincts and habits moulded by many fac tors, so that it seems reasonable to consider that such a wide spread phenomenon as migration may be due to a complex asso ciation of a number of powerful causes, some of which may have affected one species and some another, but no one of which may serve to explain the entire phenomenon as it exists at present.