Of observers whose writings are extant, Aristotle, naturalist and philosopher of ancient Greece, was one of the first to discuss the subject of bird migration. He noted that cranes traveled from the steppes of Scythia to the marshes at the headwaters of the Nile, and that pelicans, geese, swans, rails, doves, and many other birds likewise passed to warmer regions to spend the winter. In the earliest years of the Christian era, the elder Pliny, Roman naturalist, in his Historia Naturalis, repeated much of what Aristotle had said on migration, and added comments of his own concerning the movements of the European blackbird, the starling, and the thrushes.
In spite of the keen perception shown in some of his statements, Aristotle also sponsored some superstitions on bird migration that persisted for several centuries. One of these, that of hibernation, became so firmly rooted that in 1878, the American ornithologist Coues (20)[2] listed the titles of no less than 182 papers dealing with the hibernation of swallows (Hirundinidae). The hibernation theory accounted for the autumnal disappearance of certain species of birds as their passing the cold season in a torpid state, hidden in hollow trees, caves, or the mud of marshes. Aristotle ascribed hibernation not only to swallows but also to storks, kites, doves, and others. Some early naturalists wrote fantastic accounts of flocks of swallows seen congregating in the marshes until their accumulated weight bent into the water the reeds on which they clung and thus submerged the birds. It was even recorded that when fishermen in northern waters drew up their nets they sometimes had a mixed "catch" purported to consist of fish and hibernating swallows. Clarke (4) quotes Olaus Magnus, Archbishop of Upsala, who in 1555 published a work entitled "Historia de Gentibus Septentrionalibus et Natura", wherein he observed that if swallows so caught were taken into a warm room they soon begin to fly about but would live only a short time.
[2] Italic numbers in parentheses refer to the Bibliography, p. 66.
The hibernation theory survived for more than 2,000 years and is still occasionally repeated by credulous persons to account for failure to locate definitely the winter home of the chimney swifts (Chaetura pelagica), which each autumn gather in immense flocks in southern Georgia and northern Florida and then suddenly disappear. There are, however, records of occurrence during migration for a few points in the West Indies, Mexico, and Central America, and it is probable that these birds spend the winter season in the great rain-forest area of the Amazon Valley in Brazil, passing most of the daytime high in the air with other swifts that are local residents.
Aristotle was also the originator of the theory of transmutation, basing it upon the fact that frequently one species will arrive from the north just as another species departs for more southerly latitudes. From this he reasoned that although it was commonly believed that such birds were of two different species, there really was only one, and that this one assumed the different plumages to correspond with the summer and winter seasons.
Probably the most remarkable theory that has been advanced to account for migration is contained in a pamphlet mentioned by Clarke (4, v. 1, pp. 9-11), as published in 1703 under the title: "An Essay Toward the Probable Solution of this Question: Whence Come the Stork and the Turtle, the Crane, and the Swallow, when they Know and Observe the Appointed Time of their Coming." It was written "By a Person of Learning and Piety", whose "probable solution" was that migratory birds flew to the moon and there spent the winter.
Some who easily accepted the disappearance of the larger birds as migratory travelers were unable to understand how the smaller species, some of them notoriously poor fliers, could make similar journeys. They contended that the larger species, as the storks and cranes, carried their smaller companions as living freight. In some of the Mediterranean countries it is still believed that these broad-pinioned birds serve as aerial transports for the hosts of small birds that congregate on the shores of the Mediterranean Sea, awaiting opportunity for this kind of passage to their winter homes in Africa. Similar beliefs are found among some tribes of North American Indians.
Before presenting some of the present theories concerning the origin of bird migration, it seems well to consider briefly the ends served by this annual round trip between breeding grounds and winter quarters. It is apparent that the migratory habit enables a species to enjoy the summers of northern latitudes while avoiding the severity of the winters. In other words, migration makes it possible for some species to inhabit two different areas at seasons when each presents favorable conditions. In the performance of its reproductive duties every pair of birds requires a certain domain, the extent of which varies greatly in different species. Generally, however, this territory must be large enough to provide adequate food, not only for the parent birds but also for the lusty appetites that come into being with the hatching of the eggs. Thus, if all birds were to remain constantly either in tropical or in temperate regions, there might be intolerable overcrowding during the breeding season. By the spring withdrawal to regions uninhabitable earlier in the year, the migrants are assured of adequate space and ample food upon their arrival in the winter-freed North, and it may be assumed that nonmigrant species resident in the South are benefited by the withdrawal of the migrants.
Nevertheless it cannot be said that the winter or summer areas are entirely unsuited to the requirements of every migrating species at other seasons, for some individuals pass the winter in areas that are frequented only in summer by other individuals of their species. The extensive breeding ranges of such species present wide climatic variations, so that some individuals may actually be resident in a region where others of their kind are present only in winter.
The tendency in many species to move southward at the approach of winter is not always due to the seasonal low temperatures, since experiments have demonstrated that many summer insect feeders, when confined in outdoor aviaries, comfortably withstand temperatures far below zero. The main consideration is the depletion of the food supply, caused either by disappearance or hibernation of insects, or by the mantle of snow or ice that prevents access to the seeds and other forms of food found on or close to the ground or submerged in water. Possibly also the shortened hours of daylight materially restrict the ability of the birds to obtain sufficient food at a time when the cold requires an increased supply to maintain body heat. It is noteworthy that chickadees (Penthestes atricapillus) and some other of our smaller birds have no fear of Arctic weather, as their food supplies are mainly arboreal and so are always available. Also, when there is a good supply of food in the form of pine seeds in Canadian woods, nuthatches (Sitta carolinensis and S. canadensis) and crossbills (Loxia curvirostra and L. leucoptera) will remain through the winter. When these birds appear abundantly in winter at points in southern latitudes, it may be concluded that there is a shortage of their food in the North, or that they have been lured farther south by the greater abundance of this food there.
Migration has long since become a definite hereditary habit that recurs in annual cycles, probably because of physiological stimuli associated with the reproductive period. In seeking its origin it is necessary to study the history of the birds' occupation of their present ranges, and from the information available to consider what appear to be reasonable theories. The two now most commonly accepted are diametrically opposed to each other.
According to one of these, nonmigratory birds swarmed over the entire Northern Hemisphere in earlier ages, the conditions of food and habitat being such as to permit them to remain in their haunts throughout the year. The entire northern area then afforded the two important avian requirements—suitable breeding conditions and a year-long food supply. This is the condition today in the Tropics, and it is noteworthy that many tropical birds are nonmigratory. Gradually, however, in the Northern Hemisphere the glacial ice fields advanced southward, forcing the birds before them, until finally all bird life was concentrated in southern latitudes. As the ages passed and the ice cap gradually retreated, each spring the birds endeavored to return to their ancestral homes in the North, only to be again driven south at the approach of winter. As the size of the ice-covered area diminished, the journeys made became ever longer, until eventually the habits of migration were fixed to accord with the climatic conditions of the present age.
Thus, this theory supposes that today migratory birds follow the path of a great racial movement that took place in a distant past, associated with advances and recessions of the ice. The actions of the birds themselves lend some support to this theory, as every bird student has noted the feverish impatience with which certain species push northward in spring, sometimes advancing so rapidly upon the heels of winter as to perish in great numbers when overtaken by late storms. It is probable, however, that at that season the reproductive impulse urges the birds on to their northern breeding grounds.
The opposing theory is simpler in some respects and supposes that the ancestral home of all birds was in the Tropics and that as all bird life tends to overpopulation there was a constant effort to seek breeding grounds where competition would be less keen. Species that strove for more northern latitudes would be kept in check by the ice and forced to return southward with the recurrence of winter conditions. As the ice retreated, vast areas of virgin country became successively suitable for summer occupancy, but the winter habitat remained the home to which the birds returned after the nesting season. It is a fact that some species spend very little time on their breeding grounds; the orchard oriole (Icterus spurius), for example, spends only 2½ months in its summer home, arriving in southern Pennsylvania about the first week in May and leaving by the middle of July.
Both theories assume that migration is an ingrained habit, but neither is supported by positive biological data. Both have been criticized also on geological grounds, and neither can be accepted without qualification. It is apparent, however, that whether the ancestral home of any species was at the northern or at the southern limits of its present range, or even in some intermediate region, the search for conditions favorable for breeding in summer and for feeding in winter has been a principal factor underlying the origin of migration.
A modern view, based on studies of living behavior, favors the theory of photoperiodism, propounded by recent investigators as the cause of the annually induced movements of the birds. This theory holds as its major premise that quantity of light and length of day are the stimulating causes of migration. Its proponents urge that migration is a phenomenon far too regular to be created anew each season merely under stress of circumstances, such as need for food; and that it begins before the necessity for a change in latitude becomes at all pressing. Swallows, nighthawks (Chordeiles minor), shore birds, and others may start their southward movement while the summer food supply in the North is at peak abundance; while robins (Turdus migratorius), bluebirds (Sialia sialis), and others may leave an abundant food in the South in spring and press toward northern points when the food supplies there are almost entirely lacking and when severe cold and storms are likely to play havoc with the advance migrants. The regularity of arrival and departure is one of the most impressive features of migration, and since birds travel in almost strict accordance with the calendar, the proponents of the theory ask: What phenomenon to which we may attribute the stimulating impulse occurs with such precise regularity as the constantly increasing light in spring?
Experimental work has abundantly demonstrated the effect of increased light upon the growth, flowering, and fruiting of plants. Similarly, experiments with the common junco, or snowbird (Junco hyemalis), reported by Rowan (42, p. 121), resulted in increased development of the sexual organs by the end of December, although the birds were confined in outdoor aviaries in Canada and had been exposed to temperatures as low as -44° F. From the first of November until early in January, the juncos were subjected to ever-increasing light, supplied in the aviaries by electric bulbs. As regards illumination, they were thus artificially provided with conditions approximating those of spring. At the close of this period, it was found that the sexual organs of the birds had attained the maximum development normally associated with spring. With gradual reduction of the lighting over a period of little more than 1 month, the organs returned to their normal winter condition.
After a consideration of all evidence, including the fact that no ultraviolet rays were used, it was concluded that the explanation lay in the increased exercise taken during the periods of increased light. A simple test whereby certain birds were forced by mechanical means to take more exercise, the light being so reduced that there was merely sufficient glow for them to see the advancing mechanism that forced them into movement, showed that the rate of development of the sexual organs exactly paralleled that in the birds that were exposed to extended periods of illumination in the outdoor aviaries. Other features in this experiment—such as the behavior of the birds themselves—also indicated that more activity due to increased light is the governing cause of the spring development of the sexual organs. If this development be accepted as a controlling cause of migration, then this experiment must be recognized as of great importance.
Upon closer analysis, however, it is found that this theory, like those before discussed, is open to serious objections. First, some of our summer residents that migrate south for the winter do not stop in equatorial regions, where they might find the periods of day and night about equally divided, but push on beyond, some penetrating as far south as Patagonia. Also it might be asked: If the lengthening day is the stimulating factor, why should our summer birds wintering in the Tropics ever start northward, as in their winter quarters the variation in the length of day from winter to summer is imperceptible. Like all the other theories advanced, this also, as at present understood, is subject to unanswered criticism.