Читать книгу Migration of Birds - Frederick Charles Lincoln - Страница 6
ОглавлениеThe Origin of Migration
Migration has long since become a definite hereditary habit that recurs in annual cycles, probably because of physiological stimulus associated with the reproductive period. Its origin is locked in the ages of geologic time, but by study of the history of how birds came to occupy their present ranges, information becomes available from which reasonable theories may be developed and explored. The two that are most commonly accepted are diametrically opposed to each other.
Northern ancestral home theory
According to one of these hypotheses, in earlier ages nonmigratory birds swarmed over the entire Northern Hemisphere. At that time the conditions of food and habitat were such as to permit them to remain in their haunts throughout the year, that is, the entire northern area then afforded the two important avian requirements—suitable breeding conditions, and year-long food supply. This is the condition today in the Tropics, and it is noteworthy that, as a rule, tropical birds do not perform migrations. 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 the ice cap gradually retreated, and each spring the birds whose ancestral home had been in the North endeavored to return, only to be driven south again at the approach of winter. As the size of the ice-covered area diminished the journeys made became ever longer until eventually the climatic conditions of the present age became established and with them the habit of migration.
Thus, this theory supposes that today migratory birds follow the path of a great racial movement that took place in a distant past and was associated with the 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 that they perish in great numbers when overtaken by late storms. It is probable that at this season the reproductive impulse is a determining factor in driving the birds to their northern breeding grounds.
Southern ancestral home theory
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 the competition would be less keen. Species that strove for more northern latitudes were kept in check by the ice and were forced to return southward with the recurrence of winter conditions. Gradually, 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, 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 of these theories assume that migration is an ingrained habit, but both have been criticized on biological and geological grounds, so neither should be accepted without qualification as definitely accounting for the origin of bird migration. It is apparent, however, that whether the ancestral home of any species was at the northern or southern limits of its present range, or even in some intermediate region, the search for favorable conditions under which to breed in summer and to feed in winter has been the principal factor underlying the origin of migration.
Theory of photoperiodism
A modern view based on studies of living behavior, suggests also that there is good reason for believing that migration is an annually induced movement. If such be true then the theory of photoperiodism as propounded by some recent investigators should receive some consideration.
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, shore birds, and others may start their southward movement while the summer food supply in the North is at peak abundance; while robins, bluebirds, and others may leave an abundant food in the South in spring and press toward northern points when food supplies there are almost entirely lacking and when severe cold and storms are likely to cause their wholesale destruction. 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 amount of 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 reported by Rowan (1931: 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 ultra-violet 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.
Theory of continental drift
The theory of continental drift postulates an original northern land mass, called Laurasia, and a southern one, called Gondwana. According to this concept, each eventually broke into several segments which eventually became the present continents. It is further assumed that occasionally Laurasia and Gondwana drifted close to one another or were at times in actual contact. On the basis of this geological theory, Wolfson (1940) has attempted to explain the migrations of some species of birds from one hemisphere to the other, as, for example, the Greenland wheatear, Arctic tern, and several shore birds (turnstone, sanderling, knot, golden plover, and others). Acceptance of this hypothesis requires abandonment of the belief that the development of migration was the result of useful ends that were served thereby, and in its place, to give approval to the idea that migration was merely "the natural consequence of an inherent behavior pattern responding to the drifting of continental masses."
It is a strange fact that although almost all professional paleontologists are agreed that existing data oppose the theory of continental drift, those who support it contend that their case is strengthened by these same data. If, in the geologic history of the earth, there was any such thing as continental drift, it appears from the evidence available that it was before the Cretaceous period, estimated to have been about 70,000,000 years ago. Birds had then evolved but those known from fossil remains were of extremely primitive types such as Hesperornis and Ichthyornis. There is no evidence of the existence in that period of any birds that were even closely related to any of those now living. Accordingly, it is difficult to believe that the migratory patterns of existing species have been determined by events that, if they did take place, were at least 70,000,000 or more years ago.
