We may believe, therefore, that the underlying basis of migra tion is founded on a combination of a number of causes, and we may look upon the present migratory instinct as an outgrowth of all the complex circumstances that have affected birds during their entire evolution, though it seems evident that the actual routes followed by migratory birds, at least in the northern hemisphere, have been shaped during the climatic changes of the Pleistocene.
During breeding-period the bird is restricted closely to the region in which the nest is located, and though some young are free to move about when recently hatched, the parents are, as a rule, confined to one neighbourhood until their offspring may be old enough to require no further care. This close confinement is seen easily in the smaller perching birds that are held within a very limited range until the young are finally on the wing. When the young are grown the adults may linger to rear other broods, but the young, driven often by antagonism of erstwhile attentive parents, wander away, borne by their newly-developed wings, and though they may not go far, do not usually remain for long in the immediate vicinity of the nest. This is one type of wandering in its simplest form. Somewhat more complex is the condition found in some parts of Australia, where prolonged droughts occur during which water evaporates, vegetation is not developed and the majority of birds disappear. With the incidence of rains the country again becomes green and birds return. Though both kinds of movement that have been described may appear as vagrancy, yet such impulses need only to become synchronized with seasonal climatic change to become true migration.
Tropical regions offer interesting data in this connection, as though hundreds of birds found in these areas are strictly seden tary, yet there are some that shift about with changing conditions. Climatic variation within the Tropics is confined mainly to a cycle, in which periods of relatively little precipitation alternate with those of heavy rains. There is thus a seasonal shift that is influenced by changes in the vegetation. Some species of tree or vine come into flower or fruit and immediately there appear tanagers, honey-eaters and other birds hitherto absent to live upon the newly-available food.
It is idle to suppose all birds have arisen within either tropical or temperate areas. Birds, as a group, have been in existence for many million years, and during all that period there has been con stant unceasing competition between individuals. In virile spe cies individuals have been produced in abundance, and many have necessarily been forced out into new range. Some have reacted to
new conditions, or have become modified through some inherent quality, so that they have been changed so greatly that they have finally become species apart from the parent stock, and have thus set up their own loci for subsequent radiation. The complex ity of overlapping ranges under such conditions is easily apparent.
Some students have indicated that the spring and fall migra tions correspond to advance to the breeding station and subse quent retreat therefrom, and indicate correctly that the stimulus for this may be a hormone arising from physiological change in the gonads. This, however, is merely an activating principle for migration as it exists at the moment, and may not be considered the cause through which migration itself has originated.
Briefly, migration may be defined as advance and retreat, with fluctuation in conditions favourable to each species separately, which, as each form has its own reaction to its environment, has originated from varying causes. The origin of the present day regular, seasonal movements is thus complex.
Methods of Migration.—Migration may take place by day or by night, according to the species. Geese, ducks, cranes and peli cans crossing the sky, flying abreast or in angular formation, are accepted as portents of changing season throughout the world. Though seen regularly by day, these birds may also migrate by night, as we frequently hear the calls of geese and swans coming from darkened skies during the height of their movements. The smaller birds travel mainly by night, and descend on us in hordes, so that frequently we go out in early morning to find fields and hedgerows crowded with songsters that were absent the previous evening. During the proper season, when a full moon rides the sky in evening, it is possible to detect the forms of these nocturnal travellers silhouetted against the illuminated disk of light as they pass far above the earth, and occasionally to recognize a feathered friend by some peculiarity of form or wing movement. Many such observations have been made through large telescopes, and birds may be seen occasionally against the moon through ordinary field-glasses. It is probable that timid wrens, warblers and spar rows, that live ordinarily under shelter, feel greater safety in prolonged flights under the protecting cover of darkness. King birds, robins, bluebirds, bluejays and many others however regularly fly by day.