_Why Birds Migrate._--Why birds migrate we can only conjecture. Without doubt the growing scarcity of food in autumn is the controlling factor with many of them; and this would seem to be an excellent reason for leaving the region of their summer sojourn. Cold weather alone would not drive all of them southward, else why do many small birds pass the winter in northern latitudes where severe climatic conditions prevail? Should we assume the failing food supply to be the sole cause of migration, we would find ourselves at fault when we came to consider that birds leave the tropic regions in spring, when food is still exceedingly abundant, and journey northward thousands of miles to their former summer haunts. [Illustration: Young robins quarreling at their bath. Photographed in the yard of Mrs. Granville Pike, North Yakima, Washington] {65} There is a theory held by many naturalists that the migrating instinct dates back to the glacial period. According to this theory North America was inhabited originally by non-migrating birds. Then the great Arctic ice-cap began to move southward and the birds were forced to flee before it or starve. Now and then during the subsequent period the ice receded and the birds returned, only to be driven again before the next onrush of the Ice King. Thus during these centuries of alternate advance and retreat of the continental glacier, the birds acquired a habit, which later became an instinct, of retreating southward upon the approach of cold weather and coming back again when the ice and snow showed indications of passing away. _The Gathering Flocks._--To the bird student there is keen delight in watching for the first spring arrivals and noting their departure with the dying year. It is usually in August that we first observe an unwonted restlessness on the part of our birds which tells us that they have begun to hear the call of the {66} South. The Blackbirds assemble in flocks and drift aimlessly about the fields. Every evening for weeks they will collect a chattering multitude in the trees of some lawn, or in those skirting a village street, and there at times cause great annoyance to their human neighbours. Across the Hudson River from New York, in the Hackensack marshes, behind the Palisades, clouds of Swallows collect in the late summer evenings, and for many days one may see them from the car windows as they glide through the upper air or swarm to roost among the rushes. These Swallows and the Blackbirds are getting together before starting on their fall migration.