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.