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.