The opposing theory is simpler in some respects and supposes the ancestral home of Northern Hemisphere migratory birds was in the Tropics. As all bird life tends to overpopulation, there was a constant effort of young individuals to pioneer and seek breeding grounds where competition was less severe. Species better adapted to more northern latitudes moved in that direction for nesting but were kept in check by the glacial ice and 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 in the South remained the home to which the birds returned after the nesting season.
The above two theories presume that the Quaternary glaciations, which occurred 10,000 to 1 million years ago, have been the predominate influence on bird migration in North America and Europe as we observe it today. There is no doubt these extreme climatic and ecologic barriers played a part in shaping or modifying some patterns, but as Moreau (1951) has pointed out, well-developed migrations occur in parts of the world, including the Southern Hemisphere or even within the tropics, where continental glaciation was not a factor. Furthermore, migrations to fit various needs have probably been going on ever since birds could fly. The tremendous Pleistocene glaciations actually occupied less than a hundredth of the time birds have existed on the earth and probably only determined the details of migrations as we see them today (Moreau 1951).
The northern and southern ancestral home theories appear diametrically opposed to each other but Dorst (1963) concludes they are perfectly compatible. Since the phenomena probably occurred simultaneously, northern migrants then originated from two stocks: the North Temperate Zone birds sought refuge to the south during the glacial periods and the tropical avifauna expanded their range during the interglacial periods. Dorst also stated this double origin is more prevalent in North America where the tropical element is most abundant. Birds representing this element include hummingbirds, tyrant flycatchers, orioles, tanagers, and blackbirds. At some latitudes, they nest in the same area as the shorebirds which are of arctic parental stock.