Northern ancestral home theory 



According to one of these hypotheses, in earUer 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 en- 

 deavored 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 stu- 

 dent 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. 



Southern ancestral home theory 



The opposing theory is simpler in some respects and supposes that 

 the ancestral home of all birds was in the Tropics and that, as all 

 bird life tends to overpopulation, there was a constant effort to seek 

 breeding grounds where the competition would be less keen. Species 

 that strove for more northern latitudes were kept in check by the ice 

 and were 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 remained the home to which the birds returned after the nest- 



