98 THE RIDDLE OF MIGRATION 



Let US assume, then, that the Pliocene buntings 

 were forced to spread. As the centuries rolled by 

 the birds reached the Canadian plains and began to 

 feel the selective effects of a winter that they could 

 not tolerate. The birds that remained were wiped 

 out. Others, again under pressure, would replace 

 them the following spring and breed. These birds, 

 as they spread north, encountered summer con- 

 ditions progressively more favorable than the con- 

 ditions under which the southern contingent were 

 rearing their broods. They had little competition, 

 food must have been all but unlimited and they had 

 in addition an appreciably longer period of day- 

 length in which to gather it for their growing young. 

 Under these circumstances their numbers must 

 almost unquestionably have increased at a greater 

 rate than those of their southern allies. At all events 

 among the passerine species of today, the northern 

 representatives of a majority of genera either lay 

 more eggs to a clutch than the southern, or they 

 rear two or more broods in place of one. Such a 

 state of affairs can hardly be coincidence and we can 

 quite safely attribute it to a particularly favorable 

 breeding environment. The net result of the situa- 

 tion would be that the northern birds were increas- 

 ing at a greater rate than the southern but they 

 could not survive the winters unless they happened 



