112 Loomis on Birds of Chester County, South Carolina. [ad 1 



ditions are involved, particularly as pertaining to the food of the 

 young. It is hardly to he supposed that there is any vital failure 

 of food in the winter habitat (at least north of the tropics), for 

 some species that are common in winter attain their greatest 

 abundance, as migrants or breeders, after the departure of the 

 winter sojourners of the same species. 



In brief, the conclusion is reached that all southward migration 

 in North America is depopulation because of winter, and all 

 northward migration 1 is repopulation because of summer, the two 

 great migratory movements being the adjustment of bird-life to 

 the food-supply as ordered by the changing seasons — the food 

 area decreasing with the progress of the sun southward, forcing 

 birds to leave the region of their birth, and increasing with the 

 progress of the sun northward, enabling them to return to it — to 

 the region where the established equipoise between food-supply 

 and distribution may be maintained and where the conditions 

 arising from climate are perhaps better suited to the requirements 

 of the breeding season. 



Psychological Cause. — Having considered the physical 

 causes of migration — those outer conditions on which migration 

 depends — we come now in the second place to examine the 

 psychological causes — those inner facts of bird life which have 

 adjusted the migratory movements to the physical requirements. 



climate are insurmountable obstacles to successful reproduction in latitudes and alti- 

 tudes higher or lower than the normal breeding range. Extension of range, however, 

 might not be possible in such instances as the Myrtle Warbler, for the natives of the 

 soil might be better fitted to survive than the interlopers in the contest that must 

 inevitably ensue. In mountain regions the peculiar conditions may have rendered such 

 contest more equal, resulting, in lapse of time, in the present overlapping. The 

 absence of some boreal species on high mountains may be due to their inability to 

 cope with other species except under the conditions prevailing in the region of their 

 birth. Lack of powerful opposition may perhaps also account for the ranging of some 

 birds, independent of altitude, farther north or south in certain regions than in others. 

 As to the representatives that breed in high latitudes of species of wide-breeding- 

 range, they may have become so modified as to find the environment in the southern 

 portions of the breeding habitat uncongenial, but above and behind any such possible 

 cause is the necessity for dispersion. 



1 While southward migration and northward migration are more appropriate terms 

 than spring migration and fall migration, they do not cover altitudinal migration and 

 migration in an east or west direction, though both are component parts of the two 

 seasonal movements. Depopulating-migration and repopulating-migration seem more 

 expressive designations. 



