IO4 Loomis on Birds of Chester County, South Carolina. \ a ] 



species are better fitted to pass the winter in the colder parts of 

 their habitat than the southern-born, and are therefore the ones 

 that linger farthest north. If mere northern birth fitted a bird 

 for northern residence the Yellow Warbler that breeds abundantly 

 even to the shores of the Arctic Ocean might be expected to 

 winter further north than either its congener the Pine Warbler 

 or the Mockingbird, and the Gray-cheeked Thrush might be 

 expected to occur in winter in the same latitude at least as the 

 Hermit. The circumstance that Mockingbirds from below hold 

 their ground in the northward migration during far colder 

 weather than that which prompted their departure in fall disposes 

 of the idea that mere physical inability to withstand the cold is 

 the cause of their removal. A species may be common south 

 as well as common north of the region where the bulk winter. 

 If temperature alone were the controlling influence the birds at 

 the south of the centre of distribution must needs pass through 

 and leave hehind the region best suited for their winter home. 

 If cold then is not the immediate cause of removal in the 

 instances cited, insufficiency in the food-supply may reasonably be 

 assumed as the cause, not necessarily insufficiency other than 

 that occasioned by the sojourn of winter residents coming from 

 the north. To put the matter in another way, the partial passage 

 further south of Mockingbirds and other winter species has 

 apparently come about through the adjustment and equalization 

 of distribution enforced by winter, the cold simply admonishing 

 that the period of scarcity is at hand and that departure must no 

 longer be delayed. The migration of breeding Robins from 

 Chester County before the close of summer appears to be an 

 instance of adjustment in a resident species occurring long before 

 cold. 



The Mallard migrates before cold in Missouri and yet in south- 

 ern Greenland it is "Common the whole year round, but most 

 numerous in winter, when they keep in small flocks along the 

 shore" (Hagerup, Birds of Greenland, p. 17). The cold closes 

 the water courses in the Mississippi Valley and locks up the 

 food-supply for water birds, but in the latter region there is 

 open water at the mouths of the fjords. 



I have referred to the effect of snow upon the Robin, attribut- 

 ing its southward movements in winter to the covering up of the 



