no. 3637 HERMJT THRUSH — ALDRICH H 



All of the pale types (with the exception of the California to Wash- 

 ington coastal population) are from the relatively dry interior moun- 

 tain forests. All of the small types are along the humid Pacific Coast 

 or almost equally moist coastal mountains and west slopes of the 

 Rockies in southern British Columbia and northern Idaho. On the 

 other hand, the largest types are from the higher and drier mountain 

 forest of the interior. 



The large interior birds, although individually quite variable, show 

 surprisingly little average variation either in size or color over vast 

 areas despite the extremely discontinuous distribution of their moun- 

 tain coniferous summer home. Suitable breeding habitat frequently 

 is broken up into many "islands" sometimes separated by wide ex- 

 panses of desert. Such disruptions of populations might be expected to 

 offer much more of an obstacle to gene flow than the contiguous areas 

 of different ecological climax type that support quite different ap- 

 pearing populations in the vicinity of the Pacific Coast. Evidence seems 

 to support a greater importance of adaptation to environmental 

 conditions than of physiographic or ecological barriers to gene flow in 

 the development of present morphological differences in hermit 

 thrushes. On the other hand, isolation on Vancouver and Queen 

 Charlotte Islands may have reinforced the trend toward dark pigmen- 

 tation of birds adapted to moist forest conditions along the northwest 

 coast. 



Another fact, indicated by Grinnell (1901), that may bear on 

 the seeming greater effect of moist habitat on the color of the Van- 

 couver and Queen Charlotte Islands birds is that they are less mi- 

 gratory than other populations and winter chiefly along the Pacific 

 Coast in habitats similar to those in which they breed. Thus, they 

 are exposed to habitat conditions associated with a humid climate 

 throughout the entire year. On the other hand, the paler populations 

 of the Pacific Coast south of the Canadian border and those of the 

 interior mountains are less restricted in migration and winter to a 

 large extent in relatively arid environments of the southwestern 

 United States and Mexico. Thus, there is an opportunity for an 

 entirely different set of selection factors to operate on these pop- 

 ulations in winter, and these may be more critical to survival of 

 paler individuals than factors encountered on the breeding groimds, 

 as suggested by Salomonsen (1955) for certain other species. 



It would seem that the hermit thrush, which breeds in a variety 

 of habitats located in at least seven "Life Areas," each characterized 

 by different climatic climax types, is a racially variable species. 

 This variation might be expected since the hermit thrush has become 

 adapted to many different environments, a response that Miller (1956) 

 has pointed out is conducive to intraspecific variability. In some 



