1907] Beebe: Geographic Variation in Birds. 11 



Among many hundreds of rough-legged hawks sent to the 

 Smithsonian Institution from the Arctic regions, not one was in 

 the dark phase. But in the Ungava District, east of Hudson and 

 James Bay, they are said to be common. Of one hundred birds 

 killed within a short time at Toronto, five were black hawks. 

 The extreme type of each phase has the young and adult plumage 

 distinctly marked, but these are connected by individuals exhib- 

 iting every gradation of intermediate characters. 



Although not dichromatic in the sense of the different color 

 forms inhabiting the same regions, yet the common red-tailed 

 hawk, Buteo borealis, should be mentioned here as a type of sub- 

 specific geographic variation found in many groups of birds. 

 In eastern North America this hawk is subject to comparatively 

 little variation, but in the west and southward, through Mexico, 

 it presents widely different phases. In the central United States 

 a very light colored phase is known as the subspecies krideri, 

 while a Pacific and Mexican melanistic and erythrystic phase 

 has been named calurus. The young of all the forms appear to 

 be indistinguishable and the adults differ from one another in 

 neither size nor shape, but only in the pigmentation of the 

 feathers. 



As regards the group of jaegers or skuas, Stercorarius, 

 dichromatism seems to exist in almost all. S. pomarinus has two 

 very distinct color phases, one almost wholly sooty-brown, the 

 other particolored, dark above and white on the breast. This 

 condition is duplicated in S. parasiticMS but in the closely related 

 S. longicaudus, only one, the white phase, is commonly seen, al- 

 though there is one record of a dark individual of this species. 



In the two first mentioned species, a particolored bird may 

 be frequently found mated with one wholly dark, in fact it is 

 said that both partners are rarely alike in their phase of plumage. 

 As in Archihuteo, the distinction depends neither upon age, sex 

 nor season. 



In the Procellariiformes, which include the albatrosses, 

 fulmars and petrels, dichromatism is a common characteristic, 

 as among Ossifraga and Fulmarus. In the latter birds an im- 

 portant fact is the tendency toward geographical isolation of the 

 two phases, the dark phase, in both the Atlantic and Pacific birds, 

 being predominant toward the west. 



Uria troile, the common murre, has an interesting color 

 phase which is said to occur in from one out of twenty to as 

 high a ratio as one out of five individuals at the vast breeding 

 rookeries of this species. These less numerous birds were for- 

 merly thought to be distinct and were named ringvia and lacry- 



