MIGRATION AND ORIGINS 265 



The winter-resident birds of the forested parts of arctic Alaska 

 and Yukon (table 8) are well represented at Kobuk and Old 

 Crow, but only 4 of the 13 at Anaktuvuk are strictly birds of 

 the tundra, showing the restriction imposed by lack of trees. These 

 species are now adapted to life on arctic tundra and might have 

 survived under similar conditions during an ice age. Inasmuch as 

 all 25 species also now live in arctic forest, that condition would 

 have been more suitable. As for their present world distribution, 9 of 

 the species are now confined to America and 16 also extend widely 

 over Asia and Europe. Of the 10 American species 5 are dif- 

 ferentiated as races restricted to Alaska, Yukon and northwestern 

 Mackenzie. Among the 16 holarctic species are 4 races restricted to 

 the area. The differentiation of 9 populations resident in the area 

 implies that they have been long isolated from interbreeding with the 

 remainder of their species. It also indicates a certain degree of f aunal 

 homogeneity of the arctic interior of western North America from 

 the western border of forest to some line near the Mackenzie Delta 

 where contact is made with other resident races. 



Species and their Migration Routes 



Distinguishable populations of 19 migratory species of land birds 

 nest mainly in Alaska, Yukon, and western Mackenzie, as shown in 

 table 9. Of these, 3 (in Aphriza, Heteroscelus^ and Limnodromus) 

 have no nesting range outside the area except on the adjacent 

 coast of Siberia. The American populations of Oenanthe and Plec- 

 trophenax are not recognizable by taxonomic distinction ; but I have 

 indicated that the Alaskan snow buntings appear distinguishable in 

 the thickness of toes and claws. The wheatears are not taxonomically 

 distinguishable from those of Europe and Asia, although their migra- 

 tion to nest in northwestern America takes them far east of their 

 Asiatic range. Among the 14 populations assigned to taxonomic 

 races nesting only in Alaska and Yukon, I find local differentiation 

 visible in 3 racial designations that are not considered as established 

 by the A.O.U. Check-list (1957) : Branta canadensis tavemeri, 

 Leucosticte tephrocotis irvingi, and Arenaria interpres interpres. The 

 separation of the latter from A. i. morinella rests upon distinctions 

 which are difficult to make and accordingly their ranges are uncertain. 

 In spite of these reservations as to the distinction of the populations, 

 3 species and 11 races nesting mainly in northern Alaska and Yukon 

 are commonly recognized and the area is characterized by the nesting 

 of a number of distinguishable populations of migratory species. 



