268 U. S. NATIONAL MUSEUM BULLETIN 217 



Birds of 25 species that migrate overland in northwestern Canada 

 and probably through the western mountains to nest in northern 

 Alaska and Yukon, are listed in table 11a. Although swallows 

 (Hirundinidae) are widely distributed across North America during 

 migration I have included all 5 species as probably flying northward 

 along a western course to Alaska because the early northward progress 

 of the spring migration of swallows in the west fits their time of 

 arrival in Alaska and Yukon. Two of the five swallows {Tachycineta 

 and Petrochelidon) are differentiated as western races. The difficulty 

 in proposing the migratory paths of some wide-ranging species is 

 shown in the cases of the Savannah sparrow {Passer cuius) and Junco 

 {J unco), for both migrate through most of the northern states. In 

 these cases one may suspect that these migrants to Alaska come 

 from the western fringe of the widespread populations. The majority 

 of these birds can be regarded as western American and 14 of them 

 are taxonomically distinguished as western races. Of the species 

 listed in table 11a, 5 nest in the Arctic far east of Yukon, 9 have 

 already been referred to (table 9) as providing races mainly restricted 

 to nesting in Alaska and Yukon, and 11 are assigned to races which 

 nest over a considerable American area south of the Arctic. But 

 among these 25 western migrants, 20 reach the Arctic only in 

 Alaska, Yukon, and western Mackenzie, so that their most northern 

 nesting range in the west is apparently related to the favorable 

 environmental conditions of Arctic Alaska and Yukon. 



The 17 species that migrate principally east of the Rocky Mountains, 

 as shown in table lib, can be assigned to the central flyway, which is 

 routed over the watershed east of the Rocky Mountains. Among 

 these species the 2 thrushes {Turdus and Hylocichla), the kinglet 

 {Regulus), pipit (Anthus) and 5 wood warblers {Vermivora, Den- 

 droica fetechia and striata., Seiurus, and Wilsonia) also migrate 

 through paths east of the central States and Provinces. Alaska and 

 Yukon are only part of the extended nesting areas of these birds, but 

 9 of them reach the Arctic only in Alaska, Yukon and Mackenzie. 

 Again the favorable nesting environment in the western arctic is 

 apparent. 



Only 5 species nesting in Alaska and Yukon (table lie) can be 

 regarded as migrating mainly east of the Mississippi River. There is 

 some migration of each of these species west of the Mississippi but 

 they are commonly regarded as eastern birds. The arctic nesting races 

 of two of these species are restricted to Alaska and Yukon. All of 

 them also nest south of the arctic circle. 



