144 THE MIGRATIONS OF BIRDS 



tion lines are found in western North America^ par- 

 ticularly in Alaska and British Columbia. The east- 

 ern robin travels north to the headwaters of the 

 Mackenzie River, then swings over into the valley 

 of the Yukon and continues to the Kowak River in 

 northwestern Alaska. The western robin, a sub- 

 species characterized by somewhat greater size, 

 paler coloration, and lack of white spots on the tail, 

 when compared with the eastern bird, though found 

 in British Columbia and Saskatchewan, in Alaska 

 covers only the coastal district of southwestern 

 Alaska and does not penetrate into the interior. 



In work on the Stikine River region of northern 

 British Columbia and southwestern Alaska, Mr. H. S. 

 Swarth has found that certain migratory forms of 

 the interior cross the mountains to the valley of the 

 Stikine (which flows into the Pacific), and penetrate 

 far down the river into territory that would in most 

 cases be assigned on geographic grounds to the forms 

 of birds found in the coast region proper. The north- 

 ern flicker (Colaptes auratus borealis)^ the eastern 

 nighthawk (Chordeiles m. minor), Hammond's fly- 

 catcher {Empidonax hammondi), the Canada jay 

 {Perisoreus c, canadensis), the rusty blackbird 

 (Euphagus carolinus), the eastern chipping sparrow 

 {Spizella p. passerina), the eastern yellow warbler 

 {Dendroica a. aestiva), and the redstart {Setophaga 

 ruticilld), here all cross the mountains and establish 



