Evolution of Migration Routes 



From the foregoing descriptions of migration routes it will be 

 observed that the general trend of migration in most species of North 

 American birds is northwest and southeast. It is comparatively easy 

 to trace the probable steps in the evolution of the migrations of some 

 species, and some routes have developed so recently that they still 

 plainly show their origin. 



The tendency is for eastern species to extend their ranges by pushing 

 westward, particularly in the north. For example, in the Stikine 

 River Valley of northern British Columbia and southwestern Alaska 

 the eastern nighthawk, eastern chipping sparrow, rusty blackbird, 

 eastern yellow warbler, redstart, and others have established breed- 

 ing stations at points 20 to 100 miles from the Pacific Ocean. The 

 robin, flicker, slate-colored junco, blackpolled and myrtle warblers, 

 and ovenbird, all common eastern species, also are established as breed- 

 ing birds in western Alaska, the ovenbird having been detected on the 

 lower Yukon River. These birds are essentially Atlantic and Missis- 

 sippi Flyway species, however, and so do not migrate in fall by any of 

 the Pacific or Central routes, but instead retrace their journey across 

 the mountains and move southward along the broad flyways of the East. 



The red-eyed vireo, a striking example of an abundant woodland 

 bird, is essentially an inhabitant of States east of the Great Plains, but 

 an arm of its breeding range extends northwest to the Pacific coast in 

 British Columbia (fig. 23). It seems evident that this is a range exten- 

 sion that has taken place comparatively recently by a westward move- 

 ment from the upper Missouri Valley, and that the invaders retrace in 

 spring and fall the general route by which they originally entered the 

 country. 



In the case of the bobolink, a new extension of the breeding range, 

 and a consequent change in the migration of the species, has taken 

 place since the settlement of the country (fig. 19). A bird of damp 

 meadows, it was originally cut off from the Western States by the 

 intervening arid regions. But with the advent of irrigation and the 

 bringing of large areas under cultivation, small colonies of nesting 

 bobolinks have appeared at various western points, and now the species 

 is established as a regular breeder in the great mountain parks and 

 irrigated valleys of Colorado and elsewhere almost to the Pacific coast. 



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