patterns remain constant. But the landscape and the interacting 

 ecological stresses are forever changing, and we would expect the 

 adaptive behavior of birds to respond with them. One of these 

 responses to an expanding habitat is colonization of new territory and 

 expansion of a species' range with accompanying development of a 

 migratory habit. The search for favorable conditions under which to 

 breed in summer and to feed in winter, as influenced by competition 

 for space, has been the principal factor underlying the extension of 

 ranges, usually by young, nonconditioned individuals. This is 

 exemplified by the northward extension in historic times of a number 

 of species. Many of these range extensions have closely followed 

 man's settlement of the area and the subsequent changes in habitat 

 that man has made. 



From the previous descriptions of migration patterns and routes, it 

 will be observed that the general trend of migration in most northern 

 populations of North American birds is northwest and southeast. 

 Eastern species tend 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 common 

 nighthawk, chipping sparrow, rusty blackbird, yellow warbler, 

 American redstart, and others have established breeding stations at 

 points 20 to 100 miles from the Pacific Ocean. The northern race of 

 the American robin, common flickers, dark-eyed juncos, blackpoll 

 warblers, yellow-rumped warblers, and ovenbirds, all common 

 eastern species, also are established as breeding birds in western 

 Alaska. The ovenbird has even been detected on the lower Yukon 

 River, and the sandhill crane and gray-cheeked thrush have moved 

 across Bering Strait into Siberia. These birds continue to migrate 

 through the eastern part of the continent. Instead of taking the 

 shortest route south, they retrace the direction of their westward 

 expansion and move southward along the same avenues as their more 

 eastern relatives. 



The red-eyed vireo 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. 30). It seems evident this is 

 a range extension that has taken place comparatively recently by a 

 westward movement via deciduous woodland corridors, and 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 subsequent change in the migration of the species has taken 

 place since settlement by European man (Fig. 19). Because the 

 bobolink is 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 appeared at various western points. 

 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. These western pioneers must fly long 



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