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 distances east and west to reach the western edge of the route followed by the bulk of the bobolinks that breed in the northern United States and southern Canada. [Illustration: _Figure 30. Distribution and migration of the red-eyed vireo. It is evident that the red-eyed vireo has only recently invaded Washington by an extension of its breeding range almost due west from the upper Missouri Valley. Like the bobolink (Fig. 19), however, the western breeders do not take the short cut south or southeast from their nesting grounds but migrate spring and fall along the route traveled in making the extension._]