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._]
During the past few decades, various populations of Canada geese have altered their migration patterns as a result of transplanting brood stock, development of refuges or changing agricultural practices. These routes will continue to change in the coming years as long as these factors are in a state of flux. It has been shown that man can establish breeding colonies of Canada geese with young birds almost anywhere.
Europe also has several good examples of changes in migration routes through range extension. One of the best examples is the serin. During the past century, this European finch has spread its breeding range from around the Mediterranean Sea to include the entire continent. While the Mediterranean populations remain sedentary, the more northern breeding birds are migratory. Most likely, those birds that did not migrate from the North were eliminated by severe weather. Similarly, the wheatear, yellow wagtail, and Arctic warbler have extended their breeding ranges eastward across the Bering Sea into Alaska, but the wheatear, for instance, migrates all the way back across Asia to Africa where it winters with other wheatears coming from Europe, Iceland, and Greenland.