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 ovenbird has even been detected on the lower Yukon River, and the sandhill crane and gray-checked 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. 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.