The Pacific oceanic route probably is used also by the arctic terns that breed in Alaska, and possibly by those from the more western tern colonies of Canada. This species is of regular occurrence on the western coasts of both the United States and South America, indicating that the western representatives travel southward to the Antarctic winter quarters without the spectacular migration features that appear to characterize the flight of those from the eastern part of the continent ([fig. 15]).
ARCTIC ROUTES
In the discussion of the migration of the Arctic tern ([p. 31]) it was noted that this species makes a very distinct west-to-east movement across northern Canada, continuing the flight eastward across the Atlantic Ocean toward the western coast of Europe. It seems likely that there are other species, including the parasitic jaeger (Stercorarius parasiticus) that regularly breed in the northern part of the Western Hemisphere but migrate back to the Old World for their winter sojourn. Some others, as the red-legged kittiwake (Rissa brevirostris) and Ross's gull (Rhodostethia rosea), remain near the Arctic region throughout the year, retreating southward in winter only a few hundred miles. The emperor goose (Philacte canagica) in winter is found only a relatively short distance south of its breeding grounds, and eider ducks (Somateria and Arctonetta), although wintering in latitudes well south of the breeding grounds, nevertheless remain farther north than do the majority of other species of ducks ([fig. 21]).
The routes followed by these birds are chiefly coastwise, and in the final analysis may be considered as being tributary either to the Atlantic or to the Pacific coast routes. The passage of gulls, ducks, and other water birds at Point Barrow, Alaska, and at other points on the Arctic coast, has been noted by several observers, and from present knowledge it may be said that the best defined Arctic route in North America is the one that follows around the coast of Alaska.
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 (Chordeiles minor minor), eastern chipping sparrow (Spizella passerina passerina), rusty blackbird (Euphagus carolinus), eastern yellow warbler (Dendroica aestiva aestiva), redstart, and others have established breeding stations at points 20 to 100 miles from the Pacific Ocean. The robin, flicker (Colaptes auratus), slate-colored junco (Junco hyemalis hyemalis), blackpoll warbler, myrtle warbler (Dendroica coronata), and ovenbird (Seiurus aurocapillus), all common eastern species, also are established as breeding birds in western Alaska, the ovenbird having been detected on the lower Yukon River. These birds, however, do not migrate in fall by any of the Pacific routes, but instead retrace their journey across the mountains and move southward along the broad flyways of the interior.
The red-eyed vireo (Vireo olivaceus), 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 north west to the Pacific coast in British Columbia ([fig. 27]). It seems evident that this is a range extension that has taken place comparatively recently by a westward movement from the upper Missouri Valley, and that the invaders retrace in spring and fall the general route by which they originally entered the country.
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