Part 8

This account of the Mackenzie Valley-Great Lakes-Mississippi Valley route shows the great importance of this highway and also its complicated nature. It receives accretions from both the northeast and the northwest, while branch routes make wide detours toward the Pacific coast before turning back to the parent fly way. This flyway is notable for its great length, as it extends from the Arctic-coast to Patagonia; and for its width east and west, as in North America it reaches from the Allegheny Mountains westward to the Great Basin.

PACIFIC COAST ROUTE

Although it does present features of unusual interest, the Pacific-coast route is not of so great importance as some of the others described. Because of the equable conditions that prevail, many species of birds along the coast from the Northwestern States to Southern Alaska either do not migrate at all or else make relatively short journeys. This route has its origin chiefly in Alaska, the general region of the delta of the Yukon River marking its northern terminus, although a few species join it after a flight westward along the Alaskan Arctic coast (fig. 21). Some of the scoters (Melanitta and Oidemia) and other sea ducks of the north Pacific region, and the diminutive cackling goose (Branta canadensis minima), which breeds in the delta of the Yukon River, use the coastal sea route for all or most of their southward flight. The journey of the cackling geese, as shown by return records from birds banded at Hooper Bay, Alaska, has been traced southward across the Alaska Peninsula and apparently across the Gulf of Alaska to the Queen Charlotte Islands, the birds following the coast line south to near the mouth of the Columbia River. There the route swings toward the interior for a short distance before continuing south by way of the Willamette River Valley. The winter quarters of the cackling geese are chiefly in the vicinity of Tule Lake, on the Oregon-California line, and in the Sacramento Valley of California, though a few push on to the San Joaquin Valley.

Figure 24.--Probable breeding range, the winter range, and the migration route of Ross's goose. This is the only species of which all members apparently breed in the Arctic regions, migrate south through the Mackenzie Valley, and upon reaching the United States turn to the southwest rather than the southeast. The southern part of this route, however, is followed by some mallards, pintails, baldpates, and possibly by other ducks.]

A tributary of this flyway is followed by Ross's goose (Chen rossi), which is believed to breed on the Arctic islands north of Mackenzie (fig. 24). Its fall migration is across the barren grounds to Great Slave and Athabaska Lakes, where it joins thousands of other waterfowl bound for their winter homes along the eastern coast of the United States and the Gulf of Mexico. But when Ross's geese have traveled south approximately to the northern boundary of Montana, they separate from their companions, and turning to the southwest cross the Rocky Mountains and settle for the winter in California.

The route taken by the white-winged scoters that winter on the Atlantic coast already has been indicated (p. 43). Some birds of this species, however, winter on the Pacific coast from Puget Sound south to southern California. Their passage by thousands up and down the coast has been noted as far north as northwestern British Columbia. The species is known to nest in Alaska, which may be the home of some at least of the scoters that winter on the Pacific coast. If such be the case, however, it must be admitted that a part of the route taken by the birds when on migration is unknown, though very few observations are available from the interior of northern British Columbia, across which the route may lie.

The southward route of those migratory land birds of the Pacific coast that in winter leave the United States extends chiefly through the interior of California to the mouth of the Colorado River and on to winter quarters in western Mexico.

The movements of the western tanager (Piranga ludoviciana) show a migration route that is in some ways remarkable. The species breeds in the mountains from the northern part of Baja California and western Texas north to northeastern British Columbia and southwestern Mackenzie. Its winter range is in two discontinuous areas--southern Baja California and eastern Mexico south to Guatemala (fig. 25). On the spring migration the birds enter the United States about April 20, appearing first in western Texas and the southern parts of New Mexico and Arizona (fig. 26). By April 30 the van has advanced evenly to an approximate east-and-west line across central New Mexico, Arizona, md southern California. But by May 10 the easternmost birds have advanced only to southern Colorado, while those in the far West have reached northern Washington. Ten days later the northward advance of the species is shown as a great curve, extending northeastward from Vancouver Island to central Alberta and thence southeastward to northern Colorado. Since these tanagers do not reach northern Colorado until May 20, it is evident that those present in Alberta on that date, instead of traveling northward through the Rocky Mountains, which from the location of their summer and winter homes would seem to be the natural route, reached there by the Pacific coast route to southern British Columbia and thence across the mountains, despite the fact that these are still partly covered with snow at that time.

PACIFIC OCEANIC ROUTE

The route of the Pacific golden plover (Pluvialis dominica fulva) is fully as interesting and as remarkable as the elliptical course followed by its eastern cousin (P. d. dominica) (fig. 22). The breeding range of the eastern golden plover extends through Arctic America west to the northern coast of Alaska, where in the vicinity of Point Barrow it meets the nesting grounds of the Pacific form, which is really an Asiatic subspecies. It breeds chiefly in the Arctic coast region of Siberia and merely overflows onto the Alaskan coast, some of the birds probably migrating south along the coast of Asia to winter quarters in Japan, China, India, Australia, New Zealand, and Oceania, including the Hawaiian Islands, the Marquesas Islands, and the Low Archipelago. Golden plovers in migration have been observed at sea on a line that apparently extends from these islands to the Aleutians, and it therefore appears certain that at least some of the Alaskan birds make a nonstop flight across a landless sea from Alaska to Hawaii. While it would seem incredible that any birds could lay a course so straight as to attain these small oceanic islands, 2,000 miles south of the Aleutians, 2,000 miles west of Baja California, and nearly 4,000 miles east of Japan, the evidence admits only the conclusion that year after year this transoceanic round-trip journey between Alaska and Hawaii is made by considerable numbers of golden plovers.

Figure 25.--Breeding and wintering ranges of the western tanager. See figure 26 for the spring route taken by the birds breeding in the northern part of the range.]

Figure 26.--Migration of the western tanager. The birds that arrive in eastern Alberta by May 20 do not travel northward along the eastern base of the Rocky Mountains, as in that region the van has then only reached northern Colorado. Instead the isochronal lines indicate that they migrate north through California, Oregon, and Washington, and then cross the mountains of British Columbia.]

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, continning 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 obundant woodland bird, is essentially an inhabitant of States east af the Great Plains, but an arm of its breeding range extends northwest 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.

Figure 27.--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, however (fig. 23), the western breeders do not take the short cut south or southeast from their nesting grounds, but migrate spring and fall along the route traversed in making this extension.]

In the case of the bobolink, a new extension of the breeding range, and a consequent change in the migration of the species, has taken place since the spread of settlement in this country (fig. 23). 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 have appeared at various western points, and 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. In retracing their course 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, these western pioneers must fly long distances along a line that runs almost due east and west.

Similarly it is possible to sketch what seems to be the logical evolution of the remarkable routes of the golden plover (fig. 22). It may be assumed that the eastern birds of this species first followed an all-land route from the South American winter quarters through Central America, Mexico, and Texas to the western parts of the Mississippi Valley. As the migration route lengthened northward with the retreat of the ice and the bird's powers of flight developed, it would have a tendency to straighten the line and to shorten it by cutting off some of the great curve through Mexico and Texas. First a short flight across the western part of the Gulf of Mexico was probably essayed. Proving successful, this was followed by flight lines that moved farther east, until finally the roundabout curve through Texas was entirely discarded and the flight made directly across the Gulf to southern Louisiana.

As the great areas in Canada were gradually added to the bird's domain, other important factors arose, the chief being the attractiveness of the vast stretches of coast and plain of the Labrador Peninsula, which in fall offered a bountiful store of berries. The fall route therefore worked eastward to the Gulf of St. Lawrence thence southwest through the interior to the coast of Florida and across the Gulf of Mexico to the Central American mainland. A series of shortening flights followed to take out the great curve of the New England coast. A relatively short ocean flight was probably attempted, say from Cape Cod to the Bahama Islands, Cuba, and Jamaica, followed eventually by the long direct oceanic route as it is now known.

As the Labrador Peninsula in spring is bound by frost and shrouded by fog while the season advances rapidly through the interior, the oceanic route proved useful only in fall, and the spring flight continued through the Mississippi Valley. The above outline gives a probable and fairly plausible explanation of the origin of this wonderful route, particularly when it is remembered that migration routes as now known are evolutions--age-long modifications of other routes.

The evolution of the migration of the Pacific golden plover may be explained in a similar fashion. At first the route probably followed the Asiatic coast, through the Malay Peninsula and Oceania, thence east in a great curve to the Low Archipelago, with individuals and flocks dropping out to winter at many points along the way. The Siberian birds probably continue to follow this ancient flyway, but those nesting in Alaska began a long evolutionary series of flights that cut down the length of their journey by shortening the curve, until finally the transoceanic route of the present day was developed.

=VERTICAL MIGRATION=

In the effort to find winter quarters furnishing satisfactory living conditions, many North American birds fly hundreds of miles across land and sea. Others, however, are able to attain their objective merely by moving down the sides of a mountain. In such cases a few hundred feet of altitude corresponds to hundreds of miles of latitude. Movements of this kind, known as "vertical migrations", are found wherever there are large mountain ranges. In the Rocky Mountain region they are particularly notable, as chickadees, rosy finches (Leucosticte), juncos, pine grosbeaks (Pinicola), and some other species that nest in the Alpine Zone move down to the lower levels to spend the winter. It has been noted that such species as Williamson's sapsucker (Sphyrapicus thyroideus), and the western wood pewee (Myiochanes richardsoni), which nest in the higher mountains, move down to the lower regions in August following the breeding season. At this time there is a distinct tendency also among the young of mountain-breeding birds to work down to the lower levels as soon as the nesting season is over. The sudden increases among birds in the edges of the foothills are particularly noticeable when cold spells with snow or frost occur at the higher altitudes.