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.
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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, and 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.