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.
[Illustration: B4509M
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.