the one used by the blue goose, the migration route of which is
probably more nearly due north and south than that of any other
North American bird. The breeding grounds, which only in recent
years have been discovered, are mainly in the Fox Basin region of
Baffin Island and on Southampton Island. In fall these geese work
southward, chiefly along the eastern shore of Hudson Bay, and upon
reaching the southern extremity of James Bay they take off for what
is practically a nonstop flight to the great coastal marshes of
Louisiana west of the delta of the Mississippi River. In some seasons
the flocks make intermediate stops among the islands and sand bars
of the Mississippi, as they are occasionally common in the general
vicinity of Memphis, Tenn. Most of the birds push on, however, and
during the period from the first of November to the last of March
fully 90 percent of the species are concentrated in the area between
the Sabine and the Mississippi Rivers. On the return trip northward
there is sometimes a tendency for some of the blue geese to veer off
toward the northwest, as they are occasionally abundant in eastern
South Dakota and southeastern Manitoba. It is of particular interest
to note that while some other geese and many ducks start their
northward journey at the first sign of awakening spring, the blue
goose remains in its winter quarters until the season there is far
advanced, seemingly aware that its own breeding grounds in the Arctic
are still in the grip of winter.
=Great Plains--Rocky Mountain routes=
A great western highway also has its origin in the Mackenzie River
delta area and in Alaska. This is used chiefly by the pintail and the
American widgeon or baldpate, which fly southward through eastern
Alberta to western Montana. Some localities in this area, as for
example, the National Bison Range at Moiese, Mont., normally furnish
food in such abundance as to induce these birds to pause in their
migratory movement. Upon resuming travel, some flocks move almost
directly west across Idaho to the valley of the Columbia River, from
which they turn abruptly south to the interior valleys of California.
Others leave the Montana feeding and resting areas and turn
southeastward across Wyoming and Colorado to join the flocks that are
moving southward through the Great Plains (fig. 15).
Many redheads that breed in the Bear River marshes in Utah take a
westerly route across Nevada to California, but some leave these
breeding grounds and fly northeastward across North Dakota and
Minnesota to join the flocks of these ducks that come out of the
prairie regions of Canada. A few of them even travel southeastward to
the Atlantic coast. This route can be traced by the records of ducks
banded in summer in the Bear River marshes and retaken the following
fall at points in eastern Montana, Wyoming, South Dakota, North
Dakota, Minnesota, Wisconsin, Michigan, Ohio, and Maryland. Great
numbers, however, follow another route from these marshes across the
mountains in an easterly direction, where it almost immediately turns
southward through Colorado and New Mexico, and continues to winter
quarters in the Laguna Madre off the coast of Texas or in the Valley
of Mexico (fig. 17, route 6). This route also represents the travels
of many of the land birds of the Rocky Mountain region. Such birds
perform comparatively short migrations, most of them being content
to stop when they reach the middle districts of Mexico, only a few
passing beyond the southern part of that country.
Observations made in the vicinity of Corpus Christi, Tex., have
shown one of the short cuts (fig. 17, route 5) that is, in effect,
a part of the great artery of migration. Thousands of birds pass
along the coast to the northern part of the State of Vera Cruz. As
the coast of the State of Tamaulipas to the north is arid and so
entirely unsuited to the needs of birds that are frequenters of moist
woodlands, it is probable that much or all of this part of the route
is a short distance off shore. It is used by such woodland species as
the golden-winged warbler, the worm-eating warbler, and the Kentucky
warbler.
=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
southeastern 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. Some of the scoters and
other sea ducks of the north Pacific region, and the diminutive
cackling goose 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 goose, 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 goose 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: _Figure 20._--The breeding range, winter range, and
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 which is now
known to breed only in the Perry River district south of the Queen
Maud Gulf on the central Arctic coast of Canada (fig. 20). Its fall
migration appears to be southwest and south 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 the 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.
[Illustration: _Figure 21._--Breeding and wintering ranges of the
western tanager. See figure 22 for the spring route taken by the
birds breeding in the northern part of the range.]
The route taken by the white-winged scoters that winter on the
Atlantic coast already has been indicated (p. 59). 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 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. 21). 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.
22). By April 30 the vanguard 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 Albert 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 is fully as interesting and as
remarkable as the elliptical course followed by its eastern cousin
(fig. 18). 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.
[Illustration: _Figure 22._--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. 8).
=Arctic routes=
In the discussion of the migration of the Arctic tern (p. 38) 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, 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 and Ross's gull, remain near the Arctic
region throughout the year, retreating southward in winter only a few
hundred miles. The emperor goose in winter is found only a relatively
short distance south of its breeding grounds, and eider ducks,
although wintering in latitudes well south of their nesting areas,
nevertheless remain farther north than do the majority of other
species of ducks.
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,
the black brant, 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.
[Illustration: _Figure 23._--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. 19), the western breeders do not take the short cut
south or southeast from their nesting grounds, but migrate spring and
fall along the route traveled in making this extension.]
_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, eastern chipping sparrow, rusty blackbird, eastern
yellow warbler, redstart, and others have established breeding
stations at points 20 to 100 miles from the Pacific Ocean. The robin,
flicker, slate-colored junco, blackpolled and myrtle warblers,
and ovenbird, 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 are essentially Atlantic and
Mississippi Flyway species, however, and so do not migrate in fall
by any of the Pacific or Central routes, but instead retrace their
journey across the mountains and move southward along the broad
flyways of the East.
The red-eyed vireo, 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 northwest to the Pacific coast
in British Columbia (fig. 23). 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.
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 settlement of the country (fig. 19). 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. 18). 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,
there would be 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 birds'
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 along the coast to 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 Atlantic 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
in 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. This outline, although
entirely hypothetical, 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 route 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 highway, 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.
This theory of the evolution of migration routes has been questioned
by some ornithologists on the ground that it implies the possession
in some degree of reasoning powers such as would be used by human
beings. This opposition suggests that changes in migration routes
might develop suddenly following mass survival of birds that were
driven over the new route by a storm on some specific occasion.
In the language of genetics, the new route would be, in effect, a
mutation, rather than the result of an accumulation of infinitesimal
variations. There is some evidence in support of this opposing
theory. For example, information from the Hudson's Bay Co. post at
Great Whale River, on the southeastern coast of Hudson Bay, indicates
that in 1884 the snow geese suddenly changed their route from the
eastern to the western coasts of Hudson and James Bays. According to
one report, this change was caused by strong winds from the south
which caught the birds in their fall migration and caused them to
cross the entrance of James Bay from Cape Jones to the western
side; the route thus reportedly forced upon them was then used in
succeeding years.
_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 correspond 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,
juncos, pine grosbeaks, 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 and the
western wood pewee, which nest in the higher mountains, move down
to the lower regions in August following the breeding season. There
is a distinct tendency 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.
Some species that normally breed in the Hudsonian or Arctic Zones
find suitable breeding areas on the higher levels of the mountains,
as for example the pipit, or titlark, which breeds on the tundra of
Alaska and northern Canada and also south as far as Colorado on the
summits of many peaks in the Rocky Mountains. On the other hand, a
few species, as the Clark's crow, or nutcracker, nest at relatively
low altitudes in the mountains and as the summer advances move
higher up, thus performing a vertical migration that in a sense is
comparable with the post-breeding movements of herons on the Atlantic
coast. These illustrations show that the length of a migration route
may depend upon factors other than latitude.
_Vagrant Migration_
The most striking feature of the migrations of some of the herons
is a northward movement after the nesting season. The young of some
species commonly wander late in the summer and in fall, sometimes
traveling several hundred miles north of the district in which they
were hatched. The little blue heron breeds commonly north to South
Carolina, and by the last of July the young birds begin to appear
along the Potomac, Patuxent, and Susquehanna Rivers, tributary to
Chesapeake Bay. Although almost all are immature individuals, as
shown by their white plumage, an occasional adult may be noted. With
them come egrets and snowy herons and on occasion all three species
will travel in the East as far north as New England, and in the
Mississippi Valley to southeastern Kansas and Illinois. In September
most of them disappear, probably returning south by the same route.
The black-crowned night heron has similar wandering habits, and
young birds banded in a large colony at Barnstable, Mass., have
been recaptured the same season north to Maine and Quebec and west
to New York. This habit seems to be shared by some of the gulls
also, although here the evidence is not so conclusive. Herring gulls
banded as chicks at colonies in the Great Lakes have scattered in
all directions after the breeding season, some having been recovered
well north in Canada.
These movements may be considered as migration governed only by
the availability of food, and they are counteracted in fall by a
directive migratory impulse that carries back to their normal winter
homes in the south those birds that after the nesting period attained
more northern latitudes. They are not to be compared with the great
invasions of certain birds from the North. Classic examples of the
latter in the eastern part of the country are the periodic flights of
crossbills. Sometimes these migrations will extend well south into
the Carolinian Zone.