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.
[Illustration: B4506M
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.
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 (_Anthus spinoletta rubescens_),
which breeds on the tundras 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 (_Nucifraga columbiana_), 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 summer and in fall, sometimes
traveling several hundred miles north of the district in which they
were hatched. The little blue heron (_Florida caerulea caerulea_)
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 snowy herons (_Egretta
thula thula_) and egrets (_Casmerodius albus egretta_), 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 (_Nycticorax nycticorax hoactli_) 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 (_Larus argentatus smithsonianus_)
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 such birds as after the mating period have
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.
Snowy owls are noted for occasional invasions that probably are
caused by a shortage of the lemmings and rabbits that constitute
their normal food in the North. At least nine notable flights of
these birds occurred during the period 1876 to 1927. In the great
flight of 1926-27 they were noted as far south as Iowa, Ohio, West
Virginia, and North Carolina.
In the Rocky Mountain region great flights of the beautiful Bohemian
waxwing (_Bombycilla garrula pallidiceps_), are occasionally
recorded. The greatest invasion in the history of Colorado
ornithology occurred in February 1917, at which time the writer
estimated that at least 10,000 were within the corporate limits of
the city of Denver. The last previous occurrence of the species in
large numbers in that section was in 1908.
Evening grosbeaks (_Hesperiphona vespertina_) likewise are given to
performing more or less wandering journeys, and curiously enough,
in addition to occasional trips south of their regular range, they
travel east and west, sometimes covering long distances. For example,
grosbeaks banded at Sault Ste. Marie, Mich., have been recaptured on
Cape Cod, Mass., and in the following season have been re trapped
at the banding station. Banding records demonstrate that this
east-and-west trip across the northeastern part of the country is
sometimes made also by purple finches (_Carpodacus purpureus_).
=PERILS OF MIGRATION=
The period of migration is a season full of peril for birds. Untold
thousands of the smaller migrants are destroyed each year by storms,
in unfamiliar habitats, and through attacks of predatory birds,
mammals, and reptiles. If each pair of adult birds should succeed
in raising two fledglings to maturity, the population of migratory
birds would have a potential annual increase of 100 percent and the
world would soon be heavily overpopulated with them. It is evident,
therefore, that there is no such increase, and that the annual
mortality from natural causes is heavy enough to keep it in check.