simultaneously or separately. In the latter case it is usually the
males that arrive first, sometimes great flocks of male birds, as in
the red-winged blackbird, reaching a locality several days before any
of the females. This is particularly the rule in spring: The first
robins are usually found to be males, as are also the first song
sparrows, rose-breasted grosbeaks, and scarlet tanagers. This early
arrival of the males has been explained on the theory of territorial
possession, under which the male selects the area where it elects to
breed, each individual attempting to protect a definite territory
from trespass by other males of his own kind, at the same time
singing or otherwise announcing his presence and inviting the later
arriving female to examine the territory that he has selected for
nesting. The long-billed marsh wren is a noteworthy example, and the
males of this species may enthusiastically build several dummy nests
before the females arrive.
In a few species the males and females apparently arrive at the
breeding grounds together and proceed at once to nest building. In
fact among the shore birds, ducks, and geese, courtship and mating
may take place in whole or in part while the birds are in the South
or on their way north, so that when they arrive at the northern
nesting grounds they are paired and ready to proceed at once with
the raising of their families. Mallards and black ducks may be
observed in pairs as early as January, the female leading and the
male following when they take flight. Naturally these mated pairs
migrate north in company, and it was largely to protect such species
that duck shooting in spring was abolished by Federal law a number of
years ago.
Many shore birds nest well within the Arctic Circle, and it is the
opinion of ornithologists that most of these birds share, at least
in part, the habits of the phalaropes, a family in which the male
assumes the entire care of the eggs and young. If this be true, it
explains why in southern latitudes so many of the earliest fall
arrivals are females that may have deserted the breeding grounds
after the eggs were laid.
Migratory flights are frequently accomplished in close flock
formation, as with the shore birds, blackbirds, and waxwings, and
especially some of the sparrows--the snow buntings, longspurs,
juncos, and tree sparrows. Other species, however, though they travel
in flocks, maintain a very loose formation; examples are the turkey
vultures, the hawks, swifts, blue jays, swallows, warblers, and
bluebirds. Still others, the grebes, great horned owls, winter wrens,
shrikes, and belted kingfishers for example, ordinarily travel alone
and when several are found in close proximity it is an indication
that they have been drawn together by unusual conditions, such as
abundant food.
_Where Birds Migrate_
Definite evidence shows that both the length and the duration of the
migratory journey vary greatly. The bobwhite and the western quails,
the cardinal, the Carolina wren, and probably some of the titmice and
woodpeckers, which are apparently almost or quite nonmigratory, may
round out their full period of existence without at any time going
more than 10 miles from the nest where they were hatched.
=Short and undetermined migrations=
Song sparrows, meadow larks, blue jays, and some other species make
such short migrations that the movement is difficult to detect, as
individuals may be found in one area throughout the year. Thus, at
the southern part of the range there is merely a concentration in
winter, the summer individuals being entirely sedentary. Speculation
is useless on the distances of individual migration without definite
evidence concerning the precise winter quarters of birds that summer
in a particular part of the breeding range of the species, but from
the records of banded birds important evidence is becoming available.
Eventually it may be possible to say definitely just how far the song
sparrows that nest in northern New England and the Maritime Provinces
of Canada travel to their winter quarters, and whether the blue jays
of New York and the upper Mississippi Valley remain throughout the
winter in their breeding areas, or move farther south and relinquish
their places to individuals from southern Canada.
An illustration of what is now known on this subject is found in the
case of the robin. This bird occurs in the Middle Atlantic States
throughout the year, in Canada only in summer, and along the Gulf
coast only as a winter resident. On the Atlantic coast its movements
are readily ascertained, since, for example, in the section about
Washington, D. C., the breeding robin is the southern variety
(_Turdus migratorius archrusterus_) which is found there from the
first of April to the last of October, when its place is taken
(in smaller numbers) by the northern robin (_Turdus migratorius
migratorius_), which arrives about the middle of October and remains
until the following April. It is probable that a similar interchange
of individual robins occurs throughout a large part of the balance of
its range, the hardy birds from the north being the winter tenants in
the abandoned summer homes of the southern birds.
The red-winged blackbirds that nest in northern Texas are almost
sedentary, but in winter they are joined by representatives of other
subspecies that nest as far north as the Mackenzie Valley.
=Variable migrations within species=
The difference in characters between subspecies has been used by
students of migration to discover other interesting facts concerning
variations of the migratory flight between closely related birds
that breed in different latitudes. The familiar eastern fox sparrow
(_Passerella iliaca iliaca_) breeds from northwestern Alaska to
Labrador, and in winter is found concentrated in the southeastern
part of the United States. It thus travels a long distance each year.
On the west coast of the continent, however, six subspecies of this
bird breed in rather sharply delimited ranges, extending from the
region of Puget Sound and Vancouver Island to Unimak Island, at the
end of the Alaskan Peninsula. One of these, known as the sooty fox
sparrow (_P. i. fuliginosa_) breeds in the Puget Sound area and makes
practically no migration at all, while the other races, nesting on
the coast of British Columbia and Alaska, are found in winter chiefly
in California. The races that breed farthest north are in winter
found farthest south, illustrating a tendency for those birds that
are forced to migrate to pass over those so favorably located that
they have no need to leave their breeding areas, while the northern
birds settle for the winter in the unoccupied areas farther south
(fig, 7).
Another example of the same kind is the Maryland yellowthroat of
the Atlantic coast. Birds occupying the most southern part of the
general range are almost nonmigratory, residing throughout the year
in Florida, while those breeding as far north as Newfoundland go to
the West Indies for the winter, thus passing directly over the home
of their southern relatives.
[Illustration: _Figure 7._--Migration of Pacific-coast forms of the
fox sparrow. The breeding ranges of the different races are encircled
by solid lines, while the winter ranges are dotted. The numbers
indicate the areas used by the different subspecies, as follows: 1.
Shumagin fox sparrow; 2. Kodiak fox sparrow; 3. Valdez fox sparrow;
4. Yakutat fox sparrow; 5. Townsend fox sparrow; 6. Sooty fox
sparrow (After Swarth, courtesy of the Museum of Vertebrate Zoology,
University of California).]
The palm warbler (_Dendroica palmarum_) which breeds from Nova
Scotia and Maine west and northwest to southern Mackenzie, has been
separated into two subspecies. Those breeding in the interior of
Canada (_D. p. palmarum_) make a 3,000-mile journey from Great Slave
Lake to Cuba, passing through the Gulf States early in October. After
the bulk have passed, the palm warblers from the Northeastern States
and Provinces (_D. p. hypochrysea_) drift slowly into the Gulf Coast
region, where they remain for the winter. Their migratory journey is
about half as long as that of the northwestern subspecies.
There is no invariable law governing the distance of migration,
although in general it is found that where a species has an extensive
range, the subspecies that breed farthest north go farthest south to
spend the winter.
=Fall flights not far south of breeding ranges=
Some other species that have extensive summer ranges, for instance
the pine warbler, rock wren, field sparrow, loggerhead shrike, and
black-headed grosbeak, are found to concentrate during the winter
season in the southern part of the breeding range, or to occupy
additional territory that is only a short distance farther south.
The entire species may thus be confined within a restricted area for
the period of winter, and then, with the return of warmer weather,
spreads out to reoccupy the full range.
There are many species, including the tree sparrow, slate-colored
junco, and Lapland longspur, that nest in Canada and winter in the
United States; while others, including the vesper sparrow, chipping
sparrow, grackles, red-winged blackbirds, bluebirds, the woodcock,
and several species of ducks, nest in the northern United States and
move south for the winter to areas along the Gulf of Mexico. This
list includes the more hardy species, some individuals of which may
linger in protected places well within the reach of severe cold, as,
for example, Wilson's snipe or jacksnipe, which frequently is found
during subzero weather in parts of the Rocky Mountain region where
warm springs assure a food supply. More than 100 of our summer birds
leave the United States entirely and spend the winter in the West
Indies, or in Central America or South America. For example, the
Cape May warbler, which breeds from northern New England, northern
Michigan, and northern Minnesota, north to New Brunswick, Nova
Scotia, and nearly to Great Slave Lake, is concentrated in winter
chiefly in the West Indies, its metropolis at this season being the
island of Hispaniola.
=Long-distance migrations=
Some of the common summer residents are not content with a trip to
northern South America, but push on across the Equator and finally
come to rest for the winter in the pampas of Argentina, or even
in Patagonia. Thus some species that are more or less associated
with each other in summer, as nighthawks, barn swallows, cliff
swallows, and some of the thrushes, may also occupy the same general
winter quarters in Brazil. Some individual nighthawks and barn
swallows travel still farther, and of all North American land birds
these species probably have the longest migration route, as they
occur north in summer to Yukon and Alaska, and south in winter to
Argentina, 7,000 miles away. Such seasonal flights are exceeded in
length, however, by the journeys of several species of water birds,
chiefly members of the suborder of shore birds. In this group there
are 19 species that breed north of the Arctic Circle and winter in
South America, 6 of them going as far south as Patagonia, and thus
having a migration route more than 8,000 miles in length.
The arctic tern is the champion "globe trotter" and long-distance
flier (fig. 8). Its name "arctic" is well earned, as its breeding
range is circumpolar and it nests as far north as it can find a
suitable place. The first nest to be found in this region was only
7½ degrees from the North Pole, and it contained a downy chick
surrounded by a wall of newly fallen snow that had been scooped
out by the parent. In North America the arctic tern breeds south
in the interior to Great Slave Lake, and on the Atlantic coast to
Massachusetts. After the young are grown the arctic terns disappear
from their North American breeding grounds, and a few months later
they may be found in the Antarctic region, 11,000 miles away. Until
very recently the route followed by these hardy fliers was a complete
mystery, for although a few scattered individuals have been noted
south as far as Long Island, the species is otherwise practically
unknown along the Atlantic coasts of North America and South America.
It is, however, known as a migrant on the west coast of Europe and
Africa. By means of numbered bands the picture is now developing of
what is apparently not only the longest but also one of the most
remarkable of all migratory journeys.
[Illustration: _Figure 8._--Distribution and migration of the arctic
terns of eastern North America. The route indicated for this bird is
unique, as no other species is known to breed abundantly in North
America and to cross the Atlantic Ocean to and from the Old World.
The extreme summer and winter homes are 11,000 miles apart, and as
the route taken is circuitous, these terns probably fly at least
25,000 miles each year.]
Judging by the evidence at present available it seems likely that
the arctic terns of eastern North America originally found their way
here from the Old World, probably by way of Iceland and Greenland.
Consequently when the time comes for them to migrate to winter
quarters they do not go directly south as do the common and Forster's
terns, but instead they fly back eastward along their ancestral route
across the Atlantic to the shores of Europe and then go south along
the African coast to their winter home. Those that breed in the
northwestern part of the continent, as in Alaska, probably migrate
chiefly down the western coast, as the species is not infrequently
reported on the coast of California and also on the western coast of
South America.
The evidence yielded by banding consists of only six definite cases,
but their interpretation permits but one conclusion. All were banded
either as downy chicks or as nonflying immature birds. The details
of their banding and recovery are of sufficient interest to justify
citing in detail. The first was banded on July 3, 1913, at Eastern
Egg Rock, Maine,[2] and in August 1917 was found dead in the Niger
River delta, West Africa. The second was banded at the Red Islands,
Turnevik Bay, Labrador, on July 22, 1927, and was recovered near La
Rochelle, France, on October 1, 1927. The third, also banded on the
Red Islands, on July 23, 1928, was retaken at Margate, near Port
Shepstone, Natal, South Africa, on November 14, 1928. The fourth,
banded at Machias Seal Island, New Brunswick, on July 20, 1935, was
captured near St. Nazaire, France, October 8, 1935. The fifth and
sixth were banded at Machias Seal Island also; the fifth, banded
July 5, 1947, was found on November 10, 1948, at Kingfisher Creek,
Sedgefield, near Wilderness, Eastern Cape Province, South Africa,
while the sixth, banded on July 18, 1948, was picked up dead during
the latter part of September 1948 on the hills near Kyle Strome,
Sutherland, Scotland. All that remained of the bird that provides
the last-named case was a mutilated foot and it appeared that it had
been the victim of some predator. It should be pointed out that the
flights indicated in the third and fifth cases detailed above, are
the longest known for any birds. Both are between 8,000 and 9,000
miles, which in the case of No. 3 was accomplished in less than 3
months.
[2] Recorded at the time of banding as a common tern, a natural
error, as the downy young of common and arctic terns look almost
exactly alike.
Probably no other animal in the world enjoys as many hours of
daylight as does the arctic tern, since for these birds the sun never
sets during the nesting season in the northern part of the range,
while during their sojourn in the south, daylight is continuous.
During several months of the year they have 24 hours of daylight and
during the other months considerably more daylight than darkness.
_Routes of Migration_
While it is beyond question that certain general directions of flight
are constantly followed by migratory birds, it is well to remember
that the term "migration route" is to some extent a theoretical
concept concerned entirely with the lines of general advance or
retreat of a species, rather than the exact course followed by
individual birds. Even the records of banded birds usually show
no more than the place of banding and recovery, and one must
have recourse to intermediate records and to reasoning based on
probabilities to fill in details of the route actually traversed
between the two points.
There is also infinite variety in the routes covered during migration
by different species. In fact, the choice of migration highways is
so wide that it seems as if the routes of no two species coincide.
Differences in distance traveled, time of starting, speed of flight,
geographical position, latitudes of breeding and of wintering
grounds, and in other factors, all contribute to this great variation
of migration routes. Nevertheless, there are certain factors that
serve to guide the avian travelers along more or less definite lines,
and it is possible to define general lines of migration for the
majority of species.
It has been frequently observed that migrating birds have a tendency
to follow major topographic lines on the earth's surface when their
trend is in the general direction of the birds' journey. Bird
migration is generally thought of as a north-and-south movement, with
the lanes of heavier concentration following the coasts, mountain
ranges, and principal river valleys. To a considerable extent this
is the case, particularly in North America, where the coast lines,
mountain chains, and larger rivers in general run north and south.
In cases where the migration is a long one, however, the notion must
be abandoned that the birds' flight is always restricted to narrow
routes that follow river valleys and the like, as many species seem
to disregard utterly such apparently good natural highways. For
example, the Arkansas River has a general east-and-west course for a
great part of its length, and while it does constitute a route for
many perching birds en route from the Mississippi Valley to the Rocky
Mountain region, some of the hawks and many ducks and shore birds
pay the valley scant attention. They may arrest the autumn journey
to feed among the cottonwoods or along sand bars, but when ready to
resume their flight they leave the river and fly directly south over
the more or less arid region that lies between the Arkansas and the
Rio Grande.
=Wide and narrow migration lanes=
When birds start their southward migration the movement necessarily
involves the full width of the breeding range. Later there is a
convergence of the lines of flight taken by individual birds, owing
to the conformation of the land mass, and as the species proceeds
southward the width of the occupied region becomes less and less. An
example of this is provided by the common kingbird, which breeds from
Newfoundland to British Columbia, a summer range 2,800 miles wide. On
migration, however, its paths converge until in the southern part of
the United States the occupied area extends from Florida to the mouth
of the Rio Grande, a distance of only 900 miles, and still farther
south the migration path is further restricted. In the latitude of
Yucatan it is not more than 400 miles wide, and it is probable that
the great bulk of the species moves in a belt that is less than half
that width.
A migration route, therefore, may be anything from a narrow path
that adheres closely to some definite geographical feature, such as
a river valley or a coast line, to a broad boulevard that leads in
the desired direction and which follows only the general trend of the
land mass. Also it is to be remembered that whatever main routes are
described, there remains a multitude of tributary and separate minor
routes. In fact, with the entire continent of North America crossed
by migratory birds, the different groups or species frequently follow
lines that may repeatedly intersect those taken by others of their
own kind or by other species. The arterial routes, therefore, must
be considered merely as indicating paths of migration on which the
tendency to concentrate is particularly noticeable.
In considering the width of migration lanes it will be obvious that
certain species, as the knot and the purple sandpiper, which are
normally found only along the coasts, must have extremely narrow
routes of travel. They are limited on one side by the broad waters of
the ocean, and on the other by land and fresh water, both of which
are unsuited to furnish the food that is desired and necessary to the
well-being of these species.
[Illustration: _Figure 9._--Breeding and wintering ranges and
migration of Harris's sparrow, an example of a narrow migration
route through the interior of the country. The heavy broken lines
enclose the region traversed by the majority of these finches; the
light broken line encloses the country where they occur with more or
less regularity; while the spots indicate records of accidental or
sporadic occurrence.]
Among land birds that have a definite migration, the Ipswich sparrow
has what is probably the most restricted migration range of any
species. It is known to breed only on Sable Island, Nova Scotia,
and it winters along the Atlantic coast south to Georgia. Living
constantly within sound of the surf, it is rarely more than a quarter
of a mile from the outer beach, and is entirely at home among the
sand dunes and their sparse covering of coarse grass.
[Illustration: _Figure 10._--Distribution and migration of the
scarlet tanager. During the breeding season individual scarlet
tanagers may be 1,900 miles apart in an east-and-west line across the
breeding range. In migration, however, the lines converge until in
southern Central America they are not more than 100 miles apart. For
migration paths of other widths see figures 9, 11, and 12.]
Harris's sparrow supplies an interesting example of a narrow
migration route in the interior of the country (fig. 9.) This fine,
large finch is known to breed only in the region from Churchill, on
the west shore of Hudson Bay, northwest to the shores of Great Bear
Lake. Very few actual breeding records of the species are available,
but these are sufficient to indicate that the breeding range is in
the strip of country characterized by more or less stunted timber
just south of the limit of trees. When it begins its fall migration,
this species necessarily covers the full width of its breeding area.
Then it proceeds almost directly south, or slightly southeasterly,
the area covered by the majority of the birds becoming gradually
constricted, so that by the time it reaches the United States it is
most numerous in a belt about 500 miles wide, extending across North
Dakota to central Minnesota. Harris's sparrows are noted on migration
with fair regularity east to the western shore of Lake Michigan, and
west to the foothills of the Rocky Mountains, but the great bulk of
the species moves north and south through a relatively narrow path in
the central part of the continent. Present knowledge suggests that
the reason for this narrow migration range is the close association
that Harris's sparrow maintains with a certain type of habitat'
including brushy places, thickets, edges of groves, and weed patches.
While these environmental conditions are found in other parts of
the country, the region crossed by this sparrow presents almost a
continuous succession of habitat of this type. Its winter range
extends from southeastern Nebraska and northwestern Missouri, across
eastern Kansas and Oklahoma and through a narrow section of central
Texas, at places hardly more than 150 miles wide.