gray-cheeked thrush. This bird winters in Colombia, Ecuador, Peru,
Venezuela, and British Guiana and does not start its northward
journey until many other species are well on their way. It does not
appear in the United States until the last of April--April 25 near
the mouth of the Mississippi, and April 30 in northern Florida (fig.
5). A month later, or by the last week in May, the bird is seen in
northwestern Alaska, the 4,000-mile trip from Louisiana having been
made at an average speed of about 130 miles a day.
[Illustration: _Figure 4._--Migration of the Canada goose. The
northward movement keeps pace with the advance of spring, in this
case the advance of the isotherm of 35° F. agreeing with that of the
birds.]
Another example of rapid migration is furnished by the yellow, or
summer, warbler. Coming from the Tropics, the birds reach New Orleans
about April 5, when the average temperature is 65° F. Traveling north
much faster than does the season, they reach their breeding grounds
in Manitoba the latter part of May, when the average temperature is
only 47°. Encountering progressively colder weather over their entire
route, they cross a strip of country in the 15 days from May 11 to 25
that spring temperatures take 35 days to cross. This "catching up"
with spring is habitual in species that winter south of the United
States and in most of the northern species that winter in the Gulf
States. To this rule there appear to be only six exceptions--the
Canada goose, the mallard, the pintail, the crow, the red-winged
blackbird, and the robin.
[Illustration: _Figure 5._--Isochronal migration lines of the
gray-cheeked thrush, an example of rapid migration. The distance
from Louisiana to Alaska is about 4,000 miles and is covered at
an average speed of about 130 miles a day. The last part of the
journey is covered at a speed that is several times what it is in the
Mississippi Valley.]
The blue goose presents a striking example of a late but very rapid
spring migration. Practically all members of the species winter in
the great coastal marshes of Louisiana, where 50,000 or more may
be seen grazing in the "pastures" or flying overhead in flocks of
various sizes. Their breeding grounds are chiefly on Baffin Island
and on Southampton Island in the northern part of Hudson Bay, in a
region where conditions of severe cold prevail except for a few weeks
each year. The birds seem to realize that even though the season in
their winter quarters is advancing rapidly, their nesting grounds
are still covered with a heavy blanket of ice and snow. Accordingly
they remain in the coastal marshes until the last of March or the
first of April, when the local birds are already busily engaged with
the duties of reproduction. The flight northward is rapid, almost
nonstop, so far as the United States is concerned, for although the
birds are sometimes recorded in large numbers in the Mississippi
Valley, including eastern South Dakota, and in southeastern Manitoba,
there are few records anywhere along the route of such great flocks
as are known to winter in Louisiana. When the birds arrive in the
James Bay region of Canada they apparently enjoy a prolonged period
of rest, as they are not noted in the vicinity of their breeding
grounds until the first of June. During the first 2 weeks of that
month they pour into the tundra country by the thousands, and each
pair immediately sets about the business of rearing a brood.
The robin has been mentioned as a slow migrant, and as a species it
takes 78 days to make the 3,000-mile trip from Iowa to Alaska, a
stretch of country that is crossed by advancing spring in 68 days.
In this case, however, it does not mean that individual robins are
necessarily slow, for probably the northward movement of the species
depends upon the continual advance of birds from the rear, the first
individuals arriving in a suitable locality remaining to nest, while
the northward movement of the species is continued by those still to
come.
Special interest attaches to the great variation in the speed
at which birds travel in different sections of the broad flyway
extending from the Gulf of Mexico to the Arctic Ocean, by way of
the Mississippi and Mackenzie Valleys. The blackpolled warbler
furnishes an excellent example (fig. 6). This species winters in
north-central South America and migrates in April across the West
Indies to Florida. From this point some individuals fly northwest to
the Mississippi Valley, north to Manitoba, northwest to the Mackenzie
River, and thence almost due west to western Alaska. In tracing
the long route of these birds it is found that a fairly uniform
average speed of 30 to 35 miles a day is maintained from the Gulf to
Minnesota. Then comes a spurt, for a week later the blackpolls have
reached the central part of the Mackenzie Valley and by the following
week they are observed in northwestern Alaska. During the latter
part of the journey, therefore, many individuals must average more
than 200 miles a day. They use 30 days in traveling from Florida to
southern Minnesota, a distance of about 1,000 miles, and scarcely
half that time to cover the remaining 2,500 miles to Alaska. It
should be noted that the increased speed is directly associated
with the change in direction, the north-and-south course in the
Mississippi Valley being accomplished slowly while the northwesterly
course across Canada is made at a much greater speed. Increased
speed across western Canada to Alaska is also shown by many other
birds. A study of all species traveling up the Mississippi Valley
indicates an average speed of about 23 miles a day. From southern
Minnesota to southern Manitoba 16 species maintain an average speed
of about 40 miles a day. From that point to Lake Athabaska, 12
species travel at an average speed of 72 miles a day; while 5 others
travel to Great Slave Lake at 116 miles a day; and another 5 species
cover 150 miles a day to reach Alaska. This change is in correlation
with a corresponding variation in the isothermal lines, which turn
northwestward west of the Great Lakes.
[Illustration: _Figure 6._--Migration of the blackpolled warbler. The
solid isochronal lines show the places at which these birds arrive at
the same time. As the birds move northward these lines become farther
apart, showing that the warblers move faster with the advance of
spring. From April 30 to May 10 the average speed is about 30 miles
a day, while from May 25 to May 30 it is increased to more than 200
miles.]
As has been previously indicated, the advance of spring in the
northern interior is much more rapid than in the Mississippi Valley
and on the Gulf coast. In other words, in the North spring comes with
a rush and during the height of the migration season in Saskatchewan
the temperature in the southern part of the Mackenzie Valley just
about equals that in the Lake Superior area, which is 700 miles
farther south. Such conditions, coupled with the diagonal course of
the birds across this region of fast-moving spring, exert a great
influence on migration and are the chief factors in the acceleration
of speed of travel.
Variations in speed of migration in different parts of the country
are illustrated also by the movements of the cliff swallow (fig.
3), which breeds from Mexico to Alaska and winters in Brazil and
Argentina. It would be expected in spring to appear in the United
States first in Florida and Texas then in the southern Rocky
Mountain region, and finally on the Pacific coast. As a matter of
fact, however, the earliest spring records come from north-central
California, where the bird usually is common before the first
arrivals are observed in Texas or Florida. The route taken, for many
years a migration problem, was solved when it was found that these
swallows went around the Gulf of Mexico rather than across it. The
isochronal lines on the map show the more rapid advance along the
Pacific coast. By March 20, when the vanguard has not quite reached
the lower Rio Grande in Texas, the species is already north of San
Francisco in California.
=Altitude at which birds travel=
At one time students of bird migration held firmly to the theory
that normal migration takes place at heights above 15,000 feet,
reasoning (somewhat uncertainly) that flying becomes easier as
altitude is gained. Since the development of the airplane, however,
and with it man's exploration of the upper regions of the air, it
has become common knowledge that rarified atmosphere adds greatly to
the difficulties of flight. This is due not only to the reduction in
oxygen (whether for gasoline engine or the lungs of a bird) but also
to the lack of buoyancy of the rarified air. Such birds as vultures,
pelicans, cranes, and some of the hawks feel this the least, since
compared with body weight the supporting surface of their wings is
very great, but for the smaller and shorter-winged birds lack of
buoyancy at high altitudes presents a difficult obstacle in flight.
Even when flying close to the earth, small birds have to keep their
wings in rapid motion.
Another postulate favoring the high-altitude flying theory was
that the wonderful vision of birds was their sole guidance during
migratory flights; and to keep landmarks in view the birds were
obliged to fly high, particularly when crossing wide areas of water.
This will be considered in greater detail under Orientation (p. 28),
so here it will be sufficient to say that birds rely only in part
upon vision to guide them on migration. Also, it is to be remembered
that there are definite physical limitations to the range of
visibility even under perfect atmospheric conditions. Chief of these
is the curvature of the earth's surface. Thus, if birds crossing the
Gulf of Mexico to Louisiana and Florida flew at a height of 5 miles,
they would still be unable to see a third of the way across. And yet
this trip is made twice each year by thousands of thrushes, warblers,
and others.
Actual knowledge of the altitude of migratory flight is scanty,
though estimates obtained by means of the telescope, and still more
accurate data resulting from altimeter observation from airplanes,
are slowly accumulating. It is, of course, obvious that some birds
that cross mountain ranges during migration must attain a great
altitude. Observers at an altitude of 14,000 feet in the Himalayas
have recorded storks and cranes flying so high that they could be
seen only through field glasses. Being beyond the range of unaided
vision they must have been at least 6,000 feet above the observers,
or at an actual altitude of 20,000 feet above sea level. Such cases,
however, are exceptional as aviators have reported that they rarely
meet birds above an altitude of 5,000 feet.
It is now known that migration in general is performed below a
height of 3,000 feet above the earth. Some proof of this statement
is available. Observations made from lighthouses and other points
of vantage indicate that migrants commonly travel at altitudes of a
very few feet to a few hundred feet above sea or land. Sandpipers,
sanderlings, and northern phalaropes, observed in migration on the
Pacific oceanic route, have been noted to fly so low that they
were visible only as they topped a wave. Observers stationed at
lighthouses and lightships off the English coast have similarly
recorded the passage of land birds, which sometimes flew just above
the surface of the water, and rarely above 200 feet. During the
World Wars broad areas in the air were under constant surveillance
and among the airplane pilots and observers many took more than a
casual interest in birds. Of the several hundred records resulting
from their observations only 36 were of birds flying above 5,000
feet, and only 7 above 8,500 feet. Cranes were once recorded at
an altitude of 15,000 feet, while the lapwing was the bird most
frequently seen at high levels, 8,500 feet being its greatest
recorded altitude.
These observations naturally relate only to daytime travelers, but
there is no reason to believe that nocturnal migration is performed
at higher altitudes. The fact that many birds are killed each year by
striking the lanterns at lighthouses, or other man-made obstructions,
does not, however, furnish conclusive proof that low altitudes are
generally used during nocturnal flight, for it should be recalled
that these accidents occur chiefly in foggy or unsettled weather, and
also that powerful lights have a great attraction for many species
of birds. The altitude at which birds travel is affected by other
weather conditions also. For example, flight at the higher elevations
is facilitated on clear, warm days by the currents of warm air that
ascend from broad areas.
=Orientation=
There probably is no single aspect of the entire subject of bird
migration that challenges our admiration so much as the unerring
certainty with which birds cover thousands of miles of land and
water to come to rest in exactly the same spot where they spent
the previous summer or winter. The records from birds marked with
numbered bands offer abundant proof that the same individuals of many
species will return again and again to their identical nesting sites.
These data show also that many individuals migrate in fall over
the same route, year after year, making the same stops and finally
arriving at the precise thicket that served them in previous winters.
The faculty that enables these birds to point their course accurately
over vast expanses of land and water may, for want of a better
term, be called a "sense of direction." Man recognizes this sense
in himself, though usually it is imperfect and frequently at fault.
Nevertheless the facility with which experienced hunters and woodsmen
locate tiny camps or other points in forested or mountainous country,
frequently cloaked by darkness or fog, with all recognizable
landmarks obliterated seems due to this faculty. Ability to travel
with precision over unmarked trails is not limited either to birds or
to man. It is likewise possessed by many other mammals as well as by
some insects and fishes, the well-known migrations of the salmon and
the eel being notable examples.
Ability to follow a more or less definite course to a definite goal
is evidently part of an inherited faculty. Both the path and the goal
must have been determined either when the habit originated or in the
course of its subsequent evolution. The theory is sometimes advanced
that the older and more experienced birds lead the way, showing the
route to their younger companions. This explanation may be acceptable
for some species, but not for those in which adults and the young
migrate at different times. The young cowbird that is reared by
foster parents flocks with others of its kind when grown and in many
cases can hardly be said to have adult guidance in migration. An
inherited migratory instinct with a definite sense of the goal to
be reached and the route to be followed must be attributed to these
birds.
It is well known that birds possess wonderful vision. If they also
have retentive memories subsequent trips over the route may well be
steered in part by recognizable landmarks. The arguments against the
theory of vision and memory are chiefly that much migration takes
place by night and that great stretches of the open sea are crossed
without hesitation. Nevertheless, the nights are rarely so dark that
all terrestrial objects are totally obscured, and such features as
coastlines and rivers are just those that are most likely to be
seen in the faintest light, particularly by the acute vision of the
bird and from its aerial points of observation. But some birds fly
unerringly through the densest fog. Members of the Biological Survey,
proceeding by steamer from the island of Unalaska to Bogoslof Island
in Bering Sea, through a fog that was so heavy as to make invisible
every object beyond a hundred yards, recorded the fact that flocks of
murres, returning to Bogoslof, after quests for food, broke through
the wall of fog astern, flew by the vessel, and disappeared into the
mists ahead. The ship was heading direct for the island by the use of
compass and chart, but its course was no more sure than that of the
birds.
Some investigators have asserted that the sense of direction has its
seat in the ears or nasal passages and thus that the bird is enabled
to identify air currents and other phenomena. It has been found that
disturbance of the columella or the semicircular canals of the inner
ear will destroy the homing instinct of the racing pigeon, but
experiments in the form of delicate operations, or closing the ears
with wax, prove such a serious shock to the sensitive nervous system
of the bird that they cannot be considered as affording conclusive
evidence. Several years ago careful studies were made of the homing
instinct of the sooty and noddy terns, tropical species that in the
Atlantic region reach their most northern breeding point on the Dry
Tortugas Islands, off the southwest coast of Florida. They are not
known to wander regularly any appreciable distance farther north.
It was found that some were able to return to their nests on the
Tortugas after they had been taken on board ship, confined in cages
below decks, and carried northward to distances varying from 400 to
800 miles before being released. Landmarks of all kinds were entirely
lacking, and the birds certainly were liberated in a region in which
they had had no previous experience.
Possibly the "homing instinct" as shown by these terns, by the
man-o'-war birds that are trained and used as message carriers in the
Tuamotu, Gilbert, and Marshall Islands, and by the homing pigeon, is
not identical with the sense of perceptive orientation that figures
in the flights of migratory birds. Nevertheless, it seems closely
akin and is probably caused by the same impulses, whatever they
may be and however they may be received. It should be remembered,
however, that while homing may involve flight from a point that
the bird has never before visited, the flight is always to a known
point--that is, the bird's nest--while, on the other hand, the first
migratory flight is always from the region of the bird's birth to a
region it has never before visited. The spring migration might, of
course, be more nearly considered as true "homing."
Some students have leaned strongly toward the possible existence
of a "magnetic sense" as being the important factor in the power
of geographical orientation. The theory that migratory birds might
be responsive to the magnetic field of the earth was conceived as
early as 1855, when some experimental work was done in Russia, and
nearly 60 years later in France. Recently investigations in this
field have been conducted by Yeagley (1947) and by Gordon (1948)
with diametrically opposite results. The idea carries with it the
implication that contained in the bird's body is an organ that is
sensitive to the effect of its motion through the vertical component
of the magnetic field and to other related factors. In the tests by
Dr. Yeagley, 20 young homing pigeons were given training flights to
their home loft from distances up to 100 miles. Permanent magnets
were then affixed to the under side of the manus part of the wings
of half of the birds while copper plates of equivalent weight were
attached to the wings of the other half. All birds were released
singly at an air-line distance of about 65 miles from the loft. The
results were most suggestive, as only two of the birds carrying
magnets returned to the loft, whereas eight of the controls returned.
With certain minor modifications, this experiment was repeated by
Gordon. In this case 60 pigeons were used and releases were made from
points up to 58 miles, where the direction of flight was such that
the birds had to navigate across the gradient of the magnetic field.
Every bird returned to its loft on the day of release regardless of
whether it carried magnets or unmagnetized bars of the same weight.
Attempts to demonstrate the effect of radio waves on the navigational
ability of birds also have produced contradictory results. In some of
these tests, homing pigeons released near broadcasting stations have
appeared to be hopelessly confused, whereas in others, apparently
conducted in the same manner, no effects could be discerned. It is
obvious that before the electromagnetic theory can be accepted or
rejected, much additional experimental work is necessary.
In concluding this discussion of orientation it is pertinent to
point out that the migratory instinct appears to be more or less
transitory, that it is not persistent over an extended period.
Migratory birds may be arrested en route, either by natural
conditions, such as unusual food supplies, or forcibly by the act of
man, and detained until the end or nearly the end of the migratory
season, and then may not attempt to finish the journey, apparently
having lost the migratory impulse. In the fall and early winter of
1929, abundant food and an open season caused an unusual number of
mallard ducks to arrest their migration and remain in western Montana
and northern Idaho. Later, however, a heavy snowfall with subzero
temperatures suddenly cut off the food supply, with the result that
great numbers of the birds starved to death, when a flight of a few
hours would have carried them to a region of open water and abundant
food.
=Segregation during migration=
During the height of the northward movement in spring the woods
and thickets may be suddenly filled with several species of wood
warblers, thrushes, sparrows, flycatchers, and others, which
it is natural to conclude have traveled together and arrived
simultaneously. Probably they did, but such combined migration is by
no means the rule for all species.
As a group the wood warblers (_Compsothlypidae_) probably travel more
in mixed companies than do any other single family of North American
birds. The flocks are likely to be made up of several species in
spring and fall with both adults and young. Sometimes swallows,
sparrows, blackbirds, and some of the shore birds also migrate in
mixed flocks. In fall, great flocks of blackbirds frequently sweep
south across the Plains States, and occasionally one flock will
contain bronzed grackles, red-winged blackbirds, yellow-headed
blackbirds, and Brewer's blackbirds.
On the other hand many species keep strictly to themselves. It would
be difficult for any other kind of bird to keep in company with
one of such rapid movements as the chimney swift, which is rarely
found associated with any other species at any season. Nighthawks
or bullbats also fly in separate companies, as do usually crows,
waxwings, crossbills, bobolinks, and kingbirds. Occasionally, a flock
of ducks will be observed to contain several species, but generally
when they are actually on migration the individuals of each species
separate and travel with others of their own kind. The flocks of
blue geese, previously mentioned in connection with speed of flight
(p. 22), frequently have with them a few of the closely related snow
geese, particularly in the eastern part of their winter range. The
portion here is usually about 10 to 1, but farther west the numbers
of snow geese increase until they outnumber their blue relatives.
The adults of most perching birds drive the young away when they are
grown, probably to be relieved of the necessity of providing for
them, and also in order that the parents may have opportunity to rest
and renew their plumage before starting for winter quarters. The
young birds are therefore likely to drift together and, having no
further responsibility, may start south ahead of their parents. In
contrast with this indifference on the part of the adults of perching
birds, Canada geese and some others remain in family groups, the
parent birds undergoing the wing molt that renders them flightless
during the period of growth of their young, so that old and young
acquire their full plumage at the same time and are able to start
south together. The large flocks, therefore, are composed of many
families that band together, and when they separate into =V=-shaped
units it is probably correct to assume that it is an old bird that
leads the group. Where there is segregation of the sexes, the young
birds usually accompany their mothers, as is the case with some of
the ducks. After the females start to incubate their eggs, the males
of most species of ducks flock by themselves and remain together
until fall.