There probably is no single aspect of the entire subject of bird
migration that challenges our admiration for birds so much as the
unerring certainty with which they 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 of birds marked with
numbered bands afford 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 mammals as well as by
some insects and fishes, the well-known migrations of the salmon
(_Oncorhynchus_) and the eel (_Anguilla_) 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 known, however, 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 at 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 coast lines and rivers are just those that are
most likely to be seen in the faintest light, particularly by the
acute vision of a 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 (_Sterna fuscata_
and _Anoüs stolidus_), 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 deck, and
carried northward distances varying from 400 to 800 miles before
being released. Landmarks of all lands 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-of-war birds (_Fregata minor_), 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 is to 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."
At the present time some students lean strongly toward the possible
existence of a "magnetic sense" as being the important factor in the
power of geographical orientation. No direct evidence in support
of this has been obtained, but it is not impossible that there may
exist some form of physiological sensibility to the phenomena of
terrestrial magnetism. The theory as laid down (chiefly by European
investigators) is highly complex, but briefly stated it is based on
a supposed sensitiveness of birds to the magnetic influences that
cause variations in the declination and dip of magnetic needles. Some
experimental work already done lends a little support to the theory
but it is still far from established.
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.