MIGRATION OF BIRDS
“At the approach of the cold season,” Uncle Paul resumed, in his
account of bird habits and bird peculiarities, “before winter clears
the fields of insects, covers the ponds with a coating of ice, and
whitens the landscape with snow, thus cutting off the food-supply
hitherto obtainable from the earth, many birds, especially those that
live on insects or frequent bodies of water and marshy meadows, take
leave of their native land and direct their course southward, where
they will find a warmer sun and a more assured supply of food.
“They take their departure, some in large flocks, others in small
groups, or even each one separately. With no guide other than an
irresistible impulse too mysterious for us to explain, they traverse by
successive stages immense tracts of land, cross seas, and bend their
course toward the countries of the south. Africa is the rendezvous of
our birds and of European birds in general.
“After the cold season has passed, with the first fine days of spring
the same birds return to the regions where they were born, making the
journey this time in the opposite direction, from south to north. They
take possession once more of their groves and forests, their rocks and
prairies, which they know how to find with an inconceivable accuracy.
There they build their nests, rear their young, and gain strength for
the coming journey; and upon the return of cold weather they go back
again to the lands of sunshine.
“These periodical journeys are called migrations, of which there are
two each year,—that of autumn, when the birds leave us and go
southward, and that of spring, when they fly northward and come back to
us. These semi-annual flittings take place all over the earth.
“The various species do not all fix upon the same time for their
migration, but each has its own calendar, from which it departs only
very slightly. Some start well in advance of the increasing chill and
the lessening abundance of food, while others do not leave their native
land until driven by actual necessity, when the cold has become severe.
Thus our martin flies away for Africa as early as the month of August,
whereas the chimney-swallow lingers until October or even November.
“The martins forsake our turrets and old walls, our steeples and
belfries, while the summer heat is still intense and the small flies on
which they feed are still abundant. It is not, then, any lowering of
temperature that drives them away, nor is it any lack of food that
hastens their departure; but they have a secret presentiment of the
change of season that is coming in a few weeks; a deep-seated unrest,
which they cannot overcome, warns them that the hour for their
departure is drawing near.
“If one desires to witness this anxiety that torments the bird when the
time for migrating arrives, he may do so by rearing in captivity a
migratory bird caught very young. The captive, though never having
lived with its kind or had any knowledge of their migratory habits, and
furthermore having been kept in a cage with no experience of cold or
hunger, nevertheless, when the season for flitting arrives, shows
agitation and mental distress, and tries to escape from its
prison—after remaining so quiet and contented up to that time. Some
inner voice—instinct we call it—says it is time to go, and the captive
is eager to be off. If the desire is thwarted, death follows.
“To tear oneself from beloved haunts to incur the fatigues and perils
of a long journey is undoubtedly a painful decision; yet the bird
courageously submits to the inevitable, but in the hope of coming back
again some day. The strong reassuring the weak, the older ones guiding
the young, the departing flock forms itself into a caravan and takes
wing for the south. The sea is crossed, the treacherous sea from which,
at long intervals, rises an island as halting-place. Many perish in the
crossing, many reach the goal worn with hunger and spent with fatigue.
“The day for starting on this momentous journey is decided upon in a
great assembly, toward the end of August for the window-swallow, and
considerably later, even as late as November, for the chimney-swallow.
When once the date has been fixed, the window-swallows gather together
daily for several days on the roofs of tall buildings. Every few
minutes small parties detach themselves from the general conclave and
circle about in the air with anxious cries, taking a parting look at
their native haunts, and paying them a last farewell. Then they return
to their places among their companions and join in noisy chatter on the
subject of their hopes and fears, all the while preparing themselves
for the distant expedition by a careful inspection of their plumage and
a final touch to one lustrous feather after another.
“After several repetitions of these farewells a plaintive twittering
announces the fateful hour. The moment has come, it is time to start.
The flock rises, the emigrants are off for the south. If one of them
has been marked with a red string around the claw in order to be
recognized, you may be sure you will see it come back the next spring
and take possession of its nest again with little cries of joy at
finding it intact and ready for occupancy after a few repairs.
“With their vigorous wings the duck and goose, in their wild state, are
ardent travelers. On a gray day in November, when there are signs of
snow, it is not unusual to see passing from north to south, at a great
height, birds arranged in single file, or in a double file meeting in a
point, like the two branches of the letter V. These birds are a flock
of either ducks or geese in the act of migrating.
“If the flock is of no great size, the birds composing it arrange
themselves in one continuous file, the beak of each following bird
touching the tail of the preceding, in order that the passage opened
through the air may not have time to close again. But if the flock is a
large one, two files of equal length are formed, which meet at an acute
angle, the front of the moving mass.
“This angular arrangement, of which we find examples in the ship’s
prow, the plowshare, the thin edge of a wedge, and a multitude of
utensils designed for cleavage, is the most favorable for pushing
through the mass of the air with the least fatigue. If in marshaling
their flying battalions the goose and the duck had taken counsel of the
engineer’s science, they could not have managed better. But they have
no need of others’ advice: instructed by their own instinct, they
utilized long before we did the principle of the wedge.
“Moreover, to divide among all the members of the flock the excess of
fatigue incurred by the file-leader in opening a passage through the
air by strength of wing, each in turn takes the post of honor, the
forward end of the single file or the point of the angle formed by the
double file. Its term of service ended, the bird at the head retires to
the rear to recuperate, and another leader takes its place. By this
equitable division of labor the fatigue does not prove excessive for
any one bird, and the flock leaves no stragglers behind.”