To what widely different use birds put their power of flight! To the
great mass of them it is simply a means of locomotion, of getting from
one point to another. A small minority put their wing-power to more
ideal uses, as the lark when he claps his wings at heaven's gate, and
the ruffed grouse when he drums; even the woodcock has some other use
for his wings than to get from one point to another. Listen to his
flight song in the April twilight up against the sky.
Our small hawks use their power of flight mainly to catch their prey, as
does the swallow skimming the air all day on tireless wing, but some of
the other hawks, such as our red-tailed hawk, climb their great spirals
apparently with other motives than those which relate to their daily
fare. The crow has little other use for his wings than to gad about like
a busy politician from one neighborhood to another. In Florida I have
seen large flocks of the white ibis performing striking evolutions high
up against the sky, evidently expressive of the gay and festive feeling
begotten by the mating instinct.
The most beautiful flyer we ever see against our skies is the unsavory
buzzard. He is the winged embodiment of grace, ease, and leisure.
Judging from appearances alone, he is the most disinterested of all the
winged creatures we see. He rides the airy billows as if only to enjoy
his mastery over them. He is as calm and unhurried as the orbs in their
courses. His great circles and spirals have a kind of astronomic
completeness. That all this power of wing and grace of motion should be
given to an unclean bird, to a repulsive scavenger, is one of the
anomalies of nature. He does not need to hurry or conceal his approach;
what he is after cannot flee or hide; he has no enemies; nothing wants
him; and he is at peace with all the world.
The great condor of South America, in rising from the ground, always
faces the wind. It is often captured by tempting it to gorge itself in a
comparatively narrow space. But if a strong enough wind were blowing at
such times, it could quickly rise over the barrier. Darwin says he
watched a condor high in the air describing its huge circles for six
hours without once flapping its wings. He says that, if the bird wished
to descend, the wings were for a moment collapsed; and when again
expanded, with an altered inclination, the momentum gained by the rapid
descent seemed to urge the bird upwards with the even and steady
movement of a paper kite. In the case of any bird _soaring_, its motion
must be sufficiently rapid for the action of the inclined surface of
its body on the atmosphere to counterbalance its gravity. The force to
keep up the momentum of a body moving in a horizontal plane in the air
(in which there is so little friction) cannot be great, and this force
is all that is wanted. The movement of the neck and body of the condor,
we must suppose, is sufficient for this. However this may be, it is
truly wonderful and beautiful to see so great a bird, hour after hour,
without any apparent exertion, wheeling and gliding over mountain and
river.
The airplane has a propelling power in its motor, and it shifts its
wings to take advantage of the currents. The buzzard and condor do the
same thing. They are living airplanes, and their power is so evenly and
subtly distributed and applied, that the trick of it escapes the eye.
But of course they avail themselves of the lifting power of the
air-currents.
All birds know how to use their wings to propel themselves through the
air, but the mechanism of the act we may not be able to analyze. I do
not know how a butterfly propels itself against a breeze with its
quill-less wings, but we know that it does do it. As its wings are
neither convex nor concave, like a bird's, one would think that the
upward and downward strokes would neutralize each other; but they do
not. Strong winds often carry them out over large bodies of water; but
such a master flyer as the monarch beats its way back to shore, and,
indeed, the monarch habitually flies long distances over salt water when
migrating along our seacoast in spring and fall.
At the moment of writing these paragraphs, I saw a hen-hawk flap heavily
by, pursued by a kingbird. The air was phenomenally still, not a leaf
stirred, and the hawk was compelled to beat his wings vigorously. No
soaring now, no mounting heavenward, as I have seen him mount till his
petty persecutor grew dizzy with the height and returned to earth. But
the next day, with a fairly good breeze blowing, I watched two hawks for
many minutes climbing their spiral stairway to the skies, till they
became very small objects against the clouds, and not once did they flap
their wings! Then one of them turned toward the mountain-top and sailed
straight into the face of the wind, till he was probably over his mate
or young, when, with half-folded wings, he shot down into the tree-tops
like an arrow.
In regard to powers of flight, the birds of the air may be divided into
three grand classes: those which use their wings simply to transport
themselves from one place to another,--the same use we put our legs
to,--those which climb the heavens to attain a wide lookout, either for
the pleasure of soaring, or to gain a vantage-point from which to scan a
wide territory in search of food or prey, and those which feed as they
fly. Most of our common birds are examples of the first class. Our hawks
and buzzards are examples of the second class. Swallows, nighthawks, and
some sea-birds are examples of the third class. A few of our birds use
their wings to gain an elevation from which to deliver their songs--as
the larks, and some of the finches; but the robins and the sparrows and
the warblers and the woodpeckers are always going somewhere. The hawks
and the buzzards are, comparatively speaking, birds of leisure.
Every bird and beast is a master in the use of its own tools and
weapons. We who look on from the outside marvel at their skill. Here is
the carpenter bumble-bee hovering and darting about the verge-board of
my porch-roof as I write this. It darts swiftly this way and that, and
now and then pauses in midair, surrounded by a blur of whirring wings,
as often does the hummingbird. How it does it, I do not know. I cannot
count or distinguish the separate stroke of its wings. At the same time,
the chimney swifts sweep by me like black arrows, on wings apparently as
stiff as if made of tin or sheet-iron, now beating the air, now sailing.
In some way they suggest winged gimlets. How thin and scimitar-like
their wings are! They are certainly masters of their own craft.
In general, birds in flight bring the wings as far below the body as
they do above it. Note the crow flapping his way through the air. He is
a heavy flyer, but can face a pretty strong wind. His wings probably
move through an arc of about ninety degrees. The phœbe flies with a
peculiar snappy, jerky flight; its relative the kingbird, with a mincing
and hovering flight; it tiptoes through the air. The woodpeckers gallop,
alternately closing and spreading their wings. The ordinary flight of
the goldfinch is a very marked undulatory flight; a section of it, the
rise and the fall, would probably measure fifty feet. The bird goes half
that distance or more with wings closed. This is the flight the male
indulges in within hearing distance of his brooding mate. During the
love season he occasionally gives way to an ecstatic flight. This is a
level flight, performed on round, open wings, which beat the air
vertically. This flight of ecstasy during the song season is common to
many of our birds. I have seen even the song sparrow indulge in it,
rising fifty feet or more and delivering its simple song with obvious
excitement. The idiotic-looking woodcock, inspired by the grand passion,
rises upon whistling wings in the early spring twilight, and floats and
circles at an altitude of a hundred feet or more, and in rapid
smackering and chippering notes unburdens his soul. The song of ecstasy
with our meadowlark is delivered in a level flight and is sharp and
hurried, both flight and song differing radically from its everyday
performance. One thinks of the bobolink as singing almost habitually on
the wing. He is the most rollicking and song-drunk of all our singing
birds. His season is brief but hilarious. In his level flight he seems
to use only the tips of his wings, and we see them always below the
level of his back. Our common birds that have no flight-song, so far as
I have observed, are the bluebird, the robin, the phœbe, the social
sparrow, the tanager, the grosbeak, the pewee, the wood warblers, and
most of the ground warblers.
Over thirty years ago a writer on flying-machines had this to say about
the flight of sea-gulls: "Sweeping around in circles, occasionally
elevating themselves by a few flaps of the wings, they glide down and up
the aerial inclines without apparently any effort whatever. But a close
observation will show that at every turn the angle of inclination of the
wings is changed to meet the new conditions. There is continual movement
with power--by the bird it is done instinctively, by our machine only
through mechanism obeying a mind not nearly so well instructed."
The albatross will follow a ship at sea, sailing round and round, in a
brisk breeze, on unbending wing, only now and then righting itself with
a single flap of its great pinions. It literally rides upon the storm.