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St. Nicholas Magazine for Boys and Girls, September 1878
How Birds Fly
by Prof. W. K. Brooks
In our last talk about birds (in St. Nicholas for July), I told you about birds
and their nests. Now I wish to say, first, a few words about the different kinds
of birds, and then we will see how birds manage to fly. Naturalists have divided
the class, birds, into several smaller groups which are called orders. One of
these includes the birds of prey, such as the hawks, eagles, and owls. In the
picture of a bird of prey you can see the strong, hooked bill and powerful
claws, which are well fitted for seizing and tearing its prey.
The second order includes the climbing birds, such as the woodpeckers. The birds
of this order can readily be recognized, since two of the toes of each foot
point backward, to give support in climbing.
The next order, that of the perching birds, includes all our common song-birds,
such as the robin, bluebird, and blackbird, as well as a few larger birds, like
the crow.
The scratching birds form another order, including our domestic fowls and many
wild game-birds.
The next order comprises the ostrich and a few other large birds, which have
such small wings that they are unable to fly, but with very large and powerful
legs, so that they are excellent runners. Although this order includes the
largest bird at present living, there were formerly running birds very much
larger than any which now exist; for, in Madagascar and New Zealand, the bones,
and even the eggs, of gigantic birds have been found. One of these eggs was over
a foot in length, and contained more than ten quarts or as much as six ostrich
eggs or one hundred and fifty hen's eggs. A nearly complete skeleton of one of
these birds has been found, and this must have belonged to a bird fifteen feet
high, or taller than the largest elephant!
The next order includes the wading birds such as the snipe, plover, woodcock,
heron, and rail.
Another order is that of the gulls, ducks, geese, pelicans, penguins, and other
swimming birds.
Besides these living birds, fossil birds have been found in the rocks. Some of
these are very different from any species now living, and very much like
reptiles, so that it is not easy to decide whether they are to be called birds
or reptiles.
The chief peculiarity of birds is their power of flight, and, although there are
a few birds which do not fly, most of them do, and the various organs of their
bodies are all constructed in such a way as to fit them for a life in the air.
Their bodies are very solid and compact, in order that most of their weight
shall be near the place where the wings are attached. The feet, legs, head, and
neck are light, and so arranged that they may be drawn up close to the body
while the bird is flying. As the neck is long and very flexible, the body does
not need to be pliant, as with most creatures having backbones; but it is
important that the wings should have a firm support, so the bones of the back
are united. The body of a bird must also be well protected from the cold; for,
as it ascends and descends through the air, it passes through regions of very
different temperatures, and it must be provided with a thick and warm covering
in order to be able to endure these sudden changes, and one also which shall be
very light and able to shed the water; for, otherwise, a bird would be unable to
fly. The feathers of a bird answer to all these needs, and are so placed upon
the body that they form a smooth surface which does not catch against the air
when the bird is passing through it. In its rapid ascents and descents, the bird
is exposed to another danger even greater than the sudden changes of
temperature. You all know that air presses in every direction with great force,
and that we do not feel it because there is air in all parts of our bodies as
well as outside them, and the pressure of the air inside exactly balances that
of the outside air. If we should suddenly take away the outside air in any way,
such as covering a person up with an air-pump receiver, and quickly and
completely exhausting the air, the consequences of the inside pressure would be
very terrible, and if the experiment could be tried quickly enough the body
would burst like an exploding gun, with a loud noise.
The Eagle (Bird of Prey)
When people go up rapidly in a balloon or climb very high mountains, they are
troubled by a ringing noise and a feeling of great pressure in the ears and
head, and by palpitation of the heart, bleeding at the nose, and fainting. These
unpleasant and often dangerous symptoms are caused by the expansion of the air
inside their bodies. In ascending very high mountains it is necessary to go very
slowly and to stop very often, to give time for some of the expanded air to
escape, and equalize the pressure again. Now, many birds, the condor, for
example, fly over the tops of the highest mountains, and nearly all birds,
either occasionally or habitually, ascend to very great altitudes, and, unless
there were some plan for regulating the pressure of the air inside their bodies,
they would suffer great inconvenience and even pain and danger. But they are
provided with an arrangement by which the air within them can escape easily as
it expands and thus keep the pressure within just equal to that outside, so that
they can ascend and descend as rapidly as they wish, without feeling the least
inconvenience. In the body of the bird there are several large bags, like the
lungs, called air-chambers; many of their bones are hollow, and others are
pierced with long winding tubes called air-tubes. All these air-chambers and
air-tubes are connected with the lungs so that air can pass into and out of them
at each breath. The connection between these chambers and the lungs is so
complete that a wounded hawk can breathe through a broken wing almost as well as
through its mouth. When a bird mounts upward, the air inside its body gradually
expands, but the bird does not feel any inconvenience; for, at each breath, part
of the air passes from the air-chambers into the lungs, so that the pressure on
the inside does not become greater than that on the outside.
Penguins (Swimmers and Divers)
I could easily fill the whole of this chapter with an account of the different
ways in which the body of a bird is fitted for life in the air, but we have room
to examine only one of these,—the way in which the wing is adapted to its use.
Did you ever look at a bird's wing carefully, and try to find out from it the
way in which it is used? People usually suppose, either that a bird flies
because it is lighter than the air, like a balloon, or that it rows itself along
as a boat is rowed through the water. Neither of these suppositions is true. A
bird is not lighter than the air, and does not float; for when a bird is shot on
the wing it falls to the ground just as quickly as a squirrel. On the contrary,
a bird flies by its own weight, and could not fly at all if it were not heavier
than the air.
You know that when you move a large, flat surface rapidly through the air, it
meets with considerable resistance. A bird's wing is so large, and is moved so
rapidly, that the resistance of the air is enough to raise the bird a short
distance each time the wings are flapped downward; but after each down-flap
there must be an up-flap, and the air resists this just as it does the
down-flap; so, unless there were some arrangement to prevent it, the bird would
drive itself down each time it raised its wings, just as far as it had raised
itself by the down-stroke before, so that it would never get into the air at
all. To meet this difficulty, the wing is so shaped that it is concave or hollow
upon its lower surface, so that it gathers the air together and prevents it from
escaping; while the upper surface is convex or bulging, so that the air slides
off from it when the wing is moved upward. If you have ever been caught in a
sudden squall of wind with an open umbrella, you will easily understand how
great a difference in resisting power this difference in the shape of the two
sides of the wing will make. As long as you can keep the bulging side of the
umbrella pointed toward the wind, you find no difficulty in holding it; but if
the wind strikes the hollow under-side of the umbrella, it pulls so violently
that, unless you are able to turn around and face the wind, the chances are that
the umbrella will either be pulled away from you or turned inside out. But in
the latter case, the wind slides out over the edges again, so that there is no
trouble in holding on to the umbrella.
The peculiar shape of the wing is only one of the ways by which the down-stroke
is made to strike the air with more force than the up-stroke. If you will look
at a quill-feather, you will see that, on each side of the central shaft or
quill, there is a broad, thin portion, which is called the vane. The vane on one
side of the shaft is quite broad and flexible, while that on the other side is
narrow and stiff; and by looking at a wing with the feathers in their places,
you will find that they are placed so that they overlap a little, like the slats
on a window-blind. Each broad vane runs under the narrow vane of the feather
beside it, so that, when the wing is moved downward, each feather is pressed up
against the stiff narrow vane of the one beside it, and the whole wing forms a
solid sheet like a blind with the slats closed. After the down-stroke is
finished and the up-stroke begins, the pressure is taken off from the lower
surface of the wing, and begins to act on the upper surface and to press the
feathers downward instead of upward. The broad vanes now have nothing to support
them, and they bend down and allow the air to pass through the wing, which is
now like a blind with the slats open. By these two contrivances,—the shape of
the wing, and the shape and arrangement of the feathers,—the wing resists the
air on its down-stroke and raises the bird a little at each flap, but at each
up-stroke allows the air to slide off at the sides, and to pass through between
the feathers, so that nothing is lost.
Quail (Scratchers)
So much for the way in which the bird is raised into the air. Rising in the air
is not flying, for a balloon and a kite rise but do not fly. Now, how is a bird
able to move forward? This is not quite as easy to understand as the other, but
I hope to be able to make it clear to you. I must first say, however, that it is
not done by rowing with the wings, for they move up and down, not backward and
forward, and no amount of rowing up and down would drive a bird forward, any
more than rowing backward and forward would lift a boat up into the air.
You will find, if you carefully examine a bird's wing, that all the bones and
muscles are placed along the front edge, which is thus made very stiff and
strong. The quill feathers are fastened in such a way that they point backward,
so that the hind edge of the wing is not stiff like the front edge, but is
flexible and bends at the least touch. As the air is not a solid, but a gas, it
has a tendency to slide out from under the wing when this is driven downward,
and of course it will do this at the point where it can escape most easily.
Since the front edge of the wing is stiff and strong, it retains its hollow
shape, and prevents the air from sliding out in this direction, but the pressure
of the air is enough to bend up the thin, flexible ends of the feathers at the
hinder border of the wing, so the air makes its escape there, and slides out
backward and upward. The weight of the bird is all the time pulling it down
toward the earth; so, at the same time that the air slides out upward and
backward past the bent edge of the wing, the wing itself, and with it the bird,
slides forward and downward off from the confined air. You will have a much
better idea of this if you will cut out a little paper model of a bird's wing
and watch the way in which it falls through the air.
Take a sheet of stiff paper and cut it in the shape shown in the diagram above,
but considerably larger. Be very careful to have the two sides alike, so that
they shall balance each other. Now fold up the front margin of each wing, along
the dotted lines a, a, a, a, to form a stiff rim to represent the rim of bone
along the front edge of a bird's wing, and cut out a small strip of wood, about
as thick as a match and twice as long, and run this through the two slits, b, b,
to represent the body of the bird. If you hold this model about three feet from
the ground, and allow it to fall gently, you will see that, instead of falling
straight to the ground, it will slide forward, and strike the ground two or
three feet ahead of you. It is really its weight which causes it to do this, so
that the statement that a bird flies by its own weight is strictly true.
This is true, also, of insects and bats. They all have wings with stiff front
edges, and flexible hind edges which bend and allow the air to pass out, so that
flying is nothing but sliding down a hill made of air. A bird rises, then, by
flapping its wings, and it flies by falling back toward the earth and sliding
forward at the same time. At the end of each stroke of its wings it has raised
itself enough to make up for the distance it has fallen since the last stroke,
and accordingly it stays at the same height and moves forward in a seemingly
straight line. But if you watch the flight of those birds which flap their wings
slowly, such as the woodpecker, you can see them rise and fall, and will have no
trouble in seeing that their path is not really a straight line, but is made up
of curves; although most birds flap their wings so rapidly that they have no
time to fall through a space great enough to be seen. Birds also make use of the
wind to aid them in flight, and by holding their wings inclined like a kite, so
that the wind shall slide out under them, they can sail great distances without
flapping their wings at all. They are supported, as a paper kite is, by the
wind, which is continually pushing against their wings, and sliding out backward
and downward, thus lifting or holding up the bird, and at the same time driving
it forward.
A Skillful Flyer
The birds are not compelled to face the wind while they are sailing, but by
changing the position of the wings a little they can go in whatever direction
they wish, much as a boy changes his direction in skating by leaning a little to
one side or the other. Some birds are very skillful at this kind of sailing, and
can even remain stationary in the air for some minutes when there is a strong
wind; and they do this without flapping their wings at all. It is a difficult
thing to do, and no birds except the most skillful flyers can manage it. Some
hawks can do it, and gulls and terns may often be seen practicing it when a gale
of wind is blowing, and they seem to take great delight in their power of
flight.
Of all birds the albatross is the most skillful in the art of sailing in the
air. It is a large sea-bird, about the size of a swan, and has very long and
powerful wings. It lives far out upon the open ocean, hundreds of miles from
land, and spends nearly all of its life in the air, very seldom alighting upon
the water. It flies almost entirely by the aid of the wind, and sometimes does
not flap its wings for an hour at a time. Albatrosses often follow a ship clear
across the ocean, or, rather, they keep company with the ship, for as they are
able to fly one hundred miles an hour with ease, the rate at which a ship
travels is much too slow for them; so they make long journeys ahead and behind,
like a dog taking a walk with his master, returning occasionally to the ship to
pick up any food which may have been thrown overboard.
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