Bird In The Air
( Originally Published Ealry 1900's )
Mrs. Eckstorm has written much on birds, the Maine Woods, the Game Laws and some stories under the name of F. Pearson Hardy. She is the author of " The Woodpeckers" and "The Penobscot Man."
I N order to understand how the bird's wing can resist the pressure of the air, we must examine the wing-quill of some large bird. Our Christmas or Thanks-giving turkey may furnish us with a stout wing-feather, or we may pick some up in the parks in summer when the ducks and geese are moulting, or we, may, if nothing bet-ter can be obtained, pull a feather from the turkey-tail duster, remembering always that we have a tail-feather, not a wing feather. But having procured a broad-webbed feather, study it carefully. Rub your finger along the webs to test its elasticity. Notice the effect of pressing it in different directions and observe how it stretches under pressure like a piece of jersey cloth, breaking apart only under rough usage or great strain, and readily being coaxed back into place again.
What makes the web of the feather so elastic? The question is not easy to answer clearly, for a feather is complicated and its parts are minute. With the unaided eye we see too little and with the microscope we see too much. We shall understand best by taking for the first a feather whose parts can be readily made out without a microscope. An ostrich plume, for example, is made up of a multitude of little plumes, called barbs, attached to a quill, or shaft; and each of these barbs is itself a miniature plume with its own shaft and barbs, to which is given the name of barbules. Few feathers show the barbules as plainly as these plumes of the ostrich, but the ostrich's barbules are not connected, so the plumes would be wholly useless for flight even if the wings were large enough to lift the bird.
In the long feathers of the peacock's tail-coverts we see a feather that is fringed with scattered disconnected barbs near the base, but is tipped with interlocking barbs. We notice too that the barbs are set upon the shaft at an angle, so that where they come close together they over-lap like clapboards on a house ; and the barbules, being hooked at the end, catch hold of the barb next in front of them, and hold to it. Thus at the tip of the peacock's feather there is the beginning of a true web. The barbules, we notice, are all upon the upper side of the barb, or upon the upper edge of the barb-shaft, if we observe more closely; for the barb-shafts have been greatly flattened, and they lie side by side like the thin leaves we see beneath a toadstool on turning it over. This is an arrangement to give stiffness without increasing the weight, and it greatly strengthens the feather to bear the upward pressure of the air.
In the hawk and eagle this arrangement is even more remarkable, though we cannot see it so easily. And in these strong-flying birds the barbules interlock much more firmly, so that the feather is impervious to air, and is stiff enough to resist the pressure of the wind.
Without a microscope we cannot see the little barbicels. split up like shavings partly cut from a stick, and like them hooked at the ends, which reach out from the barbule to barbule binding the feathers together still more closely. Some of the other arrangements are too minute to be seen by the naked eye and not easily understood from description, but in every part we find the feather wonderfully planned to resist pressure of the air without the slightest unnecessary weight.
These little barbules have to hold tight to each other; for if they lost their grip the wind would blow up through the gap, and much of the effectiveness of the feather would be lost. If a bird is to fly well it must have firmly webbed feathers, and all flying birds have them. If the ostrich had wings as large as thunderclouds, he could not fly unless his airy plumes were replaced by good quills fit to beat down the air under them.
We observe that all the long quills overlap each other like the shingles of a roof, and that the unevenly webbed primaries lie with their narrow edge uppermost, and their wide web caught under the quill next nearer to the body. This greatly aids in making the wing air-tight; for, on the downward stroke, the wide web is pressed so firmly against the strong quill and stiff outer web of the next feather that the air cannot pass through.
But on the up stroke there is nothing to hold the weak web, which is borne down by the air, and thus the pressure on the wing is relieved. While this would happen anyway, it is such a help to the bird in flying that a special apparatus is provided along the back of the forearm for turning the secondaries on edge to let the air pass through on the upward stroke. By these arrangements the bird is able to press down a large quantity of air with every wing-beat, but is not required to lift an almost equally large amount when the wing rises. The inability to do this is what makes the bat so much less swift and capable upon the wing, although in comparison with the weight of his body his wing area is very much greater than the bird's.
As long ago as King Solomon, who was the first naturalist, " the way of the bird in the air " was one of the stock mysteries for men to wonder over. How does a bird fly? It is only recently that the secret has been discovered.
In order to fly a bird must have wings large enough to support his weight, and muscles strong enough to move his wings ; there seems to be nothing else required beyond a proper adjustment of power and supporting surface. We do not at first observe any such wonderful adaptations in the wings as we saw in the loon's foot to fit it for a life in the water—merely more or less wing, longer or shorter, pointed or rounded. But the wonderful thing is that the bird can fly at all.
Here we have the problem in its simplest form; how is an eagle, weighing ten pounds, to raise himself in the air by flapping two broad fans that spread from tip to tip some seven feet? Some say that his hollow bones and the air-sacs in his body help to lift him,— as if a bird were a balloon. But a balloon, if filled with air, would rise no more than a grocery bag blown full and tied; a balloon is always filled with gas lighter than air. An eagle can never, by any kind of puffing himself up with air, diminish that ten pounds in weight, even by a single ounce. The balloon theory finds two other obstacles —a balloon must sail before the wind, and it can travel no faster than the breeze that bears it, while the bird's speed is voluntary, and he usually prefers to fly against the wind. The bird's power to fill himself with air does not account for his flying.
Others say that the bird flies like a kite, and this is partly right. The bird's body does act very much like the string of a kite, serving as a weight to hold it steady. But the kite cannot lift the boy at the end of the string; if it could, the kite would fall just as we see it do when the string breaks.
That laughable story for boys, " Phaeton Rogers," tells us how Phaeton made his great kite draw his wagon down the road, and how the kite ran away with him while the whole town raced after to find out what the matter was. Now we know that the kite would not fly at all unless it could keep a taut string; and the faster the wagon moved the nearer it would be to outrunning the kite, so that it is hardly probable that Phaeton's wagon would travel as fast as the story says. Did you never underrun your kite and bring it down even when there was a good breeze? Now in most instances a bird outruns the breeze, and he has no stationary weight ; for his body, the weight, travels as fast as the wings. So we see that a bird does not resemble the kite.
More nearly does the bird resemble the swimmer, who supports himself in the water by striking out with his arms, pushing himself up and forward by the resistance of the water to his stroke. The bird rises and moves ahead by the forward and downward sweep of his wings, falls a little in air as he again raises them, and once more moves ahead and up with the new stroke.
Like the swimmer he advances by a series of undulations, a long incline upward when his wings press the air, a little drop downward as he raises them to get them in position once more. But the bird's stroke is different from the swimmer's. As-soon as his wings are at their highest point, they begin to turn forward and downward with a strong, even sweep that lifts the body and carries it ahead.
The air is driven backward less by the direction in which the wing is moved than by the curvature of the under surface, by its general shape, and by the rotary motion at the shoulder joint. When it is necessary to recover for the next stroke, it is wonderfully neatly done. The wing bends at the joint, leaving only half as great a resisting surface, the secondaries roll on edge, relieving still more pressure; the body drops a little by its own weight, and up flies the wing into place so quickly that the camera can get but two pictures, though it takes four of the descending stroke. Please notice carefully that the wing-beat is a forward motion ; the tip of the wing never drags far back; even when it is ready to be raised it is still on a line with the eye. The bird is always reaching ahead to cut into air not yet disturbed by his own movement.
We know that the bird rises by the resistance of the air, using his wings as levers and the air as a fulcrum. But how does he get his start? How does he guide his course? How does he stop?
Watch different birds taking flight. The old crow on the fence-rail, if there is no breeze, throws himself for-ward and drops a little, which gives him his first wing stroke with all the momentum of his falling body. When there is a wind he turns to face it, even if he intends to fly in the other direction, stretches up on his legs to his full height, and lifts his wings. The wind fills them. He leans down upon it, and his first stroke gives him headway and bears him up.
Many birds give a little leap in the air as a help in rising from the ground. From a tree it is easy for any bird to get upon the wing, but starting from a level surface the difficulties are greater and they increase with the weight of the bird, whether he be a good flyer or not. The turkey-buzzard, a majestic bird on the wing, makes a slow, ungraceful start. The eagle, even when in danger of his life, has been reported to stop to run in awkward leaps several rods because he could not at once gain momentum enough for his wings to get their leverage. The loon is habitually in a worse plight, for he can get no chance to spring from the water, and must get his momentum by running along the surface, flapping his wings. Even then his wings are too small to lift his heavy body unless there is a breeze blowing.
What is momentum?—an impulse to go ahead. A body at rest has only a tendency to stay still, its inertia, until something sets it moving. The bird starting to fly must overcome its inertia. If it can once get the going-ahead motion, all it needs to do is to hold its body in the right position and lift itself with its wings.
Holding the body in this or that position alters the direction of the bird's flight. If he wishes to rise he throws the body into a more or less vertical position, according to the angle at which he wants to ascend; if he wishes to glide down, he just lets himself fall forward. The straighter the body is held the straighter up the bird goes. The straighter it is held the more directly he descends.
If you should ever see a game bird " tower " you will notice how erect the body is. I know no flight among our American birds so nearly vertical as the towering of the ruffed grouse, but it is an exhibition not often seen unless one is with a gunner, as the birds seldom or never tower unless wounded in the head.
We have already described the forward movement of the bird in studying the stroke. Let us notice again the peculiar folding of the upraised wing and the rolling secondaries which spill the air and make the work of lifting the wing both quick and easy.
Speed in flight is attained in two ways — by the shape of the wings, and by the quickness with which they are moved. A small-winged bird may fly very fast by moving its wings with great rapidity, and a large-winged bird may be a slow flyer if it move its wings very slowly.' But if two birds move their wings the same number of times a minute, that one will fly the faster which has the longer wings, because it has the greatest leverage on the air. We shall notice too that all swift-flying birds have very strong primaries, and the stronger flyers have also very long primaries. Long wings, long primaries, strong primaries, make the work easier for the bird.
Very swift birds one may expect to find with narrow wings. The reason is that the wings are levers and their length and strength ' give them their efficiency without regard to their width. So the swifts and swallows and terns have very long, narrow wings. Birds with wings both wide and long must either be rather slow flyers or else in the habit of soaring, for which they need a large area of wing. But a very long-winged bird, even though its wings are narrow, may be able to soar if, like the albatross and man-of-war bird, its wings are long enough to furnish the required area in spite of their narrowness.
In steering the tail does most of the work, though a part of the work is, and the whole may be, done by the wings. Birds making quick evolutions are commonly long-tailed. The terns, goshawk, Cooper's and sharp-shinned hawks are good examples of this. On the other hand, the chimney-swift is rather short-tailed. Birds with short tails and longs legs usually trail the legs behind in flight, so that a boy of my acquaintance described a heron as " a big bird with only one tail feather, which was a yard long." The loon also, though his legs are not long, stretches them out behind him with the webs of the feet held close together to steer him. Finally a bird that loses his tail has to learn how to steer himself. A cat-bird that I once knew, having lost his tail by accident, was hard put to tell where he was going until he learned to steer a more certain course with his wings.
In hovering, also, the tail plays an important part. Watch the humming-bird before the flower, the king-bird over the grass, the sparrow-hawk above the hole of the meadow-mouse. You will see that the tail is held full spread and nearly at right angles to the body, unless the body itself is dropped, as it often is in the humming-birds. Thus the tail holds a large part of the air fanned back by the wings, and acts as a drag on the bird to hold him stationary or nearly so.
One of the prettiest sights I ever saw was a common tern that, attracted by my fishing, came and hovered within ten feet of me, keenly curious, his scarlet bill and feet, black cap, silvery mantle, and white body gay as a picture against the blue water of the bay. For nearly a minute he held himself as stationary as if suspended on a wire, hovering with head bent down, wings partly flexed, and his long forked tail dipped almost at right angles to the body and spread so wide that it looked nearly square across the end—a position in which the forces that naturally would have borne him ahead were balanced by others that held him back.
Stopping is accomplished by both wings and tail. A bird in swift flight, wishing to check his course immediately, spreads his tail to the fullest extent, throws up his wings, and drops as nearly vertically as his momentum will permit him. Watch pigeons and you will observe that they are experts in this method of alighting. But commonly a bird merely draws in his wings, spreads his tail more or less to cheek his motion, and comes gliding down on an easy slant.
Aside from these necessary motions the bird has a number of tricks no more a part of flying than riding on one wheel is a part of bicycling, but very pretty sport.
We sometimes see a bird glide until his momentum is gone, when, with a stroke or two, he sends himself forward and rests on his wings till the new impulse is exhausted.
Sometimes birds play with the wind, mounting by merely turning to face it, and then sliding down the breeze a short distance, when they turn once more to the wind and let it raise them. Again one bird, the tumbler pigeon, is noted for its habit of falling backward in mid-air, a habit thought by some to have its root in the method by which wild pigeons sometimes escape the onslaught of a hawk.
But the most beautiful flight trick of all is the common one of soaring. No one knows all about it, and yet it is easy to see that under most conditions the bird is playing with a breeze, letting himself be borne up as he faces it, gliding downward a little as he wheels about the circle, which will once more bring him breast to the wind. Hawks are among our best soaring land-birds; but some sea-birds excel all others in the sport, wheeling about hour after hour on motionless pinions, keeping their course and their elevation entirely by some slight adjustment of the body or by an inclination of the tail. Sailors declare that an albatross will follow a ship for days together, circling above her without rest. It is certain that on moonlight nights the man-of-war bird may be seen for hours together floating far above the sea. Nor is a soaring bird easily disturbed. I have seen a soaring goshawk, when a bullet clipped a secondary from one of its wings, answer with its wild scream of defiance, and without haste or change of motion fill out an unbroken curve of its ascending spiral.