The Flight of Birds
IN the Atlantic Monthly for June a contributor from Montana discusses what he calls ’The Mystery of the Soaring Hawk.’ It is probable that hawks do not soar otherwise in Montana than they do elsewhere; and Mr. Clough’s attempt to throw a mystery about the matter seems to me the result of hasty observation. He says that the usual explanations of the feat, such as, ‘He flies like a kite,’ ‘He rises on ascending currents of air,’ are childish and unscientific. His own explanation is given with such an air of assurance and of scientific demonstration that most readers, I fancy, will accept his theory without question. One of our great metropolitan dailies called attention to the subject editorially, and wished for such a man as Roosevelt to add his confirmation. But I am sure that Roosevelt would at once have seen through the false reasoning.
All mechanical principles are the same in the living and in the non-living machine; only in the living machine they are inherent and a vital part of it, while in the non-living the motor-power is superadded from without. The living engine stokes itself. The living phonograph winds itself up. The soaring hawk is simply a living kite that is selfregulated. It adjusts itself to the currents of air, but can no more soar on a still day than can a kite. When the wind fails, the boy raises the wind by running; but he must run as fast as the wind blows that raises his kite. So, if the breeze fails, the hawk cannot soar. He can flap his wings and keep afloat.
These facts, which seem so simple and obvious, make no impression on Mr. Clough. He is bound that his soaring hawk shall be wrapped in mystery. The same mail that brought me the June Atlantic brought me a letter from a man in Kentucky asking me to tell him ‘how the buzzard propels itself, often at a considerable distance, and sometimes in the face of a stiff wind, without flapping its wings.’ In pursuit of this knowledge, he had applied to the information bureau of a leading Southern newspaper. The reply was that the buzzard adjusts itself to the air-currents, or shifts its wings so as to take advantage of them. But this reply did not satisfy him. He forgets that the buzzard carries a propelling power within its own body, as strictly as does an airplane. Without this it would drift with the wind. In a perfectly motionless air the buzzard would have to beat its wings to keep afloat. The flying-machine is governed by the same laws of physics that govern the hawk and the buzzard.
If an airplane capable of making one hundred miles an hour starts in the face of a wind blowing one hundred miles an hour, would it not, theoretically, be lifted and remain stationary in the air? The two forces would neutralize each other. If you are rowing up-stream at the rate of five miles an hour, and the stream is flowing down at the same speed, you are making no progress; but if you look to the shore, it appears to be moving down at the same speed.
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. Notice Darwin’s phrase, ‘altered inclination’ of the wings, and his allusion to the kite.
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 flier as the Monarch beats its way back to shore.
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 two grand classes: those which use their wings simply to transport themselves from one place to another, — the same use we put our legs to, — and 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. All our common birds are examples of the first class. Our hawks and buzzards are examples of the second 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. ‘ You think that a bird beats its wings downward through the air when it flies,’ says Mr. Clough. Nearly all birds of the first class do. Members of the grouse family do so in a remarkable degree; but not so the soaring birds: they smite the air with long, flexible, extended wings. He says further, ‘The fall of the body is neutralized by the lift of the wing-beat.’ Sea birds beat the air slowly with a large wing-surface.
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.
‘A heavy-bodied domestic fowl does sometimes drive its wings down in a vain attempt to fly,’ says Mr. Clough. Does he mean to say that the barnyard fowl cannot fly? The common hen and the turkey are both strong fliers, but not soarers, and not long-distance fliers. Their short rapid flights often end in a graceful sail.
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 flier, 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 meadow lark 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 phœ, the social sparrow, the tanager, the grosbeak, the pewee, the wood warblers, and most of the ground warblers.
Mr. Clough’s explanation of his mystery seems to me forced and fanciful in the extreme. He says the hawk never sails against the wind, let him trim his pinions how he will, which is hardly consistent with a paragraph in which he calls upon us to ‘observe how sea-gulls fly with motionless wings right in the wind’s eye.’ Of course, this is not sailing as the yacht sails, but it is sailing as the hawk sails. ‘There is power here and independence to defy the wind.’ Not to defy it, but to use it. If to soar is not to fly or sail, then there is here a confusion of terms. He says that hawks prefer a still, windless day for soaring, which is a statement no careful observer will confirm.
How then does the hawk soar according to Mr. Clough? He has double engines in his breast-muscles, which he uses alternately, first on one side, then on the other; first the right limb, then the left, as we use our legs in walking. The idea of two wings working simultaneously as they do in flapping does not fit into his theory. ‘In alternate wingbeats, then, we find the solution of our problem. The hawk does not cease to exert himself, he simply changes his gait. He prefers the smooth motion of a pacer to the jolting trot of the saddle horse.’ Surelyaone-sided affair throughout. The whole truth is not in it.
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 early efforts at flying were made by imitating birds. Bird-like wings were attached to the human body and operated by the muscular efforts of the would-be flier. But all such experimenters quickly came to grief.
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.
It is a curious fact that the expanse of wing among flying creatures generally diminishes in proportion to the increase of weight. Thus it is said that the wing-surface of the gnat is at the rate of nearly fifty square feet to one pound of weight. With the dragon-fly it is less than half of that. With the sparrow it is about three feet to the pound, with the pigeon it is a little over one foot, and with the vulture it is at the rate of about four fifths of a foot to the pound. In the case of the birds, the reason of this is obvious. The smaller birds need speed, but the soaring hawks and vultures do not. The kingbird will easily overtake the crow, the hawk, and the eagle. The soaring hawks are birds of leisure, and avail themselves of the lifting power of the air-currents. Their breast-muscles are less developed than those of the game birds and common birds. The pigeon has great speed, but no soaring power.
The reason that the gnat has such a great expanse of wing-surface is probably because it has little muscular power; the same test applied to the honey bee would probably tell a different story. Yet the insects form a world by themselves; the vertebrate kingdom is governed by other laws. The lifting power of the wind on a plane surface depends upon the angle of inclination. This angle the soaring birds can change to meet the conditions. A wind blowing at the rate of ten miles an hour has a lifting power of about one half pound to the square foot. Hence, upon a soaring hawk it would be considerable.