Forty years before the invention of the movie camera there lived in France a zoologist known as Marey. Familiar with many mysteries of animal behavior he puzzled long over the problem of determining the track that is made by the wing of a bird as it flies through the air. After several unsuccessful experiments he finally struck upon the ingenious plan of focussing a camera, with shutter open, on a black velvet background; then releasing a crow, with wing tips painted white to fly across the field. When his picture was developed he found a graceful, curving streak of white, -- the first record of the trajectory of a bird in flight! Work with the modern slow-motion camera has shed further light on the subject that has aroused man's curiosity since the time of the ancient Egyptians, watching the swallows dip and wheel above the river Nile.
It has been found, for example, that the most primitive type of wing movement, that of gliding or soaring flight is based on a simple physical phenomenon known as the convecting current. Soaring flight (see illustration above) in mountainous country is the common practice of such birds as the eagle, hawk and raven. Seeming to defy all laws of gravity these dwellers of the clouds poise motionless with outspread wings above some deep ravine or talus slope. The human observer may be at a loss to explean the source of the uncanny power that supports them in their lofty habitat, but a little reflection will disclose the only possible reason, namely, a rising current of air. On brilliant days a breeze springs up with the morning sun, a wind that moves skyward from the bright reflecting surface of snowfield, lake and rocky slope, but down against the cool green surface of forest and alpine meadow. In the rising tide of air a bird has only to tilt his wings to catch the proper puff and then soar till sundown. Or, if the wind is driving strong from the west against a slanting ridge, the bird may glide above in a current of air deflected upward by the rising slope.
For our smaller birds, the tree and ground dwelling kinds, soaring flight is not as sure as flapping flight that twists and turns, stops in a moment and starts as suddenly. Here again the bird makes use of sound principles of physics, the inclined plane and screw propellor. In flying the wing tip describes a circle, or in cases where it is long and flexible, almost a graceful figure eight; both the downward and upward strokes propelling the bird forward. It is only on the downward stroke, however, that the body is lifted; at the completion of this stroke it starts to fall until again the wings beat down. Thus the body as well as the wing describes an up-and-down, undulating path.
To better understand how a bird can stay aloft for hours, possibly covering a hundred miles without rest, we must examine the body structure, interpreting each bone and muscle in its relation to the power of flight. The skull, for example, is paper thin to reduce the body weight; the larger bones of the legs and wings are hollow, air-filled tubes. Air sacks lie between the windpipe and lungs, with purpose to increase the buoyancy as well as to cool the body of the bird, hot blooded, winging through the air at perhaps ninety miles an hour. The breast or keel stands out sharply, bracing the powerful muscles that operate the wings; while on the back the lower vertebrae unite to further anchor the muscles. (In the ostrich, a bird content to remain on the ground, we find no keel; in the penguin this structure is used for swimming only.) Birds may have a development of breast muscle amounting to one fifth their total weight. Imagine a man with thirty pounds of muscle on his chest! Also in birds we find the only example of a rope and pulley in the animal world, a supple tendon running from a muscle lying below the shoulder and yet acting to raise the wing because it passes through a hole in the bone and downward again to an attachment knob on the upper part of the wing.
Wings are not unique in the bird group, being well developed on the bat - a mammal. Feathers though are imitated nowhere else; graceful, light, rigid, waterproof, warm - in short perfectly adapted to their functions. We distinguish three kinds: first, down feathers of temporary use (but persisting throughout life on the breasts of waterfowl); second, pinfeathers with smooth barbs and short quills, and third, flight or contour feathers covering most of the body.
These adaptations of body for travel through the air are shared by all birds, but among individual types there are differences suiting each to his particular environment. Birds like the swallow and gull that live in the air most of the time have long, pointed wings; whereas terrestrial birds, such as the sparrow and grouse, possess short, rounded wings that enable them to fly rapidly for short distances. Many species of birds that spend their lives mostly in water use their wings for swimming only.
It is little wonder that man has had cause to envy the marvelous perfection of bird flight, after we have seen how nicely Nature has planned the structure of wing and body for travel thought the air. Man, it is thought by some anthropologists, has arrived at his present dominant place in the animal kingdom through the development of increasing skill with his hands; yet who has not expressed at some time or another in his life a willingness to give up his earthly cunning for the ability to soar overhead in the clouds with the birds!
Victor Scheffer, Ranger-
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