They are known as Apteria, or naked tracts, and Pterylae, or feather tracts. With but one or two exceptions all birds have large, bare tracts or spaces, from which feathers do not grow these tracts in many cases, being nearly equal in the aggregate area to the feathered tracts (Plate xxvx). The body of a bird is not covered thickly with feathers growing evenly from the skin.
The action of soaring is practically that of sliding down the air until by closing the planes it once more rises, without effort, the air rushing under the convex wing.
Thus the muscular power of the downward stroke being so much greater almost all the muscles of flight upon a bird will be found upon the under surface of the body.īy means of the broad ligament at the base of the quills, the flight feathers can be turned at an angle in a manner similar to a Venetian blind. The upward stroke, however, does not nullify this by bringing the bird down again, because the broad section of the feather is now unsupported and allows a certain quantity of the air to pass through the wing. During a downward stroke of the wing the broad section will be pressed tightly against the narrow section of the feather above it, making one complete surface, and the passage of the air along this enables the bird to rise. This fact is commonly overlooked by the student and painter of birds, with the result that a considerable portion of the beauty of the bird is unappreciated, namely the graceful lines of the wing (Plate xxiv).Īgain, it will be noticed that the quill of the principal flight feathers is not placed exactly in the centre, and in overlapping the broader section will always be found to be underneath (Plate xxv). If the under or concave surface is exposed to the wind, the resistance is enormous as compared to the bulging or convex side, and, in a similar manner, the effort to lower the wing of a bird is so much greater as compared to the upward stroke when one realises that the wing of a bird is convex upon the upper surface and concave below. The enormous difference this makes may be gathered if one considers the effect upon an umbrella by the wind. But each upward stroke would be in danger of neutralising the effect of the downward stroke were it not for two things-the overlap of the feathers and the convex and concave surfaces of the wings.
It is obvious that if a bird is to support itself by the downward blow of the wings upon the air, it must at the end of each downward stroke lift the wings upwards again, so as to prepare for the next downward movement.
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