Our Bird Comrades

by Leander S. Keyser

Another mechanical contrivance deserves attention. An extremely elastic cord reaches over from the shoulder to the wrist joint, supporting a fold of skin that occupies the deep angle of the elbow, and that is covered with short, fluffy feathers. When the bird is flying, this cord is stretched and forms the front edge of that section of the wing. But, now, suppose the wing is closed, will not this cord make a cumbersome fold, flapping loosely in the angle of the elbow? Such would, indeed, be the case, did not its extreme elasticity enable it to contract to the proper length, so as to keep the wing's border straight and smooth.

Without the feathers the wing would be useless as an instrument of flight. The shorter plumes that shield the bases of the long quill feathers are called the coverts, which are found on both the upper and under surfaces of the wing. They are divided into several sets, according to the position they occupy, and are called the "primary coverts" (because they overlie the bases of the primaries), the "greater coverts," the "middle coverts," and the "lesser coverts." Forming a vast expansion of the bony and fleshy framework are the quills, or flight-feathers, called collectively the "remiges." These plumes mainly determine the contour of the wing, and constitute a thin, elastic surface for striking the air—one that is sufficiently resilient to give the proper rebound and yet firm enough to support the bird's weight. The longest quills are those that grow on the hand or outer extremity of the wing and are known as the primaries. What are called the secondaries are attached to the ulna of the forearm, while the tertiaries occupy the humerus and are next to the body. All these feathers are so placed relatively that the stiff outer vane of each quill overlaps the more flexible inner vane of its successor, like the leaves of certain kinds of fans, thus presenting an unbroken surface to the air. As to the structure of these plumes, they combine firmness, lightness, and mobility, the barbs and barbules knitting the more flexible parts together, so that they do not separate, but only expand, when the wing is unfolded.

While the primary purpose of wings is flight, there is quite a number of notable exceptions. A concrete example is the ostrich, whose wings are too feeble to lift it from the ground, but evidently aid the great fowl in running, as it holds them outspread while it skims over the plain, perhaps using them mainly as outriggers or balancing poles in its swift passage on its stilt-like legs. The penguins convert their wings into fins while swimming through the water, the feathers closely resembling scales.

There are birds of many kinds, and therefore a great variety of wings and modes of flight. Birds with short, broad, rounded wings, with the under surface slightly concave and the upper surface correspondingly convex, usually have comparatively heavy bodies, and race through the air with rapid wing-beats and rather labored flight, and compass only short distances. Among the birds of this kind of aërial movement may be mentioned the American meadowlark, the bob-white, and the pheasant. Other species propel themselves in rapid, gliding, and continued flight by means of long, narrow, and pointed wings, like the swifts, swallows, and goatsuckers, while many others, notably herons, hawks, vultures, and eagles, are distinguished by a vast alar expansion in proportion to their weight, and hence are able to sustain themselves in the air by sailing, with only a slight stroke at rare intervals. Such birds as the stormy petrel and the frigate-bird have wings that are broad, convex, and of great length in contrast with the lightness and small bulk of their bodies, for which reason they are able to sustain themselves in the air for days without rest. It is even thought that some of these wonderful birds of the limitless ocean sleep on the wing, though how such an hypothesis could be proved it would be difficult to say.

Even in this day of scientific research and astuteness, it must not be supposed that everything about the mechanics of avicular flight is understood. We may readily comprehend how a bird, without fluttering its wings, can poise in the air; but how can it move forward or in a circle, and even mount upward, without a visible movement of a pinion? And this some birds are able to do without reference to the direction of the ethereal currents. That, I venture to say, is still a mystery. It almost seems as if some of the masters of aërial navigation in the bird world were gifted with the ability to propel themselves forward by a mere act of volition.

An interesting article on the subject of bird flight appeared not long ago in one of the foremost periodicals of the country, a part of which is here quoted to show what a puzzling problem we have before us:

Recent developments in aërial navigation have renewed interest in the comparative study of the mechanical principles involved in the flying of birds. There is one exceedingly puzzling law in regard to birds and all flying creatures, the solution of which may work far-reaching influences in the construction of flying craft.

"This law, which has thus far perplexed scientists, is that the heavier and bigger the bird or insect, the less relative wing area is required for its support. Thus the area of wing surface of a gnat is forty-nine units of area to every one of weight. In graphic contrast to that, a condor (Sarcorhamphus gryphus) which weighed 16.52 pounds had a wing surface of 9.80 square feet. In other words, though the gnat needs wing surface in a ratio of forty-nine square feet per pound of weight, a great condor manages to sail along majestically with .59 of a square foot to at least a pound of weight. The unexplained phenomenon persists consistently throughout the whole domain of entomology and ornithology. Going up the scale from the gnat, it is found that with the dragon fly this ratio is 30 to 1, with the tipula, or daddy-longlegs, 14.5 to 1, the cockchafer only 5.15 to 1, the rhinoceros beetle 3.14 to 1.