SOARING BIRDS.

Otago Witness, Issue 3773, 6 July 1926, Page 78

SOARING BIRDS.

By

CETPTOS.

(Fo* ths Witness.) One of the unsolved problems, probably, too, one that never will be wholly accounted for, is that connected with the most remarkable form of bird flight known as “soaring.” But few birds have mastered the technique of this method of apparently effortless movement. Perhaps the finest performer is the adjutant stork. For the first hundred feet or so after leaving the ground he rises by rapid and powerful strokes of the wings, and then, apparently without the slightest effort or the suspicion of a wing beat, he sweeps round in great spirals, gaining some ten or twenty feet with each gyration, the wings and tail all the while being fully extended. During the first part of the turn he dips slightly downward. At the end of the descent he sweeps round and faces the wind, which carries him upward. Round and round he goes, mounting ever higher and higher, until at last he attains a height of about two miles.

The vultures closely rival the adjutant stork in this art, but, whereas tlie latter goes aloft apparently for the mere delight of it, the vulture soars for his very existence. In the wide African plains dead, bodies are not to be found everywhere, and since the vulture will eat nothing that has not already died, he must therefore roam widely for his food. Possessed of powers of sight far beyond that of the average bird even, he mounts the skies for the purpose of taking observations. If no meal be in sight he turns his attention to such of his fellows as may be up on the same errand miles away. Should he see one swooping earthwards he instantly follows. This accounts for the mysterious manner in which vultures invariably gather in numbers at a feast, where, shortly before, no bird was to be seen.

Eagles, falcons, buzzards, pelicans, albatrosses, and ravens are chief among the comparatively few soarers. Just Ibow they contrive to so balance their bodies and wings that the wind should lift their bulk after a certain initial impetus has been given might be partially understood—man lias contrived certain small successes along these lines with gliders—but to attempt to explain how, for hours on end in any sort of weather, calm or rough, they can not only raise themselves, but can propel themselves in any direction, and at any height, is impossible at present. The adjutant stork soars aloft after flying up the first*.hundred feet or so, and gaining much speed. The albatross, so well known to all sea-goers, by means of his ribbon-like wings, which may measure up to eleven feet from tip to tip, moves or soars in all directions, but usually low down near the sea. Mr Froude graphically describes this: “The albatross wheels in circles round and round, and for ever round the ship —now far behind, now sweeping past in a long rapid curve, like a perfect skater on a perfect field of ice. There is no effort; watch as closely as you will, you will rarely see, or never see, a stroke of the mighty pinion. The flight is generally near the water, often close to it. You lose sight of the bird as he disappears in the hollow between the waves, and catch him again as he rises over the crest; but how he rises, and whence comes the propelling force, is, to the eye, inxeplicable. He alters the angle at which the wings are inclined. Usually they are parallel to the water and horizontal; but when he turns to ascepd, or makes a change in # his direction, the wings then point at an angle, one to the sky, Jtlie other to the water.”

Again, Darwin, in liis “Journal of a Voyage Round the World,” describes the flight of the largest of all vultures, the condor of the Andes, a heavier bird than the albatross, but with a smaller wing span of about nine feet. “When tlie condors are wheeling in a flock round and round any spot their flight is beautiful. Except when rising from the ground', I do not recollect ever having sqen one of these birds flap its wings. Near Lima I watched several for nearly half an hour, without once taking off my eyes; they moved in large circles, descending and ascending without giving a single flap. As they glided close over my head I intently watched, from an oblique position, the outlines of the great terminal feathers of each wing; and these separate feathers, if there had been the least vibratory movement, would have appeared as if blended together; but they were seen distinctly against the blue sky. The head and neck were moved frequently, and apparently with force, and the extended wings seemed to form the fulcrum on which tho movements of the neck, body, and the tail acted. If the

bird wished to descend, the wings for a moment collapsed; and then again expanded with an altered inclination. The momentum gained by the rapid descent seemed to urge the bird upward with the even and stately movement of a paper kite. In the case of any bird soaring, its- motion must be sufficiently rapid so that the action of the inclined surface of its body may counter-balance its gravity. The force to keep. up the momentum of a body moving in a horizontal plane in tlie 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 apparent exertion, wheeling and gliding over mountain and river.” It is difficult to imagine, however, that, as Darwin suggests, the movements of the head and body of the condor alone supply the propelling force. The same or similar movements, if these were the motive, would be observed in the actions of other soaring birds, blit no one has commented' upon them so far as I can find out. When one thinks of the tremendous power of wing stroke needed to propel the average bird mere movements of head and body do not appear sufficient. Nevertheless, in place of a better theory, this must stand as the only apparent movement observed in soaring birds, which could supply any propelling force.

One sometimes hears the skylark described as “soaring” upwards when performing that wonderful musical ride which lias made him so famous. If one watches closely as he ascends spirally, one cannot help but notice that his wings are never still, but move rythmically, and appear almost to be beating time to his music as he goes. In true soaring the wings are scarcely ever moved, except when altering in angle to the surface of the earth or sea. The upward progress of a soaring bird must be, of necessity, comparatively slow. However, once sufficient height is obtained, and the bird wishes to descend, the velocity of the plunge is often terrific. I have described previously how the black sliags of Lake Ellesnftere have been observed literally to fall about two mile* to their nests. The frigate birds of tropical seas, and the gannet of northern waters are famous for tlieir wonderful dives from great heights. The gannet, from a considerable height, will suddenly shoot straight downward with the speed of lightning, on a shoal of fish, and with half-spread wings disappear with a terrific plunge beneath the ~ surface, to emerge an instant later with his prey. Fishermen occasionally float a herring on a board in the cruel hope that a gannet will dive for it. The force of the impact results in the instant death of the bird. Two birds of our own country, which also dive froui a height, are the kingfisher and the tern.