Animal Locomotion; or, walking, swimming, and flying / With a dissertation on aëronautics

by James Bell Pettigrew

Fig. 105.

Captain Hutton makes the following remarkable statement regarding the albatross:—“I have sometimes watched narrowly one of these birds sailing and wheeling about in all directions for more than an hour, without seeing the slightest movement of the wings, and have never witnessed anything to equal the ease and grace of this bird as he sweeps past, often within a few yards, every part of his body perfectly motionless except the head and eye, which turn slowly and seem to take notice of everything.”[98]

“Tranquil its spirit seem’d and floated slow;
Even in its very motion there was rest.”[99]

As an antithesis to the apparently lifeless wings of the albatross, the ceaseless activity of those of the humming-bird may be adduced. In those delicate and exquisitely beautiful birds, the wings, according to Mr. Gould, move so rapidly when the bird is poised before an object, that it is impossible for the eye to follow each stroke, and a hazy circle of indistinctness on each side of the bird is all that is perceptible. When the humming-bird flies in a horizontal direction, it occasionally proceeds with such velocity as altogether to elude observation.

The regular and irregular in Flight.—The coot, diver, duck, and goose fly with great regularity in nearly a straight line, and with immense speed; they rarely if ever skim or glide, their wings being too small for this purpose. The woodpecker, magpie, fieldfare and sparrow, supply examples of what may be termed the “irregular” in flight. These, as is well known, fly in curves of greater or less magnitude, by giving a few vigorous strokes and then desisting, the effect of which is to project them along a series of parabolic curves. The snipe and woodcock are irregular in another respect, their flight being sudden, jerky, and from side to side.

Mode of ascending, descending, turning, etc.—All birds which do not, like the swallow and humming-birds, drop from a height, raise themselves at first by a vigorous leap, in which they incline their bodies in an upward direction, the height thus attained enabling them to extend and depress their wings without injury to the feathers. By a few sweeping strokes delivered downwards and forwards, in which the wings are made nearly to meet above and below the body, they lever themselves upwards and forwards, and in a surprisingly short time acquire that degree of momentum which greatly assists them in their future career. In rising from the ground, as may readily be seen in the crow, pigeon, and kingfisher (fig. [102], p. 183), the tail is expanded and the neck stretched out, so that the body is converted into an inclined plane, and acts mechanically as a kite. The centre of gravity and the position of the body are changed at the will of the bird by movements in the neck, feet, and tail, and by increasing or decreasing the angles which the under surface of the wings makes with the horizon. When a bird wishes to fly in a horizontal direction, it causes the under surface of its wings to make a slight forward angle with the horizon. When it wishes to ascend, the angle is increased. When it wishes to descend, it causes the under surface of the wings to make a slight backward angle with the horizon. When a bird flies up, its wings strike downwards and forwards. When it flies down, its wings strike downwards and backwards. When a sufficient altitude has been attained, the length of the downward stroke is generally curtailed, the mere extension and flexion of the wing, assisted by the weight of the body, in such instances sufficing. This is especially the case if the bird is advancing against a slight breeze, the effort required under such circumstances being nominal in amount. That little power is expended is proved by the endless gyrations of rooks and other birds; these being continued for hours together. In birds which glide or skim, it has appeared to me that the wing is recovered much more quickly, and the down stroke delivered more slowly, than in ordinary flight—in fact, that the rapidity with which the wing acts in an upward and downward direction is, in some instances, reversed; and this is what we should naturally expect when we recollect that in gliding, the wings require to be, for the most part, in the expanded condition. If this observation be correct, it follows that birds have the power of modifying the duration of the up and down strokes at pleasure. Although the wing of the bird usually strikes the air at an angle which varies from 15° to 30°, the angle may be increased to such an extent as to subvert the position of the bird. The tumbler pigeon, e.g. can, by slewing its wings forwards and suddenly throwing back its head, turn a somersault. When birds are fairly on the wing they have the air, unless when that is greatly agitated by a storm, completely under control. This arises from their greater specific gravity, and because they are possessed of independent motion. If they want to turn, they have simply to tilt their bodies laterally, as a railway carriage would be tilted in taking a curve,[100] or to increase the number of beats given by the one wing as compared with the other; or to keep the one wing extended while the other is partially flexed. The neck, feet, and tail may or may not contribute to this result. If the bird wishes to rise, it tilts its entire body (the neck and tail participating) in an upward direction (fig. [59], p. 126; fig. [102], p. 183); or it rises principally by the action of the wings and by muscular efforts, as happens in the lark. The bird can in this manner likewise retain its position in the air, as may be observed in the hawk when hovering above its prey. If the bird desires to descend, it may reverse the direction of the inclined plane formed by the body and wings, and plunge head foremost with extended pinions (fig. 106); or it may flex the wings, and so accelerate its pace; or it may raise its wings and drop parachute-fashion (fig. [55], p. 112; g, g of fig. [82], p. 158); or it may even fly in a downward direction—a few sudden strokes, a more or less abrupt curve, and a certain degree of horizontal movement being in either case necessary to break the fall previous to alighting (fig. 107, below). Birds which fish on the wing, as the osprey and gannet, precipitate themselves from incredible heights, and drop into the water with the velocity of a meteorite—the momentum which they acquire during their descent materially aiding them in their subaqueous flight. They emerge from the water and are again upon the wing before the eddies occasioned by their precipitous descent have well subsided, in some cases rising apparently without effort, and in others running along and beating the surface of the water for a brief period with their pinions and feet.

Fig. 106.—The Pigeon (Treron bicincta, Jerdon), flying downwards and turning prior to alighting. The pigeon expands its tail both in ascending and descending.—Original.