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CHAPTER XX.
GENERAL OBSERVATIONS ON BIRDS.
"Birds, the free tenants of land, air, and ocean,
Their forms all symmetry, their motions grace;
In plumage delicate and beautiful;
Thick without burthen, close as fishes' scales,
Or loose as full-blown poppies to the breeze."
The Pelican Island.
917. THE DIVISIONS OF BIRDS are founded principally on their habits of life, and the natural resemblance which their external parts, especially their bills, bear to each other. According to Mr. Vigors, there are five orders, each of which occupies its peculiar place on the surface of the globe; so that the air, the forest, the land, the marsh, and the water, has each its appropriate kind of inhabitants. These are respectively designated as BIRDS OF PREY, PERCHERS, WALKERS, WADERS, and SWIMMERS; and, in contemplating their variety, lightness, beauty, and wonderful adaptation to the regions they severally inhabit, and the functions they are destined to perform in the grand scheme of creation, our hearts are lifted with admiration at the exhaustless ingenuity, power, and wisdom of HIM who has, in producing them, so strikingly "manifested His handiwork." Not only these, however, but all classes of animals, have their peculiar ends to fulfil; and, in order that this may be effectually performed, they are constructed in such a manner as will enable them to carry out their conditions. Thus the quadrupeds, that are formed to tread the earth in common with man, are muscular and vigorous; and, whether they have passed into the servitude of man, or are permitted to range the forest or the field, they still retain, in a high degree, the energies with which they were originally endowed. Birds, on the contrary, are generally feeble, and, therefore, timid. Accordingly, wings have been given them to enable them to fly through the air, and thus elude the force which, by nature, they are unable to resist. Notwithstanding the natural tendency of all bodies towards the centre of the earth, birds, when raised in the atmosphere, glide through it with the greatest ease, rapidity, and vigour. There, they are in their natural element, and can vary their course with the greatest promptitude—can mount or descend with the utmost facility, and can light on any spot with the most perfect exactness, and without the slightest injury to themselves.
918. THE MECHANISM WHICH ENABLES BIRDS to wing their course through the air, is both singular and instructive. Their bodies are covered with feathers, which are much lighter than coverings of hair, with which quadrupeds are usually clothed. The feathers are so placed as to overlap each other, like the slates or the tiles on the roof of a house. They are also arranged from the fore-part backwards; by which the animals are enabled the more conveniently to cut their way through the air. Their bones are tubular or hollow, and extremely light compared with those of terrestrial animals. This greatly facilitates their rising from the earth, whilst their heads, being comparatively small, their bills shaped like a wedge, their bodies slender, sharp below, and round above,—all these present a union of conditions, favourable, in the last degree, to cutting their way through the aërial element to which they are considered as more peculiarly to belong. With all these conditions, however, birds could not fly without wings. These, therefore, are the instruments by which they have the power of rapid locomotion, and are constructed in such a manner as to be capable of great expansion when struck in a downward direction. If we except, in this action, the slight hollow which takes place on the under-side, they become almost two planes. In order that the downward action may be accomplished to the necessary extent, the muscles which move the wings have been made exceedingly large; so large, indeed, that, in some instances, they have been estimated at not less than a sixth of the weight of the whole body. Therefore, when a bird is on the ground and intends to fly, it takes a leap, and immediately stretching its wings, strikes them out with great force. By this act these are brought into an oblique direction, being turned partly upwards and partly horizontally forwards. That part of the force which has the upward tendency is neutralized by the weight of the bird, whilst the horizontal force serves to carry it forward. The stroke being completed, it moves upon its wings, which, being contracted and having their edges turned upwards, obviate, in a great measure, the resistance of the air. When it is sufficiently elevated, it makes a second stroke downwards, and the impulse of the air again moves it forward. These successive strokes may be regarded as so many leaps taken in the air. When the bird desires to direct its course to the right or the left, it strikes strongly with the opposite wing, which impels it to the proper side. In the motions of the animal, too, the tail takes a prominent part, and acts like the rudder of a ship, except that, instead of sideways, it moves upwards and downwards. If the bird wishes to rise, it raises its tail; and if to fall, it depresses it; and, whilst in a horizontal position, it keeps it steady. There are few who have not observed a pigeon or a crow preserve, for some time, a horizontal flight without any apparent motion of the wings. This is accomplished by the bird having already acquired sufficient velocity, and its wings being parallel to the horizon, meeting with but small resistance from the atmosphere. If it begins to fall, it can easily steer itself upward by means of its tail, till the motion it had acquired is nearly spent, when it must be renewed by a few more strokes of the wings. On alighting, a bird expands its wings and tail fully against the air, as a ship, in tacking round, backs her sails, in order that they may meet with all the resistance possible.
919. IN THE CONSTRUCTION OF THE EYES of birds, there is a peculiarity necessary to their condition. As they pass a great portion of their lives among thickets and hedges, they are provided for the defence of their eyes from external injuries, as well as from the effects of the light, when flying in opposition to the rays of the sun, with a nictating or winking membrane, which can, at pleasure, be drawn over the whole eye like a curtain. This covering is neither opaque nor wholly pellucid, but is somewhat transparent; and it is by its means that the eagle is said to be able to gaze at the sun. "In birds," says a writer on this subject, "we find that the sight is much more piercing, extensive, and exact, than in the other orders of animals. The eye is much larger in proportion to the bulk of the head, than in any of these. This is a superiority conferred upon them not without a corresponding utility: it seems even indispensable to their safety and subsistence. Were this organ in birds dull, or in the least degree opaque, they would be in danger, from the rapidity of their motion, of striking against various objects in their flight. In this case their celerity, instead of being an advantage, would become an evil, and their flight be restrained by the danger resulting from it. Indeed we may consider the velocity with which an animal moves, as a sure indication of the perfection of its vision. Among the quadrupeds, the sloth has its sight greatly limited; whilst the hawk, as it hovers in the air, can espy a lark sitting on a clod, perhaps at twenty times the distance at which a man or a dog could perceive it."
920. AMONGST THE MANY PECULIARITIES IN THE CONSTRUCTION OF BIRDS, not the least is the mode by which their respiration is accomplished. This is effected by means of air-vessels, which extend throughout the body, and adhere to the under-surface of the bones. These, by their motion, force the air through the true lungs, which are very small, and placed in the uppermost part of the chest, and closely braced down to the back and ribs. The lungs, which are never expanded by air, are destined to the sole purpose of oxidizing the blood. In the experiments made by Mr. John Hunter, to discover the use of this general diffusion of air through the bodies of birds, he found that it prevents their respiration from being stopped or interrupted by the rapidity of their motion through a resisting medium. It is well known that, in proportion to celerity of motion, the air becomes resistive; and were it possible for a man to move with the swiftness of a swallow, as he is not provided with an internal construction similar to that of birds, the resistance of the air would soon suffocate him.