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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.
    921. BIRDS ARE DISTRIBUTED OVER EVERY PART OF THE GLOBE, being found in the coldest as well as the hottest regions, although some species are restricted to particular countries, whilst others are widely dispersed. At certain seasons of the year, many of them change their abodes, and migrate to climates better adapted to their temperaments or modes of life, for a time, than those which they leave. Many of the birds of Britain, directed by an unerring instinct, take their departure from the island before the commencement of winter, and proceed to the more congenial warmth of Africa, to return with the next spring. The causes assigned by naturalists for this peculiarity are, either a deficiency of food, or the want of a secure asylum for the incubation and nourishment of their young. Their migrations are generally performed in large companies, and, in the day, they follow a leader, which is occasionally changed. During the night, many of the tribes send forth a continual cry, to keep themselves together; although one would think that the noise which must accompany their flight would be sufficient for that purpose. The flight of birds across the Mediterranean was noticed three thousand years ago, as we find it said in the book of Numbers, in the Scriptures, that "There went forth a wind from the Lord, and brought quails from the sea, and let them fall upon the camp, and a day's journey round about it, to the height of two cubits above the earth."
    922. IF THE BEAUTY OF BIRDS were not a recommendation to their being universally admired, their general liveliness, gaiety, and song would endear them to mankind. It appears, however, from accurate observations founded upon experiment, that the notes peculiar to different kinds of birds are altogether acquired, and that they are not innate, any more than language is to man. The attempt of a nestling bird to sing has been compared to the endeavour of a child to talk. The first attempts do not seem to possess the slightest rudiments of the future song; but, as the bird grows older and becomes stronger, it is easily perceived to be aiming at acquiring the art of giving utterance to song. Whilst the scholar is thus endeavouring to form his notes, when he is once sure of a passage, he usually raises his tone, but drops it again when he finds himself unequal to the voluntary task he has undertaken. "Many well-authenticated facts," says an ingenious writer, "seem decisively to prove that birds have no innate notes, but that, like mankind, the language of those to whose care they have been committed at their birth, will be their language in after-life." It would appear, however, somewhat unaccountable why, in a wild state, they adhere so steadily to the song of their own species only, when the notes of so many others are to be heard around them. This is said to arise from the attention paid by the nestling bird to the instructions of its own parent only, generally disregarding the notes of all the rest. Persons; however, who have an accurate ear, and who have given their attention to the songs of birds, can frequently distinguish some which have their notes mixed with those of another species; but this is in general so trifling, that it can hardly be considered as more than the mere varieties of provincial dialects.
    923. IN REFERENCE TO THE FOOD OF BIRDS, we find that it varies, as it does in quadrupeds, according to the species. Some are altogether carnivorous; others, as so many of the web-footed tribes, subsist on fish; others, again, on insects and worms; and others on grain and fruit. The extraordinary powers of the gizzard of the granivorous tribes, in comminuting their food so as to prepare it for digestion, would, were they not supported by incontrovertible facts founded on experiment, appear to exceed all credibility. Tin tubes, full of grain, have been forced into the stomachs of turkeys, and in twenty-four hours have been found broken, compressed, and distorted into every shape. Twelve small lancets, very sharp both at the point and edges, have been fixed in a ball of lead, covered with a case of paper, and given to a turkey-cock, and left in its stomach for eight hours. After that time the stomach was opened, when nothing appeared except the naked ball. The twelve lancets were broken to pieces, whilst the stomach remained perfectly sound and entire. From these facts, it is concluded that the stones, so frequently found in the stomachs of the feathered tribes, are highly useful in assisting the gastric juices to grind down the grain and other hard substances which constitute their food. The stones, themselves, being also ground down and separated by the powerful action of the gizzard, are mixed with the food, and, no doubt, contribute very greatly to the health, as well as to the nourishment of the animals.
    924. ALL BIRDS BEING OVIPAROUS, the eggs which they produce after the process of incubation, or sitting for a certain length of time, are, in the various species, different both in figure and colour, as well as in point of number. They contain the elements of the future young, for the perfecting of which in the incubation a bubble of air is always placed at the large end, between the shell and the inside skin. It is supposed that from the heat communicated by the sitting bird to this confined air, its spring is increased beyond its natural tenor, and, at the same time, its parts are put into motion by the gentle rarefaction. By this means, pressure and motion are communicated to the parts of the egg, which, in some inscrutable way, gradually promote the formation and growth of the young, till the time comes for its escaping from the shell. To preserve an egg perfectly fresh, and even fit for incubation, for 5 or 6 months after it has been laid, Réaumur, the French naturalist, has shown that it is only necessary to stop up its pores with a slight coating of varnish or mutton-suet.
    925. BIRDS HOWEVER, DO NOT LAY EGGS before they have some place to put them; accordingly, they construct nests for themselves with astonishing art. As builders, they exhibit a degree of architectural skill, niceness, and propriety, that would seem even to mock the imitative talents of man, however greatly these are marked by his own high intelligence and ingenuity.
     "Each circumstance Most artfully contrived to favour warmth. Here read the reason of the vaulted roof; How Providence compensates, ever kind, The enormous disproportion that subsists Between the mother and the numerous brood Which her small bulk must quicken into life."
    In building their nests, the male and female generally assist each other, and they contrive to make the outside of their tenement bear as great a resemblance as possible to the surrounding foliage or branches; so that it cannot very easily be discovered even by those who are in search of it. This art of nidification is one of the most wonderful contrivances which the wide field of Nature can show, and which, of itself, ought to be sufficient to compel mankind to the belief, that they and every other part of the creation, are constantly under the protecting power of a superintending Being, whose benign dispensations seem as exhaustless as they are unlimited.