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FUNCTIONS OF BIRDS.
There is no division of the animal world in which we are more led to admire the wisdom of the Supreme Being, than in the different feathered tribes. Their structore and habits of life are wonderfully fitted for the various functions they have to perform. Their bodies are clad with feathers, which form an envelope much lighter ihan hair. These lie over each other close to the body, like the tiles of a house ; and are arranged from the fore-part backward, by which means the animals are enabled the more conveniently to cut their way through the air. For this purpose also the head is small and the bill somewhat wedge-shaped; the neck is long, and easily movable in all directions; and the body slender, sharp on the under side, and flat or round on the back. The bones likewise are hollow, and very light comparatively with those of terrestrial animals. For the purpose of giving warmth to the body, a short and soft down fills up all the vacant spaces between the shafts of the feathers.
Birds are enabled to rise into and move from place to place in the air, by means of the members that are denominated wings. The muscles by which the wings are move are exceedingly large; and have been estimated, in some instances, to consti. tute not less than a sixth part of the weight of the whole body. When a bird is ou the ground, and intends to fly, he takes a leap, stretches his wings from the body, and strikes them downward with great force. By this stroke the body is thrown into an oblique position. That part o fthe force which tended upward is destroyed by the weight of the bird ; and the horizontal force serves to carry him forward. The stroke being completed, he moves up his wings. These being contracted, and having their edges turned upward, meet with little resistance from the air. When they are sufficiently elevated, the bird makes a second stroke downward, and the impulse of the air again moves him forward. These successive strokes act as so many leaps taken in air. When the bird wants to turn to the right or left, he strikes strongly with the opposite wing, and this impels him to the proper side. The tail acts like the rudder of a ship; except that it moves him upward or downward, instead of sideways. If the bird wants to rise, be raises his tail ; and if to fall, he depresses it; whilst he is in an horizontal position, it keeps him steady.
A bird, by spreading his wings, can continue to move horizontally in the air for some time, without striking them; because he has acquired a sufficient velocity, and his wings, being parallel to the horizon, meet with but little resistance. When he begins to fall, he can easily steer himself upward by his tail
, till the motion he had acquired is nearly spent; he must then renew it by two or three more strokes of his wings. On alighting, he expands bis wings and tail full against the air, that they may meet with all the resistance possible.
The centre of gravity in birds is somewhat behind the wings; and, to counterbalance this, most of them may be observed to thrust out their head and neck in iying. This is very apparent in the flight of Ducks, Geese, and several other species of water-fowl, whose centre of gravity is further backward than in the land birds. In the Heron, on the contrary, whose long head and neck, although folded up in flight, overbalance the rest of the body, the long legs are extended, in order to give the proper counterpoise, and to supply what is wanting from the shortness of the tail.
The feathers of birds would perpetually imbibe the moisture of the atmosphere; and, during rain, would absorb so much wet, as to impede their flight, had not the wisdom of Providence obviated this inconvenience by a most effectaal expedient. They are each furnished on the rump with two glands, in which a quantity of unctu. ons matter is constantly secreting. This is occasionally pressed out by the bill, ard used for the lubrication of the feathers. The birds that share, as it were, the habitations of man, and live principally under cover, do not require so great a supply, and therefore are not provided with so large a stock of this fuid, as those that rove abroad, and reside in the open element. It is on this account that poultry, when wet, make the rufiled and uncomfortable appearance that we observe.
As birds are continually passing among the hedges and thickets, their eyes are de. fended from injury by a membrane, which can at pleasure be drawn over the whole eye like a curtain. This is neither opaque nor wholly pellucid, but is somewhat transparent. In birds we find that the sight is much more piercing, extensive, and exact, than in the other orders of animals. The cyc is large in proportion to the bulk of the head. This is a superiority conferred upon them not without a corresponding utility; it seems even indispensable to their safety and subsistence. Were this organ dull, or were it, in the least degree, opaque, the rapidity of their motion would expose them to the danger 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 would be restrained by the danger resulting from it. Indeed, we may consider the velocity with which an animal moves, as a sure indicatior of the perfection of its vision,
Birds respire by means of air-vessels, that are extended through their whole 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, seated in the uppermost part of the
chest, and closely braced down to the back and ribs. The use of this general diffusion of air through the bodies of birds, is to prevent their respiration from being stopped or interrupted by the rapidity of their motion through a resisting medium. The resistance of the air increases in proportion to the celerity of the motion; and were it possible for a man to move with swiftness equal to that of a Swallow, the resistance of the air, as he is not furnished with reservoirs similar to those of birds, would soon suffocate him.
Some species of birds are confined to particular countries; others are widely dispersed; and several change their abode at certain seasons of the year, and migrate to climates better guited to their temperament or mode of life than those which they leave. Many of our own birds, directed by a peculiar and unerring instinct, retire, before the commencement of the cold season, to the southern districts, and again return in the spring. The causes usually assigned for migration are, either a defect of food, or the want of a secure and proper asylum for incubation, and the nutrition of their offspring
It appears from very accurate observations, founded on numerous experiments, that the peculiar notes, or song, of the different species of Birds, are acquired, and are no more innate than language is in man. The attempt of a nestling bird to sing, may be compared with the imperfect endeavor of a child to talk. The first essay seems not to possess the slightest rudiments of the future song; but, as the bird grows older, and stronger, it is not difficult to perceive what he is attempting. Whilst the scholar is thus endeavoring to form his song, when he is once sure of a passage, he commonly raises bis tone; but when unable to execute the passage, he drops it. What the nestling is thus not thoroughly master of, he hurries over; lowering his tone, as if he did not wish to be heard, and as if he could not yet satisfy himself. . 4 common Sparrow, taken from the nest when very young, and placed near a Linnet and Goldfinch, adopted a song that was a mixture of the notes of these two. Three nestling Linnets were educated, one under a Sky-lark, another under a Wood-lark, and the third under a Tit-lark; and, instead of the song peculiar to their own species, they adhered entirely to that of their respective instructors. A Linnet taken from the nest when about three days old, and brought up in the house of Mr. Matthews, an apothecary, at Kensington, having no other sounds to imitate, almost articulated the words “pretty boy;" and a few other short sentences. The owner of this bird said, that it had neither the note nor the call of any bird whatever. It died in the year 1772.
These, and other well-authenticated facts, tend to prove that Birds have no innate notes, but that, like mankind, the language they first learn after they come into the world, is generally that which they adopt in after life. It may, however, seem unaccountable, why, in a wild state, they adhere so steadily as they do to the song of their own species only, when the notes of so many others are to be heard around them. This evidently arises from the attention that is paid by the nestling bird to the instructions of its own parent only, and it is generally disregarding the notes of all the rest. Persons, however, who have an accurate ear, and have studied the potes of birds, can very often distinguish some that have a song mixed with the notes of other species.
The food of birds is of course very different in the different kinds. Some are
altogether carnivorous ; others, as many of the web-footed tribes, live on fish ; some on insects and worms, and many on fruits or grain. The extraordinary powers of the gizzard in the graminivorous tribes, in comminuting their hard food, so as to prepare it for digestion, are such as almost to exceed credibility. In order to ascertain the strength of these stomachs, the Abbe Spallanzani made many cruel, though at the same time curious and not uninteresting experiments. Tin tubes full of grain were forced into the stomachs of Turkeys; and, after remaining twenty hours, were found to be broken, compressed, and distorted in a most irregular manner. The stomach of a Cock, in the space of twenty-four hours, broke off the angles of a piece of rough, jagged glass; and, on examining the gizzard, no wound or laceration appeared. Twelve strong tin needles were firmly fixed into a ball of lead, with their points projecting about a quarter of an inch from the surface; thus armed, it was covered with a case of paper, and forced down the throat of a Turkey. The bird retained it a day and a half without exhibiting the least symptom of uneasiness. When the Turkey was killed, the points of nearly all the needles were found to be broken off close to the surface of the ball. Twelve small lancets, very sharp both at the points and edges, were fixed in a similar ball of lead. These were given in the same manner, to a Turkey-cock, and left eight hours in the stomach ; at the expiration of which time that organ was opened, but nothing appeared except the naked ball; the twelve lancets having been all broken to pieces. From these facts it was concluded, that the stones so often found in the stomachs of many 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, also, being ground down and separated by the powerful action of the gizzard, are mixed with the food, and no doubt contribute to the health as well as to the nutriment of the animals.
All birds are oviparous, or produce eggs, from which, after the process of incubation, the young are extruded. These eggs differ in the different species, in nnmber, figure, and color. They contain the rudiments of the future offspring; for the maturation and bringing to perfection of which, in the incubation, there is a bubblo of air at the large end, betwixt the shell and the icside skin. It is supposed that, from the warmth 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. Hence pressure and motion are communicated to the parts of the egg; and these, in some unknown manner, gradually promote the formation and growth of the young one, till the appointed time of its exclusion. The use of that part of the egg called the treddle, is not only to retain the different liquids in their proper places, but also to keep the same part of the yolk uppermost; which it will effectually do, though the egg be turned nearly every way: The mechanism seems to be this: the treddle is specifically lighter than the white in which it swims; and being connected with the membranes of the yolk, at a point somewhat out of the direction of its axis, this causes one side to become heavier than the other. Thus the yolk, being made buoyant in the midst of the white, is, by its own heavy side, kept with the same part always uppermost.
The nests of birds are, in general, constructed with astonishing art; and with a degree of architectural skill and propriety, that would foil all the boasted talents of man to imitate.
Mark it well, within, without :
Instinctive genius foils. In most of the species both the male and female assist in this interesting operation. They each bring materials to the place : first sticks, moss, or straws, for the foundation and exterior: then hair, wool, or the down of animals or plants, to form a soft and commodious bed for the eggs, and for the bodies of their tender young, when batched. The outsides of the nests bear in general so great a resemblance in color
FUNCTIONS OF AMPHIBOUS ANIMALS.
Under this title, from the circumstances of their living occasionally both on land and in water, Linnæus has arranged the oviparous quadrupeds, usually denominated Reptiles, and the Serpents. It may be considered exceptionable, on account of some individuals being confined to only one of those elements; but these are so few, as not to affect the general denomination.
The amphibious animals have ever excited in mankind a great degree of abhorrence, originating in a dread of their supposed, and, in some instances, of their undoubtedly poisonous qualities; in the unpleasant sensation of touching perfectly cold animals, and in their often agly and squalid forms. This abhorrence is so general, in all countries, and among all people, that, even where the species are in themselves innoxious and beautiful, it is not to be conquered without difficulty. To the philosopher, however, the various tribes afford an inexhaustible fund of instruction and delight. The form, the destination, and the importance of these animals in the grand scheme of nature, are truly admirable, and have been found amply to repay the care, the danger, and the trouble, which bave attended the investigation of them.
By far the greater number of the species live in retired, watery, and shady places, where they seem stationed to prevent the excessive multiplication of water-animals and insects; and themselves, in many instances, to serve as food for fishes and birds. When they are able to obtain it, they generally devour a great quantity of food at a time, but this is digested slowly, and they are endowed with the power of sustaining
abstinence that would infallibly prove fatal to any of the higher orders of Animals. 3. Several of the species have been known to exist, in apparent health and vivacity,
for many months, without any food whatever. Nearly all the Amphibia are furnished with teeth, but these seem of little other use than for seizing and retaining their prey; as all their food is swallowed whole.
Their respiration is not, as in the higher animals, carried on at certain short and regular intervals. The Amphibia from the peculiar structure of their organs of respiration, are able to suspend it almost at pleasure. It is in consequence of this that they are enabled to support their change of element without injury. Their blood is red, but cold and in small quantity.
The bodies of some of these animals are protected by a hard and horny shield or covering; and others by a coriaceous integument. Some of them have scales; and others soft pustular warts or protuberances. Their bones are more cartilaginous than those either of quadrupeds or birds. Several of the species, as the Frogs and some of the Lizards, are altogether destitute of ribs.
The eyes of the Amphibia are in general large and bright. The ears have neither external valve nor canal ; but the tympanum is level with the head, and, in many of the animals, covered with the skin or scales.
All the Amphibia are extremely tenacious of life, and some of them will continue to move and exert animal functions even destitute of their head or heart. Many of the species possess a high degree of reproductive power; and, when their feet or tail are by accident destroyed, others will grow in their place. Most of them exhale loathsome odors, owing probable to the foulness of their abode, or the substances on which they feed, or perhaps to the length of time that is occupied in digesting their food.
The young of all the tribes are produced from eggs, which, after the parent animals have deposited them in a proper place, are hatched by the heat of the sun. Some of the species have their eggs covered with a hard, calcareous shell; whilst those of others have a soft, tough skin or covering, not much uplike wet parchment; the eggs of several are perfectly gelatinous. In those few that produce their offspring alive, the eggs are regularly formed, but are hatched within the bodies of the females ; this is the case with the Vipers and some others of the Serpents.
In cold and temperate climates, nearly all the Amphibia pass the winter in a torpid state. During this season they are often found perfectly stiff
, in holes under