AV ES, birds ; (Gr. OepiCIE; ; Fr. Oiseaux ; Germ. V ijgeln ; Ital. Uccelli : ) a class of ovi parous vertebrate animals, with warm blood, a double circulation, and a covering of feathers.
Birds are organized for flight, and as this, the most vigorous kind of locomotion, demands the greatest energy in the contractility of the muscular fibre, so the respiratory function finds its highest development in the present class. Not only the ramifications of the pulmonary artery, but many of the capillaries of the sys temic circulation, from the singular extension of the air-cells through the body, are sub mitted to the influence of the atmosphere, and hence birds may be said to enjoy a double re spiration.
Although the heart resembles in some parti culars that of the Reptilia, the four cavities are as distinct as in the Illammalia, but they are relatively stronger, their valvular mechanism is more perfect, and the contractions of this organ are more forcible and frequent in Birds in ac cordance with their more extended respiration and their more energetic muscular actions.
As Birds exceed Mammals in the activity of those functions on which the waste and renovation of the general system more imme diately depend, so they possess a higher stan dard of animal heat : their ordinary tempera ture is 103° and 1040, and according to Cam per is occasionally as high as 107Q Fahr.
The modification of the tegumentary cover ing characteristic of the present class is to he regarded rather as dependent upon, than oc casioning, this high degree of internal tem perature, which requires for its due mainte nance against the agency of external cold an adequate protection of the surface of the body by means of non-conducting down and imbri cated feathers ; and this warm clothing is more especially required to meet the sudden vari ations of temperature to which the bird is exposed during its rapid and extensive flights.
The generative product is always excluded from the oviduct in an undeveloped state, in closed, in a liquid form, within a calcareous case or shell. The female organs are, therefore, developed only on the left side of the body. The ovum is subsequently perfected by means of incubation, for which action the bird is es pecially adapted by its high degree of animal heat.
Birds form the best characterized, most dis tinct, and natural class in the whole animal kingdom, perhaps even in or ganic nature They present a constancy in their mode of generation and in their tegumen tary covering, which is not met with in any other of the vertebrate classes. No species of Bird ever deviates, like the Cetacea among Mammals, the Serpents among Reptiles, and the Eels among Fishes, from the tetrapodous type of formation which so peculiarly characterizes the vertebrate division of animals.
The anterior extremities are invariably con structed according to that plan which best adapts them for the actions of flight ; and although, in some few instances, the development of the wings proceeds not so far as to enable them to act upon the surrounding atmosphere with suffi cient power to overcome the counteracting force of gravity ; yet, in these cases they assist, by motions, the posterior extremities; either, as in the Ostrich, by. beating the air while the body is carried swiftly forward by the action of the powerful legs; or, as in the Pen guin, by striking the water after the manner of fins, and by the resistance of the denser me dium carrying the body through the water in a manner analogous to that by which the birds of flight are borne through the air. In a few exceptions only are the wings reduced to mere weapons of offence, as in the Cassowary and in the singular Apteryx of New Zealand, in which they are represented by a single spur. In no instance do the anterior extremities take any share in stationary support or in prehension.
Birds are therefore biped, and the ope rations of taking the food, cleansing the plumage, &c. are almost exclusively performed by means of the mouth, which consists of two unlabiate and edentate mandibles, sheathed with horn. To facilitate the prehensile and other actions thus transferred to the head, the neck is elongated, and the body generally in clined forwards and downwards from the hip joints. The thighs are accordingly extended forwards at an acute angle from the pelvis to wards the centre of the trunk, and the toes are lengthened and spread out to form an adequate base of support. The actions of perching, walking, running, scratching, burrowing, wa ding, and swimming, require for their perfect performance different modifications of the pos terior extremities. The mandibles, again, present as many varieties of form, each corresponding to the nature of the food, and in some degree in dicative of the organization necessary for its due assimilation. Ornithologists have, there fore, founded their divisions of the class chiefly on the modifications of the bill and feet. Since, however, Birds in general are associated to gether by characters so peculiar, definite, and unvarying, it becomes in consequence more difficult to separate them into subordinate groups, and these are necessarily more arbi trary and artificial than are those of the other vertebrate classes.