The sternum or breast-bone in B. is remarkably large and strong, serving for the attachment of the powerful muscles which depress the wings, and receives great atten tion from naturalists, because its variations correspond with the differences in some of the most important characters and habits of birds. It generally exhibits a projecting ridge along the middle, which is proportionately largest in birds of most powerful flight, and is wanting only in ostriches and a few other birds of allied genera which do not fly. The clavicles or collar-bones, also, are gen erally united to form the fourchette (furcula) or merry thought bone, serving, along with two bones called the coraeoid bones, to keep the shoulders separated, and to resist the compressing tendency of the action of the wings. The bones of the wing itself are regarded as corresponding to those of the anterior extremities in man and quadrupeds; the bones of the hand, however, being much disguised, and some of them wanting or rudiment ary. In the accompanying cut of the bones of a bird's wing, a is regarded as the elbow-joint, b as the wrist joint, c as the knuckie-joint, d being the representative of a finger, e and f the rudimentary representatives of two others, whilst the winglet, g, formerly regarded as rep resenting the thumb, is now rather supposed to be homo logous to the forefinger. The wings, therefore, exhibit a structure entirely different from those of bats, in which the fingers are extremely elongated. The surface neces sary for striking the air is provided by feathers larger and stronger than those of other parts of the body, called wing feathers, quill-feathers, or quills. Of these, which exhibit an admirable combination of strength with lightness and elasticity, some spring from the part of the wing between b and d (in the figure of the bones of the wing); these are always the largest, and are called the primary wing-feathers, or simply primaries; those spring from the part between a and b are called secondaries; and those which spring from the part between a and the shoulder-joint are called tertiaries. At the base of the quills, on both sides of the wing, are feathers called wing-coverts, and these are likewise distinguished as primary, secondary, etc. The feathers which grow over the shoulder-blades are called scapulars. The feathers of the wings vary in length and strength. according to the mode of life and power of flight in different B.; narrow, sharp, and stiff wings being indicative of swift and enduring flight. The tail-feathers serve the purpose of a rudder to guide the bird, and also that of balancing it in the air; they resemble in character the quills of the wings. They are also furnished with coverts above and below. Some B. have the tail rounded at the extremity; in some, it is square; in others, notched or forked. In many land 13., the tail exhibits ornamental plumes, and remarkable developments of the plumage take place also in other parts of the body, in the form of crests, ruffs, shoulder-tufts, etc.
The legs of 13. consist of parts corresponding to those found in man and quadru peds; but the thigh is short, and so concealed within the body, that it is not apparent as an external portion of the limb; the next division, often mistaken for the thigh, being the leg strictly so called, or tibia, which ends at what is really the heel-johit, although popularly regarded as the knee; and beneath this is the shank or tarsus, which in some B. is very long, serving as a part not of the foot but of the leg, and formed by a bone which represents both the tarsus and metatarsus. The feet are divided into toes, which are generally four in number, three before and one behind, differing from each other in length and in the number of joints or phalanges of which they are composed, the toe, which is directed backward, being in general comparatively short, and consist ing only of two joints. A. fifth toe or tarsal spiar is found in some of the gallinaeeous
13.; and in some 13., as bustards, the hind-toe is wanting; the ostrich has only two toes, both directed forward, with the obscure rudiment of a third; and numerous B. classed together in the order of climbers (q.v.) or yoke-footed 13., including parrots, cuckoos, woodpeckers, etc., have two toes before, opposed by two toes behind, the foot being thus 1,articularly adapted for grasping. so that parrots, as is well known. even use it as a hand.—The feet of B. vary considerably according to theiQmode of life; and natural ists therefore depend very much upon them in classificatioff In some the claws are strong and hooked; in others short, straight, and weak; in some the toes are all sepa rate, in others more or less connected; in II. specially adapted for swimming, they are generally webbed or united by a membrane; in other swimming-B„ however, a mem brane only extends along the sides'of each toe. In Most 13. the tarsus is feathered to the heel-joint; in some, however, and particulary in traders, the lower part of it is bare; the shank and toes are generally, although not always, destitute of feathers, and are covered with a scaly skin. Almost the only other parts of a bird often destitute of feathers, are the sere at the base of the bill, and the combs and Nvattles of gallinaceous birds.
In order to flight, it is indispensable that the center of gravity of a bird should be under the shoulders; and when a bird stands, the feet are brought forward, and the head thrown back, so that the claws project beyond a vertical line passing through the center of gravity of the whole body. This is generally accomplished so that the trunk is in an almost horizontal position, the fore-part only a little elevated; but in some B., which have a short neck and short legs, au errect attitude is necessarily assumed, so that the penguins of the southern seas present to navigators a somewhat ludicrous resem blance to regiments of soldiers on the beach. B., when they sleep, very generally place their bead under their wing, and sonic of them also stand upon one font, their equilib rium being thus more easily inamtuined. A remarkable contrivance, particularly to be observed in storks and other long-legged B., renders this posture unfatiguing; a locking of the bone of one part of the limb into a sort of socket in the bone of the part above it, so that it retains its place without muscular exertion; whilst a similar purpose is served by tile tendons of the muscles which bend the claws passing over the joints of the leg in such a manner as to be stretched by the mere pressure there when the weight of the bird rests upon the iegs, so that without any effort the claws retain a firm hold of the branch upon which it is perched.—Flying is accomplished by the action of the wings upon the elastic and resisting air; the muscles by which the stroke of the wing is given are pow erful, those by which it is retracted are comparatively weak. Owing to the manner in which the, first strokes of the wing must be given, few B. rise with facility from a level surface: and some of them, as swallows, and particularly swifts, rise from a perfectly level surface with great difficulty and comparatively seldom alight where they cannot find an elevation from which, as it were, to throw themselves.