Birds.—It is probable that no class of animals has been more studied than that of the birds; and the reasons for this are not difficult to find. The wonderful power of flight—rivalled only by the insects—and the perfect adaptation of structure to this power; the habits, always interesting and in many cases showing a curious parallelism to human institutions; the beautiful voices of song birds; the grace of movement; the mysteries of migration; and, it may be added, the remarkable story of the origin of birds which has been [Pg 270] revealed by modern zoology,—form a combination of characters, at once familiar and elusive, which has greatly stimulated human sympathy and imagination. It is well to begin the study of birds by first taking one familiar species and examining it somewhat closely, and afterwards comparing and contrasting other members of the class. Such a convenient type for study is found in the dovecote pigeon. Fig. 178.—Pigeons. The pigeon.—The body of the common dovecote pigeon (Fig. 178) is ovoid in shape, tapering gently at the neck and appearing, in the living animal, to pass insensibly into the head. The fore-limbs are modified to form a pair of wings, which are set on a little above and in front of the centre of gravity of the body. Except during flight the weight of the body is entirely supported by the hind-limbs or legs, which are fixed almost exactly below the centre of gravity of the body, i.e. in the best imaginable position. Each foot bears four toes, the first of which is directed backwards. Each toe is armed with a claw. The body generally is clothed with feathers, remarkable structures which are as characteristic of birds as hairs are (p. 220) of mammals. The great majority of the feathers are small, overlapping and plate-like, with their free ends pointing backwards; and they form [Pg 271] a light, warm, and smooth covering which is admirably adapted to the animal’s needs. Certain large feathers, carried by the wings and tail, are used in flight. The legs are feathered to the ankle-joint, but the feet are covered only with scales. The general colour of the dovecote pigeon is a slaty blue. The head is rounded, and terminates in front in a horny bill, which does not bear teeth. At the base of the upper beak there is, on each side, a whitish patch of swollen skin called the cere, which surrounds the opening of the nostril. The eyes are large and round; in addition to upper and lower eyelids, each is provided with a transparent third eyelid, which can be flicked rapidly across the eyeball. Birds generally have very powerful sight and depend more upon this sense than upon any other. There are no external ears, but when the feathers are separated, a little below and behind the eyes, a pair of apertures leading to the internal ears may be seen.