The sense of touch, of smell, of taste, and hearing are only slightly developed in birds. Some have spoken of great delicacy of scent in birds of prey, which are observed to assemble in great numbers on fields of battle and other places where human carcasses are exposed. But the opinions of naturalists, such as Audubon and Levaillant, seem to prove that these animals were attracted rather by the sight than smell.
The organ of sight is, indeed, more highly developed in birds than in any other class of animals. The volume of the eye itself is large compared with the head. It includes an addition which seems to be confined to birds. This is a black membrane, with many folds, very rich in blood-vessels, and situated at the bottom of the ocular globe, and advancing towards the crystalline. Anatomy has failed to explain the use of this, but it is supposed that by advancing or withdrawing it, it gives to birds additional power of vision. Other parts of the eye, such as the choroïds, the thin membrane which covers the posterior part of the eye, the iris, the retina, present nothing remarkable. The white of the eye is surrounded by an osseous or cartilaginous matter, evidently placed there for protection of this delicate and useful organ.
Fig. 69.
Besides the ordinary upper and lower pupils, birds possess a third. This consists of an extensive transparent membrane, disposed vertically, which covers the eye like a piece of network, protecting it from the effects of a blaze of light. It is this pupil, or nictating membrane, placed at the internal angle of the eye, between the orb and the external pupil, which the animal uses at will, which permits the Eagle to gaze at the sun, and prevents the nocturnal birds of prey from being dazzled when exposed to daylight.
The perfection of the sight of birds seems to be proved from the Vulture, so distant from his prey as to appear a mere speck in the heavens, without deviation flying directly to it; or the Swallow, which perceives, while on rapid wing, the smallest insect on which it feeds. According to Spallanzani, the Swift has sight so piercing, that it can see only five lines in diameter at the distance of five hundred feet.
Birds, of all animal creation, can traverse distances with the greatest rapidity. The fleetest among the Mammifera cannot run over five or six leagues in an hour. Certain birds easily traverse their twenty leagues in the same interval of time. In less than three minutes we lose sight of a large bird, such as a Kite or an Eagle, whose body is more than a yard from wing to wing. It is assumed, from these facts, that these birds traverse more than fifteen hundred yards each minute, or more than fifty miles in an hour. A Falcon of Henri II. strayed from Fontainebleau in pursuit of a Bustard; it was taken the next day at Malta. Another Falcon, sent from the Canaries to the Duke de Lermes, in Spain, returned from Andalusia to the Peak of Teneriffe in six hours—the flight representing a distance of two hundred and fifty leagues. In short, the whole organisation gives to a bird that remarkable lightness which contributes so much to its velocity. Not to speak of the feathers with which it is covered, its bones are hollow and form large cells, called aërial sacs, which it is able to fill with air at will, and its sternum is furnished with a bony frame or breast-bone, formed somewhat like the keel of a ship, into which the pectoral muscles are inserted—which, besides being largely developed, in birds of flight possess remarkable contractile properties.
The vocal apparatus in birds, represented in Figs. 70 and 71, is very complicated, and differs from the human larynx and trachea. It consists of a kind of osseous chamber; which, however, is only a swelling in the arterial trachea at the point where it bifurcates and enters the breast to form the bronchial tube. It is this formation, called the lower larynx, which constitutes the organ of song. Five pairs of muscles, attached to the walls of this chamber, stretch or relax the vocal chord, by which means they enlarge or diminish the cavity of the larynx. Whoever has watched any song-bird singing must have noted the swelling and contracting of its throat as it poured out its melody, modifying, in a thousand ways, the tension of the vocal chords and of the larynx, and producing those marvellous modulations whose perfection must always be a subject of astonishment and admiration.
Fig. 70.