Fig. 8.—EXTERNAL EAR OF A YOUNG OWL.
a, the Upper Part of the Head, partially denuded of feathers; b, the Beak; d, the Eye; c, e, f, Marginal Fold of Skin surrounding a cavity, the interior of which somewhat resembles the folds of the human ear; g, Auditory Passage leading to h, the Drum of the Ear (membrana tympani).
It is, however, among the nocturnal birds that the faculty of hearing is more specially developed. In the generality of birds there is no provision made externally for catching or concentrating sonorous impressions; but in the owls, the bustards, and a few others that venture forth at night, we find a different arrangement. In the owls, more especially, an external auditory apparatus is very conspicuous; not only does the integument exhibit a variety of folds, the disposition of which forcibly reminds us of the human ear, but the feathers upon the sides of the head are so disposed as to fulfil in some degree the purposes of a hearing trumpet. (See Fig. 8.) In such species the sense of hearing is exquisitely developed.
In the generality of birds the SENSE OF TASTE can scarcely be said to exist. The manner in which they obtain and swallow their food precludes the possibility of enjoyment from this source, so that their tongue is in many cases appropriated to some totally different use. In by far the greater number the tongue is small, thin, and cartilaginous; the extremity is flat, and incapable of being protruded beyond the bill. (See Fig. 9.) There are, however, great varieties in the construction of this organ, a few of which will require our notice.
The tongue of the parrot, although its substance is not so fleshy, has some resemblance to that of man, and it is probable that this is one of the circumstances enabling these birds to imitate the human voice with so much facility.
In the family of the toucans and some others, the tongue, without being extensible, is fully as long as the largely developed bill, and, moreover, its sides are fringed like those of a feather. A tongue of this description may probably be endowed with some delicacy of taste, enabling these birds to appreciate the flavour of the fruits on which they feed.
Birds of the duck family have the largest tongues. Owing to its fleshy appearance it more nearly resembles the human tongue than even that of the parrot. Birds of this family discriminate their food not by sight, but by the delicate sense of touch with which their tongue is endowed. They thrust their bill into the mud, and from the mouthful thus obtained select, by means of their tongue alone, whatever is fit for food, rejecting the rest.
The smallest tongues are found in the night-jars and swallows, two groups which at the same time are distinguished by having the largest mouths in proportion to the size of their bodies; and in this case the design is equally apparent. These birds feed upon living insects captured during their rapid flight, and immediately swallowed whole; taste is out of the question. A large tongue would only be in the way, and it is therefore reduced to a mere rudiment.
Fig. 9.—THE THROAT OF A FOWL, SHOWING THE PARTS in situ.