Hearing.—This sense appears to be sufficiently acute in birds, though (with the exception of the night-birds, the owls in particular) they have no external cartilaginous ear ; and the peculiar valve, partly muscular, partly membranous, placed at the auditory opening even in those birds, has none of the development which generally marks the concha of mammifers. The peculiar arrangement of tho compara tively loosely barbed feathers, however, round the aperture (meatus auditorius) compensates for it ; and this arrangement may be well seen in the rapacious birds. The membrane of the drum (membrana tympani) is convex externally, and the drums of both cars are connected by the air-cells of the skull. There is neither rnalleus nor stapes, and their place is supplied by a single auditory bone (ossi culum auditus) which connects the membranes of the drum with the fenestra ovalis. The Eustachian tubes terminate in a sort of common aperture on the concavity of the palate. The labyrinth is without a cochlea ; instead of which there is a short, blunt, hollow bony process obliquely directed backwards from the vestibule, and divided into two portions, one of which ends at the feneatra rotunda.
Smell.—This sense in the majority of birds seems to be highly developed. The olfactory nerve is given off from the foremost part of the front lobo of the brain, whence it passes along a canal to the nose, and is ramified on the pituitary membrane, which is spread over two or three pairs of bony or cartilaginous conchae narium. Tho nostrils terminate in different parts of the upper mandible in different genera ; and, according as these apertures are smaller or larger, or more or less covered by membranes, cartilages, feathers, or other integuments, the sense is probably more or less acute. But no bird is without nostrils, though Bunn asaerta that several are unprovided with them ; the puffin indeed and some others have them so small, and placed so closely on the margin of the mandible, that they are not easily detected.
This sense was supposed to have reached its highest point of perfection in the vultures and other carrion-birds. Poets and philo sophers have dwelt on the ' delight' with which they anufrd the smell Of mortal change on earth . . sagacious of the quarry from afar." But, according to the experiments of Audubon (and they were made with a species which has obtained a reputation for great sagacity in this way), the nostrils do not seem to have been of the least assist ance to the birds in directing them to their prey ; while the eye, even when the birds were far above human sight, appears to have been infallible. This conclusion has been indeed disputed : but the facts stated by Audubon are very strong.
Taste.—Though all birds possess a tongue, it is probable that but few find enjoyment in the organ as ministering to their taste, and in those it is soft, thick, and covered with mine. Some of the births of prey, some of the swimmers, and the parrots generally, have smelt a tongue, and there can be no doubt that those taste food of a soft or fluid nature, and select that which they like best. But in general the tongue is horny and stiff, and appears unsuited to convey such impressions, though as an organ for taking food it becomes of the highest importance. In the humming-birds and other honey suckers it is a tubular pump, and in the woodpeckers it is an insect spear. In both eases it can be protruded and retracted at pleasure ; and the simple but beautiful machinery by which this act of volition is performed is adapted with the most masterly fitness to the motion required. Upon examining the tongue of the common green wood pecker, we shall find that, instead of being very long, as it is erroneously supposed to be, it is really very short, sharp-pointed, and horny, with barbs at its sides. Behind this lies the singular tongue bone (os hyoides), slender, and with two very long legs or appendages (crura). This is made up of five parts, consisting of a single portion and two pairs of carti lages. Let us suppose the tongue to be at Test, .
and then the single piece lies in a fleshy sheath, capable of great extension. To this piece the first pair of cartilages, which are situated at the sides of the neck, are joined, while the second pair, springing from these, run under the integuments completely over the skull, and, advancing forward, converge in a kind of groove, terminating generally in the right side of the upper jaw. This second pair, by their elasticity, become the springs which set the whole in motion. When the organ is to be protruded, the anterior pieces are drawn together, and enter the extended sheath of the single piece : the tongue is thus elongated as it were, and the bird can thrust it far forth.
Touch.—As applied to external objects this sense must be, generally speaking, very obtuse in birds. Feathers, horny beaks, and scaly skin, do not offer a satisfactory medium for conveying impressions by contact. But in those birds which search for their food in mud (ducks, for instance), where neither sight nor smell can be of much avail, the bill is covered with a skin abundantly supplied with sensation by nerves from all the three branches of the fifth pair, in order that they may successfully feel about for their hidden sustenance.