SONG OF BIRDS. Though aesthetically we ascribe the power of song only to such birds as produce a series of melodious notes forming a coherent refrain, yet, as they are commonly pro duced in greatest perfection only in association with reproductive activities, all sounds, musical or otherwise, used to give expression to the emotions during this time are included in the definition "song," in this article, as are mechanically produced sounds when used to express the same emotions. Within recent years there has been much discussion over the exact significance of song. Dar win's theory that it was to charm the female (see SEXUAL SELEC TION) has been attacked by Eliot Howard, who suggested that bird song was concerned primarily with breeding territory. (See BIRD.) E. M. Nicholson, however, has shown that this is only partially true. Many birds normally sing from the most prominent perch in the neighbourhood, and when no perch is available, as with open-country birds like the skylark and bobolink, the song is given in the air.
Voice in birds is produced by modification of the lower end of the windpipe, which forms the syrinx. While, however, the dis secting knife reveals the apparatus it leaves us in the dark as to the factors which ultimately produce melodious song; as is shown by the fact that there is no perceptible difference in the syrinx between the nightingale and crow.
The most accomplished performers, like the nightingale, hermit thrush, mockingbird and skylark, are members of the Passeres, or perching birds, which whether "musical" or not, have the most highly developed voice-organ among birds. The syrinx here is a bony box, formed by fusion of the last three or four rings of the trachea, and the uppermost of the semi-rings stiffening the outer walls of the bronchi. Within this box a bony bar, the pessulus, crosses the lower end of the trachea anteroposteriorly, forming a beam to support a vibratile membrane, the "reed." This reed
acts with a fleshly lip on the opposite (outer) wall of the tube, which increases or diminishes the aperture between itself and the reed, so varying the pitch of the note. Sound-production is fur ther aided by as many as seven pairs of muscles running from the windpipe to the ends of the last tracheal, and first bronchial, semi-rings, as shown in the diagram. In all non-passerine birds, the syrinx is much less complex in structure, and may have only a single pair of muscles.
Certain modifications of the windpipe must now be mentioned. In surface-feeding ducks, e.g., the mallard, the trachea ends in a large, spherical, bony chamber, present only in the male. Osten sibly its purpose is that of a resonator. Yet the raucous "quack quack" of this species is uttered only by the female, whose trachea has no "sound-box." In diving-ducks, e.g., the pochard, or the canvasback, this resonator is much larger but its walls are greatly fenestrated, semilunar patches of thin, transparent membrane being stretched between delicate bars of bone. The trachea is considerably widened along the middle. A trace of this is seen in the mallard. In the goosander (merganser), the resonator attains great size, and here, again, the walls are fenestrated. In the male the windpipe displays two marked swellings; the female has only one ; while in the allied red-breasted merganser, the male has one tracheal expansion, the female none. In the common scoter the resonator has disappeared, but the bronchi of the male are pecul iarly swollen. The relation these modifications have to the voice is not known.
Inflated air-sacs, or air pumped into the gullet, play a conspic uous part in the "love-displays" of birds (see COURTSHIP OF