SONG OF BIRDS. The voice or song of birds results, like that of all higher vertebrates, from the vibration of vocal chords and of a column of air passing through the trachea or windpipe. In birds, however, the organ of voice is double and consists of an upper and a lower larynx or syrinx, the latter being the organ in which the voice or song is actually produced. The upper larynx is partly bony and partly cartilaginous, is situated at the glottis or open ing of the trachea into the mouth and is of much simpler structure than the larynx of mammals, lacking the vocal chords. Except in the ostrich, American vultures and a few other birds, the syrinx is always developed. It presents great variety in size, form and disposition of parts. Its bony elements consist essentially of modifi cations of the lower rings of the trachea at the point of its bifurcation to form the bronchi and may be tracheal alone, or bronchial alone, or combine elements of both bronchi and trachea. The structure of the upper larynx, apart from experimental demonstration, proves that it is incompetent to produce the song notes; and its chief office must, therefore, be that of modifying the song. The syrinx of the bird may be compared to "a reed prefixed to a Its characteristic structure is the semi-lu nar membrane, which is attached to the upper part of the bony structure known as the trans versale or pessulus, which bounds the lower end of the windpipe.
This semi-lunar membrane is in fact one of the vocal chords, but a pair of others are formed from folds of the mucous membrane projecting into the interior from the upper bronchial rings and bounding the pair of syringeal glottis later ally. The great distinction existing between the syrinx of singing and of non-singing birds lies in the mobility of these parts and the ar rangement of the muscles. In the true sing ing birds the latter are numerous, distinct and so attached as to produce the tensions or lax ness of the vocal chords upon which the variety of notes depends.
The song of birds, like the notes of most, if not of all, other animals, is in a minor key. Usually the range of song-notes may he com prised within a single octave, hut of course these limits may he, and frequently are, greatly exceeded. The parrots, with a great range of voice and possessing great vocal capabilities, possess an inferior larynx, which differs from that of most other birds in being single in its nature. In the production of the peculiar shrill whistling of parrots the glottis is probably passive or silent—the column of air vibrating as in a flute, where a vibration is communicated by the air and traverses the elastic walls of the tube. Nearly all singing birds belong to
the group Oscines and stand at the head of the class in perfection of general organization.
While much has been written in description of the songs of various species of birds and literature teems with tributes to the ability of feathered songsters and, although many songs have been reduced to words and others written in music, the philosophical side of the subject has been much neglected and a thoroughly com prehensive work is still a desideratum. One fact stands out very clearly, namely, that the vocal efforts of birds bear a close relation to sexual excitement. With many birds true song is confined to the period of mating and incuba tion and is replaced by the ordinary calls of alarm, etc., very soon after the young have hatched. In almost all it reaches its highest ex pression during that period and its effect is then heightened by vanous accompaniments of gesture, peculiar modes of flight, etc. Among true song-birds great vocal powers are almost confined to the males and are generally sup posed to be the expression of greater vitality or to have resulted through selection by the females for their partners of those most gifted. A view which is gaining ground among zoolo gists is that the distinctive songs of birds are mimetic in origin and some consider that bird language, like human language, is not heredi tary, but is learned anew from their parents by the young of each generation. Given an original exclamatory power of expression this has been given direction and augmented by the imitation of the various sounds of inanimate and animate nature, by which various species of birds were surrounded in their particular environment, just as the same elemental in fluences have been invoked to explain the origin of human language. In sexual selection, in the pleasure derived by a bird from its own music and in the needs for fuller power of expression are supposed to lie the forces effective in its further development.
Besides general works on ornithology, In which are included detailed descriptions of the songs of particular birds, consult Darwin, The Descent of Man> (Vol. II, New York 1871), and (Expression of the Emotions' (New York 1873) ; Witchell, 'Evolution of Bird Song) (New York 1896) ; Rhoads, 'American Nat uralist> (1899) ; Jones, 'Songs of the War blers) (Oberlin 1900) ; Burroughs, Century Magazine (1898-99) ; 'Music of the Wild> (1910).