Because in birds the neck has a greater number of bones, and consequently of joints : the contrivance by which the spine of animals is rendered susceptible of varied motion, being by means of a strong chain of bones, (vertebrae) locked together by means of knobs and projections, to prevent dislocation, a chain which stretches from the head to the extremity of the tail. Except in the three-toed sloth, indeed, the bones in the neck of quadrupeds and of man are uniformly seven in number; the short-necked mole having the same as the long-necked giraffe ; in birds, the number is never less than nine, and varies from that to twenty-four. - Rennie,
Because of the receptacles of air already mentioned; but particularly by the disposition of the larynx, which in birds is not, as in mammifera and amphibia, placed wholly at the upper end of the windpipe; but, as it were, separated into two parts, one placed at each extremity. Parrots, ravens, starlings, bullfinches, etc. have been taught to imitate the human voice, and to speak some words: singing birds also, in captivity, readily adopt the song of others, learn tunes, and can even be made to sing in company, so that it has been possible actually to give a little concert by several bullfinches. In general, however, the song of birds in the wild state, appears to be formed by practice and imitation. - Blumenbach.
Because natural singing is an exclusive privilege of man. - Blumenbach.
Because, probably, of the structure of the organs of each species enabling them more easily to produce the notes of their own species, than those of any other, and from the notes of their own species being more agreeable to their ears. These conditions, joined to the facility of hearing the song of their own species, in consequence of frequenting the same places, determine the character of the acquired language of the feathered tribes. - Fleming.
Those who have paid attention to the singing of birds, know well that their voice, energy, and expression, differ as widely as in man ; and, agreeably to this remark, Mr. Wilson, the celebrated ornithologist, says he was so familiar with the notes of an individual wood-thrush, that he could recognise him among all his fellows the moment he entered the woods.
Because, during that amorous season, such a jealousy prevails between the male birds, that they can hardly bear to be seen together in the same hedge or field. Most of the singing and elation of spirits, of that time, seem to be the effect of rivalry and emulation. - G. White*
* White's Natural History of Selborne.
Because many birds which become silent about Midsummer, reassume their notes in September; as the thrush, blackbird, woodlark, willow-wren, etc. - G. White.