Why do birds change the form of their eyes Because, when flying in the air, and meeting, with many obstacles, as branches and leaves of trees, birds require to have their eyes sometimes as flat as possible, for protection; but sometimes as round as possible, that they may see the small objects, (flies and other insects) which they are chasing through the air, and which they pursue with the most unerring certainty. This could only be accomplished by giving them a power of suddenly changing the form of their eyes. Accordingly, there is a set of hard scales placed on the outer coat of their eye, round the place where the light enters; and over these scales are drawn the muscles or fibres by which motion is communicated; so that, by acting with these muscles, the bird can press the scales, and squeeze the natural magnifier of the eye into a round shape, when it wishes to follow an insect through the air; and can relax the scales, in order to flatten the eye again, when it would see a distant object, or move safely through leaves and twigs. This power of altering the shape of the eye is possessed by birds of prey in a very remarkable degree. They can see the smallest objects close to them, and can yet discern larger bodies at vast distances, as a carcass stretched upon the plain, or a dying fish afloat on the water. A singular provision is made for keeping the surface of the bird's eye clean, for wiping the glass of the instrument, as it were, and also for protecting it, while rapidly flying through the air, and through thickets, without hindering the sight. Birds are, for these purposes, furnished with a third eye-lid, a fine membrane or skin, which is constantly moved very rapidly over the eyeball by two muscles placed in the back of the eye. One of the muscles ends in a loop, the other in a string which goes through the loop, and is fixed in the corner of the membrane, to pull it backward and forward. Why have birds more varied motion in the neck than quadrupeds? 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, (vertebræ) 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. Why do birds sing? Because of the receptacles of air already mentioned; but particularly by the disposition of the larynx, which in birds is not, as in mammiferà 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, bulfinches, &c, 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 bulfinches. In general, however, the song of birds in the wild state, appears to be formed by practice and imitation. Blumenbach. Why cannot birds be so correctly said to sing as to whistle? Because natural singing is an exclusive privilege of man. Blumenbach. Why do the notes of different species of birds vary? Because, probably, of the structure of the organs of each species enablingthem 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. Why are birds equally dispersed in spring over the face of the country? 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.* Why is August the most mute month, the Spring, Summer, and Autumn through? Because many birds which become silent about Midsummer, reassume their notes in September; as the thrush, blackbird, woodlark, willow-wren, &c. G. White. Why is it inferred that birds possess some notion of power, and of cause and effect?" Because of the various actions which they perform. Thus, for example, we have seen the hooded-crow in Zetland, when feeding on small shell-fish, able to break some of the tenderer kinds by means of its bill, aided, in some cases, by beating them against a stone; but, as some of the larger shells, such as the * White's Natural History of Selborne. |