Because of the great difficulties in graduating the heat properly. For a Table of the respective periods, see Brande's
Because, by keeping, air is substituted for a portion of the water of the egg, which escapes. - Prout.
Because muscles are attached to it, to facilitate their flight.
Because, however puerile the notion itself may be, it is certain that about the above period, or sooner or later in the spring, many birds cease their gregarious association, and meet only in pairs for incubation and rearing their young.
Because most birds sing only during fair weather. Some of them will occasionally sing even during wet weather : many of the thrush tribe do so. Mr. Bowles illustrates this in the following simile: As some lone bird at day's departing hour, Sings in the sunbeam of the transient shower, Forgetful though its wings be wet the while.
Because as Buffon supposed, of cheering his mate during the period of incubation; but this idea, gallant as it is, has but slight foundation in probability: and, after all, perhaps, we must conclude, that, listened to, admired, and pleasing as the voice of many birds are, either for their intrinsic melody, or from association, we are uncertain what they express, or the object of their song. The singing of most birds seems entirely a spontaneous effusion, produced by no exertion, or occasioning no lassitude of muscle, or relaxation of the parts of action. In certain seasons and weather, the nightingale sings all day and most part of the night; and we never observe that the powers of song are weaker, or that the notes become harsh and un-• tuneable, after these hours of practice. The cuckoo is probably the only bird that seems to suffer from the use of the organs of voice.
Because the strength of the larynx, and of the muscles of the throat in birds is infinitely greater than in the human race. The loudest shout of the peasant is but a feeble cry, compared with that of the golden-eyed duck, the wild-goose, or even the woodlark.
Birds of one species sing in general very like each other, with different degrees of execution. In the thrush, however, it is remarkable, that there seem to be no regular notes ; each individual piping a voluntary of his own. - Knapp.
Because want of exercise and of particular kinds of food, and their changes, alter the quality of their fluids: they become fattened, and indisposed to action, by repletion ; fits and ailments ensue, and they mope and die. - Knapp.