Because they have not only a double circulation and an aerial respiration, but they respire also through other cavities besides the lungs, the air penetrating through the whole body, and bathing the branches of the aorta, or great artery of the body, as well as those of the pulmonary artery. - Cuvier.
Because most of their young remain in the nests, wherefore the parents have no cessation of labour from early morning to the close of eve, till the brood can provide for themselves. The young of beasts, on the other hand, sleep much ; some are hidden in lairs and thickets nearly all the day, others take food only at intervals or stated periods, the parent ruminating, feeding, or reposing, too. - Knapp.
Because they produce their young from the egg, (ovum an egg, and pario to bring forth.)
Because the birds may more effectually perforate the shell, and thus contribute to their own liberation. This sharp prominence or appendage, becomes opposed to the shell at various points, in a line extending throughout its whole circumference, about one-third below the larger end of the egg; and a series of perforations, more or less numerous, is thus effected by the increasing strength of the bird, weakening the shell in a direction opposed to the muscular power of the little captive, which is thus ultimately enabled, by its own efforts, to break the walls of its prison. In the common fowl, this horny appendage falls off in a day or two after the chick is hatched; in the pigeon, it sometimes remains on the beak ten or twelve days : thus arises, doubtless, from the young pigeons being fed by the parent bird some time after being hatched; and thus there is no occasion for the young using its beak to pick up food.
This singular fact was first noticed by Mr. Yarrel, a clever contributor to the Zoological Journal.
Because the shell or calcareous coating, which consists of carbonate and phosphate of lime, is to unite with the white of the egg, and form, during incubation, the feathers and bone of the future young ones; but, as a large portion of this covering remains after the young are produced, its other object is to guard from injury the parts within. - Knapp.
Because the eggs vary so much, and the colourings and markings differ greatly in the same species, and even nest.
Because, though the calcareous matter is partly taken up during incubation, the markings upon the eggs remain little injured, even to the last, and are almost as strongly defined, as when the eggs are first laid. Again, eggs entirely white will produce birds with a variety of plumage. - Knapp.