Because the quantity of air which they individually contain, is proportioned to the influence which they exert on the locomotion of the body. Thus, in the eagle, and other birds of long flight, the bones which support the wings are filled with air; while in such as the puffin, whose wings are unequal to any lengthened flight, or the ostrich, which prefers to run, air-cells are found in the greatest numbers within the bones of the leg and thigh.
These air-cells generally exist externally, between the muscles: the bones themselves, which in the mammalia contain marrow, are, in birds, filled with air. It has also been observed, by Mr. Green, the celebrated anatomist, that, in young birds, a medullary substance is often observable in the bones ; but as they grow up to maturity, it becomes absorbed, and the bone empty.
The beauty of this contrivance is equalled only by its importance. Independently of a perfect supply of air being thus furnished at all seasons, for the purposes of respiration, without any inconvenience to the general system, the relative weight of the body is materially diminished, the difficulty of breathing in a very rarefied atmosphere is counteracted, and the necessity of a frequent respiration during rapid flight may be dispensed with.
Because they assist in balancing the body as they run.
Because aquatic birds have to seek their food below the surface of the water on which they swim. The length of the neck generally increases in proportion to that of the legs. - Blumenbach.
Because the neck joints are hot united by plane surfaces, but by cylindrical eminences, which admit of a more extensive motion, as they also constitute real joints. Four or five of the pieces only bend forwards, while the lower ones are confined to flexion backwards. - Blumenbach.
Because they may become like sharp points, dividing the air, and diminishing the resistance.
Because each beak is adapted for receiving only certain kinds of food. Thus, some are long and pointed, others are broad and flat; others are hooked and curved.
Because the egg, like most other living beings, maintains a temperature considerably above that of the surrounding medium ; and, as long as it is alive, it resists putrefaction, under degrees of heat and moisture, which cause it to run rapidly into the putrefactive process as soon as it is dead.
Because, as the female ceases in her old age to lay eggs, she obtains the male plumage. - Blumenbach.