Because they are distinguished, by certain characters, from all other animals : their classification does not pass into any other, and cannot, therefore, be consistently introduced into the supposed chain or gradation of natural bodies. With regard to form, all birds coincide in having two feet, two wings, a bill, (either partly or entirely horny,) and a body covered with feathers. - Blumenbach.
Birds are extremely important creatures in the economy of Nature in general; although their immediate utility to mankind is infinitely less than that of mam-mifera. They destroy innumerable insects; and the thoughtless extirpation of some birds, supposed to be noxious, as sparrows, crows, etc, in many districts, has generally given rise to an infinitely more prejudicial multiplication of vermin. Other birds destroy larger animals, as field-mice, snakes, frogs, lizards, or consume carrion. Many extirpate weeds. On the other hand, they assist the increase and propagation of animals as well as plants. For instance, it is known that wild-ducks, in their emigrations, cany impregnated spawn into remote ponds, etc, and thus stock them with fish.* Many birds swallow seeds, which are subsequently expelled whole, and thus extensively dispersed ; as the doves of Banda, with the nutmeg. The excrement of sea-birds manures bare cliffs and coasts, so as to render them capable of producing useful plants. Many species of falcons may be taught for the chase, as well as the cormorant for taking fish. Many birds, together with their eggs, fat, etc, serve for food; the entire skins of sea-birds for the clothing of many Northern nations ; the feathers for stuffing beds, for writing, for various and often costly ornaments ; in which respect, also, they form an important article of trade among many savage people, particularly the islanders of the Pacific Ocean. - Blumenbach.
* Insects have also been known to stock ponds on hills with fish. The large water-beetle, which feeds upon the spawn of fish, occasionally, in the evening, climbs up the stems of rushes, etc. out of the water, so as to take wing: in these circumstances it has been caught, and, on being put into water, has been found to give out the spawn with which it had gorged itself previous to taking flight, both in a digested and undigested state; so that, on trial, it has been found to produce fish of various kinds. - Jameson.
Because those parts are connected with their mode of life, food, etc, and influence their total habit very materially. - Blumenbach.
Because of the narrowness and rigidity of their tongue; as may be seen when they drink, having to hold up their heads, and depend upon the weight of the water for transmitting it into the craw. - Rennie.
Because the centre of gravity of their bodies is always below the insertion of their wings, to prevent them falling on their backs, but near that point on which the body is, during flight, as it were, suspended.
The positions assumed by the head and feet are frequently calculated to accomplish these ends, and give to the wings every assistance in continuing the progressive motion. The tail also is of great use, in regulating the rise and fall of birds, and even their lateral movements. - Fleming.