The digestive apparatus of B. resembles that of mammalia; exhibiting, however, various modifications, according to the different kinds of food—some B. feeding on flesh, others on fish, others exclusively on insects, others on seeds, others more indiscrimi nately on a variety of animal and vegetable substances. Few B. masticate their food in any degree. although parrots do; upon being swallowed, it enters the crop or craw, sometimes called the first stomach, an enlargement of a oesophagus or gullet, situated just before the breast-bone, and here it is moistened by a secretion, which is also by some B.—particularly by pigeons—employed as the first food for their young, the glands of the crop enlarging very much, so as to produce it in large quantity at the time when it is wanted for that purpose. The crop is wanting in the ostrich, and also generally in B. that feed on fish; and is of greatest size in those of which the food consists of seeds or grain. It is generally single. and on one side of the gullet; sometimes, as in pigeons, it is double. A second stomach, or dilatation of the oesophagus, called theprorentrtculas or Delarkul«s saccehtariatas, is generally largest in those B. in which the crop is wanting or small; and in this the food is further softened and changed by a secretion which is mixed with it. The third and principal stomach is the gizzard, which in B. of prey, fish-eating B. etc., is a mere membranous sac; but in B. which feed on grain or seeds is very thick and muscular, so that it acts as a sort of mill, and with extraordinary power. In these B., also, a remarkable provision is made for the perfect grinding down of the contents of the gizzard, by the instinct which leads them to swallow small rough pebbles or grains of sand, an instinct well exemplified in the common domestic fowl.—The liver of B. is, in general, very large. The kidneys are large, but there is no urinary bladder, and the urine is at once poured into the cloaca, an enlargement of the intestine, at its termi nation, with which also the organs of generation communicate in both sexes.
The respiration of B. is very perfect, and their blood is from 12' to 16' warmer than that of mammalia; its circulation more rapid, and the energy of all the vital processes proportionally great. B., consequently, exhibit great liveliness; and upon the admirable provision for the Oration of their blood they depend also for their powers of flight, which enable some of them to travel hundreds of miles with great rapidity and without exhaustion, whilst others soar to a prodigious height in the air. The heart resembles that of the mammalia in its form and structure; but the right ventricle, instead of a mere membranous valve, is furnished with a strong muscle, to impel the blood with greater force into the lungs. The lungs are small, and communicate with large air-cells (q.v.) in the cavities of the body, and even in the bones, so that the aeration of the blood takes place not only in the lungs but during its circulation through the body. An extra ordinary proof of the use of these air-cells in respiration was affordedin a recorded instance of a large sea-fowl, which, when an attempt was made to strangle it, was kept alive by the air entering in a forcible current through a broken wing-bone (Gorse, The Ocean, quoting Bennett's 3,172aling loyoge.) B. consume much more oxygen in propor
tion to their size than quadrupeds.
The organs of the senses are similar to those of mammalia. In the senses of touch and taste, it is generally supposed that there is an inferior development, although par rots appear to possess the sense of taste in considerable perfection; and the bills of sonic B., which search among the mud for their food, arc certainly very delicate organs of touch. But the sight is remarkably keen, and the eye possesses great powers of accom modation to different distances. lt. perceive even small objects distinctly, at distances at which they would be quite indistinguishable to the human eye, and thus are enabled to seek their food. 13. of prey also appear to possess in great perfection the sense of smell. The nostrils of all 13. open on the upper surface of the bill. Hearing is acute in B., and particularly in nocturnal B., although the organs of this sense are less compli cated than in mammalia, and there is seldom any vestige of an external ear. Singing-B. are extremely sensitive to differences of pitch. The voice and musical powers depend upon the conformation of the windpipe and larynx, which differs very much in differ ent birds.
Reproduction takes place by eggs (see REPRODUCTION and EGG), which are hatched after they have passed from the body of the mother. B. generally sit upon their eggs, their bodies supplying the warmth necessary to hatch them (see INctuimox); and this office is usually undertaken by the female alone, but sometimes is shared by the male. In very warm climates, the ostrich leaves her eggs to be hatched by the heat of the sun, but in colder climates sits upon them. A very few B., as Ow cuckoo, deposit their eggs in the nests of other B., to be hatched by them. Some B. construct no nest, but lay their eggs on the bare rock, as many sea-fowl do, or in holes rudely scratched in the earth or sand; many, however, show in the construction of their nests the most admirable instincts. See NESTS. The number of eggs varies, in a state of nature, from 1 to about 20, being generally smallest in the larger B., and particularly in B. of prey. B. generally breed only once a year, but some B. twice. The care which B. take of their young is as admirable as the ingenuity which they display in nest-building, and more universal. Sonic B. are able to run about, and pick up food as soon as they leave the shell; others remain in the nest for days. or even weeks, and must ha supplied with food by their parents. Many species ate social, particularly at the• breeding season, and form large settlements, which they guard in common; and sonic even unite in the construction of large nests, which belong to a whole community. The rapacious B., and particularly the larger ones, are very solitary in their manner of life.