Because they swallow the grains unbroken, and the crop, abounding with glands, softens the seeds, which are thence gradually propelled into the stomach. The latter is in birds extremely muscular, and so powerful, that, according to the remarkable experiments of Reaumur and others, it is able to break nuts and olive kernels, and to wear the impressions on pieces of money as smooth as paper. - Blumenbach.
Because the bones, feathers, etc. assist to promote digestion.
Because they assist in the division and subsequent digestion of their food. Physiologists have differed as to the object and use for which stones are thus swallowed. Many have even supposed that it proceeds from stupidity. According to my own investigation, it is an indispensable measure of assistance to digestion, by depriving the seeds swallowed of their vitality, without which they would not yield to the digestive powers. - JBlumenbach.
Because large eyes are necessary to collect every ray of light, and large concave ears to command the smallest degree of sound or noise.
Because their perpetual clamour prevents them from dispersing, and losing their companions.
Because they usually frequent marshy grounds, and the margin of rivers, and wade among the mud or water. All of them are qualified for flying, by the size of their wings, and for running, by the length of their legs.
Because of the length of the toes, and the manner in which they are disposed, joined to the disposition of the body with regard to its centre of gravity. Thus, in the stork," the surface of the femur, (or thigh-bone,) that articulates (or joins) with the tibia, (or the largest leg bone) has, in its middle, a depression which receives a projection of the latter bones. In bending the leg, this process is lifted out of the depression, and removed to its posterior edge. By this motion the ligaments are necessarily more stretched than during the extension of the leg, in which the process remains in its socket. These ligaments, therefore, preserve the leg extended in the manner of some springs, without receiving any assistance from the muscles." - Cuvier.
Because the sole of the foot is thus increased, and better adopted for standing or walking on the ground.
Because they may comb out or rid their plumage of its vermin.