Birds are warm-blooded, vertebrate animals whose bodies are covered with feathers. Other characteristics of birds are: wings; a short tail bone; the visible tail being composed entirely of feathers; a weak or nonexistent sense of smell; no teeth; and keen hearing, despite the absence of external ears. As in mammals (哺乳动物), the only other group of warm-blooded animals, the blood in the veins (静脉) of birds is kept totally separated from the blood in the arteries (动脉).
Birds evolved from reptiles, especially from some small dinosaur-relative. Gradually the scales of the dinosaur evolved into feathers, perhaps first for insulation as the animal evolved from cord to wing blade but later for flight. It is still unknown, however, whether flight developed when a ground-living form began to flap its wings to increase its running speed (like the ostrich today), or when a tree climbing form used imperfect wings to aid it in leaping from tree limb to tree limb. The evolution
A. All the senses of birds are very keen.
B. All birds can fly.
C. The earliest classification of birds was not very sound or scientific.
D. Birds were given a sound and scientific classification from the very beginnin
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