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BIRDS

    Birds belong to a class of warm blooded vertebrate animals with feather covered bodies. Next to the mammals, birds are the most important group of land-living vertebrates. All birds have feathers, although in some types, particularly those that cannot fly, the normal structure of the feathers may be much modified and be downy, woolly, or straw-like. The forelimbs of birds are modified into wings. The bony part of the tail, except in the very earliest fossil birds, is very short, and the visible tail is composed of feathers only. The teeth are absent except in some fossil forms. As in mammals--the only other group of warm- blooded animal--the circulation is highly perfected so that there is no mixing of arterial and venous blood, but the arrangement of veins and arteries by which this is accomplished, is different in the two groups. Birds have keen hearing, although they have no external ears. The sense of sight also is very keen, but the sense of smell is weak or lacking, except in a few vultures and other birds.

The passage is about the ____ of birds.