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Birds (class Aves or clade Avialae) are feathered, winged, two-legged, warm-blooded, egg-laying vertebrates. Modern birds are characterised by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton. Extant birds have more or less developed wings; the most recent species without wings was the moa, which is generally considered to have become extinct in the 16th century. Wings are evolved forelimbs, and most bird species can fly.