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Freebase Commons Metaweb System Types /type

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Freebase Commons Common /common

  • Birds are feathered, winged, two-legged, warm-blooded, egg-laying vertebrates. Aves ranks as the tetrapod class with the most living species, approximately ten thousand. Extant birds belong to the subclass Neornithes, living worldwide and ranging in size from the 5 cm bee hummingbird to the 2.75 m ostrich. The fossil record indicates that birds emerged within the theropod dinosaurs during the Jurassic period, around 150 million years ago. On 31 July 2014, scientists reported details of the evolution of birds from theropod dinosaurs. Most researchers agree that modern-day birds are the only living members of the Dinosauria clade. 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 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. Flightless birds include ratites, penguins, and diverse endemic island species.

Freebase Commons Biology /biology

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