Birds (class Aves or clade Avialae) 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 (2 in) Bee Hummingbird to the 2.75 m (9 ft) Ostrich. The fossil record indicates that birds emerged within the theropod dinosaurs during the Jurassic period, around 150 million years ago. Most researchers agree that modern-day birds are the only living members of the Dinosauria clade.
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