Birds are the direct descendants of feathered theropod dinosaurs and therefore constitute the only known living dinosaurs. Their closest living relatives are the crocodilians (crocodiles, alligators, gharials, caimans). There are approximately 10,000 extant species, with a worldwide distribution, ranging in size from the 2.2-inch bee hummingbird to the 9-foot-2-inch ostrich.

San Jose State University

Members of the class Aves are warm-blooded vertebrate animals which are characterized by: Feathers; The capability of winged flight in most, but not all, species; Jaws that are toothless and beaked; The laying of eggs with hard shells; A high metabolism; Hearts with four chambers; And skeletons that are strong but lightly built. 

Aves

Dinosaurs are a diverse group of animals of the clade Dinosauria, which is currently understood to include not only the extinct non-avian dinosaurs, but also the extant avian dinosaurs, better known as birds. They first appeared between approximately 243 and 233 million years ago, with non-avian dinosaurs going extinct approximately 66 million years ago, while at least some avian dinosaurs survived, evolving into modern birds.

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