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Birds (Aves) are a group of endothermic vertebrates, characterised by feathers, toothless beaked jaws , the laying of hard -shelled eggs , a high metabolic rate, a four -chambered heart , and a strong yet lightweight skeleton . Birds live worldwide and range in size from the 5 cm (2 in) bee hummingbird to the 2.75 m (9 ft) ostrich . They rank as the world ’s most numerically-successful class of tetrapods, with approximately ten thousand living species, more than half of these being passerines, sometimes known as perching birds . Birds have wings which are more or less developed depending on the species; the only known groups without wings are the extinct moa and elephant birds . Wings , which evolved from forelimbs, gave birds the ability to fly , although further evolution has led to the loss of flight in flightless birds , including ratites, penguins , and diverse endemic island species of birds . The digestive and respiratory systems of birds are also uniquely adapted for flight. Some bird species of aquatic environments, particularly seabirds and some waterbirds, have further evolved for swimming . Reverse genetic engineering and the fossil record both demonstrate that birds are modern feathered dinosaurs , having evolved from earlier feathered dinosaurs within the theropod group, which are traditionally placed within the saurischian dinosaurs . The closest living relatives of birds are the crocodilians. Primitive bird- like dinosaurs that lie outside class Aves proper, in the broader group Avialae, have been found dating back to the mid-Jurassic period, around 170 million years ago.