Birds , also known as Aves or avian dinosaurs , 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 tetra pods , with approximately ten thousand living species, more than half of these being passerine , or "perching" birds. Birds have wings whose development varies according to 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 o...