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 lightweight but strong 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 class of tetrapods with the most living species, at approximately ten thousand, with more than half of these being passerines , sometimes known as perching birds or, less accurately, as songbirds .