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.
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.
Birds live worldwide and range in size from the 5 cm (2 in) bee hummingbird to the 2.