Aves; Print; 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 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.; 1654-1693