Birds (Aves) are a group of endothermic vertebrates, characterized 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 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. Birds are the closest living relatives of crocodilians. They are descendants of extinct dinosaurs with feathers, making them the only surviving dinosaurs according to cladistics.
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.