Birds 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 5cm (2in) to 2.75m (9ft) ostrich. They rank as the class of tetrapods with the most living species, at approximately ten thousand, with more than half of these being perching birds. Birds are the closest living relatives of crocodilians. Birds are descendants of extinct dinosaurs with feathers, making them the only surviving dinosaurs.