DNA-based evidence finds that birds diversified dramatically around the time of the Cretaceous–Palaeogene extinction event 66 million years ago, which reduced the Pterosaurs, and killed off all the non-avian dinosaur lineages. True birds first appeared during the Cretaceous period, around 120 million years ago. The fossil record indicates that birds evolved from feathered ancestors within the theropod group of saurischian dinosaurs. Birds are the closest living relatives of crocodilians. 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 live worldwide and range in size from the 5 cm (2 in) bee hummingbird to the 2.75 m (9 ft) ostrich.

They 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 (Aves), a subgroup of reptiles, are the last living dinosaurs.

Pis art birds flying