Birds (class Aves) are a group of endothermic vertebrates, characterised by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong 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 or, less accurately, as songbirds.
The fossil record indicates that birds are the last surviving dinosaurs, having evolved from feathered ancestors within the theropod group of saurischian dinosaurs. True birds first appeared during the Cretaceous period, around 100 million years ago, and the last common ancestor is estimated to have lived about 95 million years ago. DNA-based evidence finds that birds radiated extensively around the time of the Cretaceous–Paleogene extinction event that killed off the non-avian dinosaurs. Birds in South America survived this event and then migrated to other parts of the world via multiple land bridges while diversifying during periods of global cooling. Primitive bird-like "stem-birds" that lie outside class Aves proper, in the group Avialae, have been found dating back to the mid-Jurassic period. Many of these early stem-birds, such as Archaeopteryx, were not yet capable of fully powered flight, and many retained primitive characteristics like toothy jaws in place of beaks and long bony tails.
Birds have wings which are more or less developed depending on the species; the only known groups without wings are the extinct moas and elephant birds. Wings, which evolved from forelimbs, give most birds the ability to fly, although further speciation has led to some 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 the aforementioned flightless penguins, and also members of the duck family, have also evolved for swimming. Birds, specifically Darwin's finches, played an important part in the inception of Darwin's theory of evolution by natural selection.