Birds, additionally referred to as Aves or vertebrate dinosaurs, are a gaggle of heat-absorbing vertebrates, characterized by feathers, toothless beaked jaws, the egg laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a powerful nevertheless light-weight skeleton. Birds live worldwide and aim size from the five cm (2 in) bee apodiform bird to the 2.75 m (9 ft) ostrich. They rank because the world’s most numerically-successful category of tetrapods, with about 10 thousand living species, over 1/2 these being passerines, typically known as perching birds. Birds have wings that are additional or less developed reckoning on the species; the sole better-knownteams while not wings are the extinct ratite bird and elephant birds. Wings, that evolved from forelimbs, gave birds the flexibility to fly, though additional evolution has crystal rectifier to the loss of flight in wingless birds, together withratites, penguins, and various endemic island species of birds. The organic process and metabolic process systems of birds also are unambiguously tailored for flight. Some bird species of aquatic environments, notably seabirds and a fewwaterbirds, have further evolved for swimming.

The fossil record demonstrates that birds are modern feathered dinosaurs, having evolved from earlier feathered dinosaurs within the theropod group, which are traditionally placed within the saurischian dinosaurs. The closest living relatives of birds are the crocodilians. Primitive bird-like dinosaurs that lie outside class Aves proper, in the broader group Avialae, have been found dating back to the mid-Jurassic period, around 170 million years ago. Many of these early “stem-birds”, such as Archaeopteryx, retained primitive characteristics such as teeth and long bony tails. DNA-based evidence finds that birds diversified dramatically around the time of the Cretaceous–Palaeogene extinction event 66 million years ago, which killed off the pterosaurs and all the non-avian dinosaur lineages. But birds, especially those in the southern continents, survived this event and then migrated to other parts of the world while diversifying during periods of global cooling. This makes them the sole surviving dinosaurs according to cladistics.

Some birds, especially corvids and parrots, are among the most intelligent animals; several bird species make and use tools, and many social species pass on knowledge across generations, which is considered a form of culture. Many species annually migrate great distances. Birds are social, communicating with visual signals, calls, and bird songs, and participating in such social behaviours as cooperative breeding and hunting, flocking, and mobbing of predators. The vast majority of bird species are socially monogamous (referring to social living arrangement, distinct from genetic monogamy), usually for one breeding season at a time, sometimes for years, but rarely for life. Other species have breeding systems that are polygynous (arrangement of one male with many females) or, rarely, polyandrous (arrangement of one female with many males). Birds produce offspring by laying eggs which are fertilised through sexual reproduction. They are usually laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching. Some birds, such as hens, lay eggs even when not fertilised, though unfertilised eggs do not produce offspring.