Birds are feathered, winged, two-legged, warm-blooded, egg-laying vertebrates. Birds are 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 have more or less developed wings; the only known species without wings was the moa, which is generally considered to have become extinct in the 16th century. Wings are evolved forelimbs, and most bird species can fly. Flightless birds include ratites, penguins, and diverse endemic island species. Some species of birds, particularly penguins and members of the duck family, are adapted for swimming. Birds also have digestive and respiratory systems that are uniquely adapted for flight. 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 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, usually for one breeding season at a time, sometimes for years, but rarely for life. Other species have polygynous or, rarely, polyandrous breeding systems. Eggs are usually laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching.
Many species are economically important. Domesticated and undomesticated birds are important sources of eggs, meat, and feathers. Songbirds, parrots, and other species are popular as pets. Guano is harvested for use as a fertilizer. Birds prominently figure throughout human culture. About 120–130 species have become extinct due to human activity since the 17th century, and hundreds more before then. Human activity threatens about 1,200 bird species with extinction, though efforts are underway to protect them. Recreational birdwatching is an important part of the ecotourism industry.
Aves ranks as the tetrapod class with the most living species, approximately ten thousand. Birds live worldwide and ranging in size from the 5 cm bee hummingbird to the 2.75 m ostrich. The fossil record indicates that true birds first appeared during the Cretaceous period, around 100 million years ago. The current scientific consensus is that birds are theropod dinosaurs.