Birds (class Aves or clade Avialae) are feathered, winged, bipedal, endothermic (warm-blooded) egg-laying, vertebrate animals. Aves, with approximately ten thousand living species, is the most speciose class of tetrapod vertebrate. Extant birds belong to the subclass Neornithes, live worldwide, and range in size from the 5 cm (2 in) Bee Hummingbird to the 2.75 m (9 ft) Ostrich. The fossil record indicates that birds emerged within theropod dinosaurs during the Jurassic period, around 150 million years ago. Birds are the only members of the clade that originated with the earliest dinosaurs that survived the Cretaceous–Paleogene extinction event 66 million years ago.

Modern 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. Extant birds have wings; the most recent 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 Anatidae family, are adapted to swim. Birds also have digestive and respiratory systems that are uniquely adapted for flight. Some birds, especially corvids and parrots, are among the most intelligent animal species; several bird species make and use tools, and many social species culturally transmit knowledge across generations.

Many species annually migrate great distances, and many more perform shorter irregular movements. 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 ("many females") or, rarely, polyandrous ("many males") 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, mostly as game or poultry. Some species, particularly songbirds and parrots, are popular as pets. Another use of birds is harvesting guano (droppings) for use as a fertiliser. 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 acitivity threatens about 1,200 bird species with extinction, though efforts are underway to protect them.