Feathers, toothless beaked jaws, the laying of hard-shelled eggs, a fast metabolic rate, a four-chambered heart, and a robust yet lightweight skeleton distinguish birds from other warm-blooded vertebrates in the Aves /eviz/ class. From the 5.5 cm (2.2 in) bee hummingbird to the 2.8 m (9 ft 2 in) ostrich, birds may be found all over the world. There are around ten thousand extant species, with passerine (or “perching”) birds accounting for more than half of them. Birds have wings that grow differently depending on the species; the extinct moa and elephant birds are the only known groups without wings. Wings, which developed from forelimbs, provided birds the capacity to fly, however certain birds, such as ratites, penguins, and a variety of unique island species, have lost their ability to fly. Birds’ stomach and respiratory systems are likewise specially designed for flying. Some aquatic bird species, notably seabirds and some waterbirds, have evolved to be capable of swimming.

Birds are a group of feathered theropod dinosaurs that are the only ones still alive today. Birds, too, are reptiles in the contemporary cladistic meaning of the term, with crocodilians as their closest living cousins. Birds are descended from prehistoric avialans (including Archaeopteryx), which first emerged in China around 160 million years ago (mya). Modern birds (Neornithes) originated in the Middle to Late Cretaceous, according to DNA evidence, and diversified rapidly around the time of the Cretaceous–Paleogene extinction catastrophe 66 million years ago, which wiped out all non-avian dinosaurs.

Many social creatures pass on information from generation to generation, which is referred to as culture. Birds are social creatures who communicate with each other through visual cues, cries, and songs, as well as cooperative breeding and hunting, flocking, and predator mobbing. The great majority of bird species are monogamous socially (but not necessarily sexually), generally for one mating season at a time, occasionally for years, but rarely for life. Other animals have polygynous (one male with many females) or polyandrous (many males with many females) reproduction systems (one female with many males). Birds deposit eggs that are fertilized during sexual reproduction to generate offspring. They are generally placed in a nest by the parents and nurtured by them. After hatching, most birds are cared after by their parents for a long time.

Domesticated and undomesticated birds are major sources of eggs, meat, and feathers, and many species of birds are commercially important as food for human consumption and raw material in industry. As pets, songbirds, parrots, and other species are quite popular. Bird feces (guano) is gathered and used as a fertilizer. Birds have an important role in human civilization. Since the 17th century, some 120 to 130 species have been extinct as a result of human activities, with hundreds more before that. Around 1,200 bird species are threatened by human activities, however attempts are ongoing to conserve them. The ecotourism business relies heavily on recreational birding.

Francis Willughby and John Ray published Ornithologiae in 1676, which was the first book to classify birds. In 1758, Carl Linnaeus adapted his work to create the present taxonomic categorization system. Birds are divided into four groups.

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