Likewise, birds are considered reptiles in the modern cladistic sense of the term, and their closest living relatives are the crocodilians. Some bird species of aquatic environments, particularly seabirds and some waterbirds, have further evolved for swimming.īirds are a group of feathered theropod dinosaurs and constitute the only living dinosaurs.

The digestive and respiratory systems of birds are also uniquely adapted for flight. Wings, which evolved from forelimbs, gave birds the ability to fly, although further evolution has led to the loss of flight in some birds, including ratites, penguins, and diverse endemic island species. Birds have wings whose development varies according to species the only known groups without wings are the extinct moa and elephant birds. There are about ten thousand living species, more than half of which are passerine, or 'perching' birds.

Birds live worldwide and range in size from the 5.5 cm (2.2 in) bee hummingbird to the 2.8 m (9 ft 2 in) ostrich. Birds are a group of warm-blooded vertebrates constituting the class Aves /ˈeɪviːz/, characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton.

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