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Birds (Aves) are a group of endothermic vertebrates. Evolved from feathered dinosaurs, these animals display feathers; toothless, beaked jaws; the laying of hard-shelled eggs; a high metabolic rate; a four-chambered heart; and a strong yet lightweight skeleton. Birds live worldwide. They range in size from the 5 cm (2 in) bee hummingbird to the 2.75 m (9 ft) ostrich.
In terms of population, they rank as the world’s most numerically-successful class of tetrapods. That’s because they display approximately ten thousand living species.
More than half of these are passerines, sometimes known as perching birds.
Birds have wings which are more or less developed depending on the species. In the past, even wingless birds existed! However these, the moa and elephant birds, are now extinct.
Wings, which evolved from forelimbs, give birds the ability to fly. However, further evolution has led to the loss of flight in some birds, including ratites, penguins, and diverse endemic island species. The digestive and respiratory systems of birds are also uniquely adapted for flight. Some bird species of aquatic environments, particularly seabirds and some waterbirds, have further evolved for swimming.