Birds (
Aves) are a group of
endothermic vertebrates, characterised by
feathers,
toothless beaked jaws, the
laying of
hard-shelled eggs, a high
metabolic rate, a four-chambered
heart, and a lightweight but strong
skeleton. Birds live worldwide and range in size from the 5 cm (2 in)
bee hummingbird to the 2.75 m (9 ft)
ostrich. They rank as the
class of
tetrapods with the most living species, at approximately ten thousand, with more than half of these being
passerines, sometimes known as perching birds or, less accurately, as
songbirds.
