Birds (
class Aves) are a group of
endothermic vertebrates, 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. Birds live worldwide and range in size from the
bee hummingbird to the
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.
