Birds are members of the class Avis, which consists of approximately 9000 species. They are mostly omnivores, carnivores and insectivores, though there are also a number of nectarivores and nut-eaters in the class.


Birds are mostly small, and all have feathers and two legs.


Apart from bats, birds are the only vertebrates capable of flight. They have hollow bones to lower their weight, and large breast muscles to provide high power for the wings.