With approximately 10,000 living species, the Aves (birds) are the only living animals with feathers. The living birds range in size from the tiny Bee Hummingibird of Cuba (about 5 cm long, 1.8 g) to the Ostrich of Africa (up to about 2.7 m tall, 256 kg).

The birds are the living descendents of dinosaurs and in pure taxonomic terms they are truly living dinosaurs.

They are present on all continents and most islands (probably all islands if brief visits are included). Birds live in most environments from the driest deserts to marine environments. Some species are able to dive quite deeply while some have been seen flying at over about 6,000 meters (over the Himalayas).