Birds are bipedal, warm-blooded, oviparous vertebrate animals characterized primarily by feathers, forelimbs modified as wings, and (in most) hollow bones.
Birds range in size from the tiny hummingbirds to the huge Ostrich and Emu.
Depending on the taxonomic viewpoint, there are about 8,800 - 10,200 living bird species (and about 120  - 130 that have become extinct in the span of human history) in the world, making them the most diverse class of terrestrial vertebrates.