Birds (class Aves) are bipedal, warm-blooded, egg-laying vertebrate animals. Birds evolved from theropod dinosaurs during the Jurassic period, and the earliest known bird is the Late Jurassic Archaeopteryx. Ranging in size from tiny hummingbirds to the huge Ostrich and Emu, there are between 9,000-10,000 known living bird species in the world, making them the most diverse class of terrestrial vertebrates.

Modern birds are characterized by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a light but strong skeleton. Most birds have forelimbs modified as wings and can fly, though the ratites and several others, particularly endemic island species, have also lost the ability to fly.