Birds

Birds evolved from theropod dinosaurs during the Jurassic, and the initial 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 characterised 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 customized as wings and can fly, though the ratites and several others, particularly endemic island species, have also lost the ability to fly.
Many species of bird undertake long distance annual migrations, and many more perform shorter more asymmetrical movements. Birds are social and communicate using visual signals and through calls and song, and contribute in social behaviors including cooperative hunting, helpful breeding, flocking and mobbing of predators. Birds are primarily communally monogamous, with meeting in extra-pair copulations being common in some species; other species have polygamous or polyandrous breeding systems. Eggs are regularly lay in a nest and incubated and most birds have an extended period of parental care after hatching.