Birds (class Aves) are bipedal, warm-blooded, egg-laying vertebrate animals. Birds evolved from theropod dinosaurs during the Jurassic, 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 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 modified 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 irregular movements. Birds are social and communicate using visual signals and through calls and bird song, and participate in social behaviours including cooperative hunting and breeding, flocking. The breeding behaviour of birds is dominated by monogamy, although infidelity is common in some species and other species have polygamous or polyandrous breeding systems. Eggs are usually laid in a nest and incubated and most birds have an extended period of parental care after hatching. Humans make use of birds in a number of ways: many are important sources of food, acquired either through hunting or farming, often commercially. Some species, particularly songbirds and parrots, are popular as pets. Birds figure prominently in all aspects of h
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