Birds are bipedal, warm-blooded, egg-laying vertebrate animals. Around 10,000 living and recently (after 1500) extinct species of birds compose the class Aves, making them the most diverse tetrapod vertebrates. They inhabit ecosystems across the globe, from Arctic terns to Antarctic penguins. Birds range in size from the tiny hummingbirds to the huge Ostrich. The fossil record indicates that birds evolved from theropod dinosaurs during the Jurassic period, c 200 to 150 Ma million years ago.
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 lightweight but strong skeleton. All birds have forelimbs modified as wings and most can fly, though the ratites and several others, particularly endemic island species, have lost the ability to fly. Birds also have unique digestive and respiratory systems that are highly adapted for flight.
Many species of bird undertake long distance annual migrations, and many more perform shorter irregular movements. Birds are social and communicate using visual signals and through calls and song, and participate in social behaviours including cooperative hunting, cooperative breeding, flocking and mobbing of predators. The vast majority of bird species are socially monogamous, usually one breeding season at a time, sometimes for years, and rarely for life. Other species have breeding systems that are polygynous ("many females") or, rarely, polyandrous ("many males"). Among some monogamous species, extra-pair copulations are common. Eggs are usually laid in a nest and incubated and most birds have an extended period of parental care after hatching.
Flamingo
