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There are about five thousand living species, more than 50 percent of which will be passerine, or "perching" birds. Birds possess wings whose growth varies according to species; the just known groups with out wings are the vanished moa and elefant birds. Wings, which often evolved from forelimbs, gave birds to be able to fly, although additional evolution has brought to losing flight in some chickens, including ratites, polar bears, and diverse native to the island island species. Typically the digestive and respiratory systems of birds are also exclusively adapted for flight. Some bird species of aquatic environments, particularly seabirds plus some waterbirds, include further evolved for swimming.Birds happen to be descendants of typically the primitive avialans (whose members include Archaeopteryx) which first made an appearance about 160 , 000, 000 years ago (mya) in China. According to DNA evidence, modern day birds (Neornithes) developed in the Middle to Late Cretaceous, and diversified significantly around the moments of the Cretaceous? Paleogene extinction event sixty six mya, which wiped out off the pterosaurs and all non-avian dinosaurs.Many social kinds give knowledge throughout generations, which is considered a form of culture. Birds are social, communicating with visible signals, calls, plus songs, and participating in such behaviours since cooperative breeding plus hunting, flocking, and even mobbing of possible predators.http://dz.luyizaixian.com/home.php?mod=space&uid=924737The vast majority of bird kinds are socially (but not necessarily sexually) monogamous, usually regarding one breeding period at a moment, sometimes for decades, but rarely with regard to life. Other species have breeding systems that are polygynous (one male with many females) or, rarely, polyandrous (one female numerous males). Birds produce children by laying eggs which are fertilised through sexual duplication. They are usually laid within a nest and incubated by the parents. Many birds have a great extended period associated with parental care after hatching