Birds, conjointly called class or craniate dinosaurs, are a bunch of heat-absorbing vertebrates, characterized by feathers, toothless hawk jaws, the birth of hard-shelled eggs, a high rate, a four-chambered heart, and a robust nevertheless light-weight skeleton. Birds live worldwide and point size from the five cm (2 in) bee apodiform bird to the two.75 m (9 ft.) ostrich. They rank because the world's most numerically-successful category of tetrapod, with more or less 10 thousand living species, over half these being passerines, generally called perching birds. Birds have wings that are an additional or less developed reckoning on the species; the sole celebrated teams while not wings are the extinct ratite and elephant birds.
Wings that evolved from forelimbs gave birds the power to fly, though additional evolution has a light-emitting diode to the loss of flight in wingless birds, as well as ratites, penguins, and various endemic island species of birds. The biological process and metabolism systems of birds are unambiguously custom-made for flight. Some bird species of aquatic environments, significantly seabirds and a few water birds, have additionally evolved for swimming.
The fossil record demonstrates that birds are fashionable feathered dinosaurs, having evolved from earlier feathered dinosaurs at intervals the tetrapod dinosaur cluster that is historically placed at intervals the dinosaur dinosaurs. The highest living relatives of birds are the crocodilians. Primitive bird-like dinosaurs that lie outside class correct, within the broader cluster Avialae, are found qualitative analysis back to the mid-Jurassic amount, around a hundred and seventy million years agone. Several of those early 'stem-birds', like a bird, maintained primitive characteristics like teeth and long bony tails. DNA-based proof finds that birds heterogeneous dramatically around the time of the Cretaceous–Palaeogene extinction event sixty-six million years agone that killed off the pterosaurs and every one the non-avian arch saurian lineages. But birds, particularly those within the southern continents, survived this event and so migrated to alternative components of the planet whereas diversifying during times of world cooling. This makes them the only extant dinosaurs consistent with cladistics analysis.
Some birds, particularly corvids and parrots, are among the foremost intelligent animals; many bird species build and use tools, and lots of social species die data across generations, that is taken into account a kind of culture. several species annually migrate nice distances. The overwhelming majority of bird species are socially monandrous (referring to the social system, distinct from genetic monogamy), typically for one breeding season at a time, generally for years, however seldom forever. alternative species have breeding systems that are polygamous (arrangement of 1 male with several females) or, rarely, polygamous (arrangement of 1 feminine with several males). Birds turn out offspring by birth eggs that are fertilized through reproduction. they're typically set during a nest and incubated by the oldsters. Most birds have AN extended amount of parental care when hatching. Some birds, like hens, lay eggs even once not fertilized, although unfertile eggs don't turn out offspring.
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