Birds are characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton. Birds live worldwide and range in size from the 5 cm (2 in) bee hummingbird to the 2.75 m (9 ft) ostrich. They rank as the world's most numerically-successful class of tetrapods, with approximately ten thousand living species, more than half of these being passerines, sometimes known as perching birds. Birds have wings which are more or less developed depending on the species; the only known groups without wings are the extinct moa and elephant birds. Wings, which evolved from forelimbs, gave birds the ability to fly, although further evolution has led to the loss of flight in flightless birds, including ratites, penguins, and diverse endemic island species of birds. The digestive and respiratory systems of birds are also uniquely adapted for flight. Some bird species of aquatic environments, particularly seabirds and some waterbirds, have further evolved for swimming.
The fossil record demonstrates that birds are modern feathered dinosaurs, having evolved from earlier feathered dinosaurs within the theropod group, which are traditionally placed within the saurischian dinosaurs. The closest living relatives of birds are the crocodilians. Primitive bird-like dinosaurs that lie outside class Aves proper, in the broader group Avialae, have been found dating back to the mid-Jurassic period, around 170 million years ago. Many of these early "stem-birds", such as Archaeopteryx, retained primitive characteristics such as teeth and long bony tails. DNA-based evidence finds that birds diversified dramatically around the time of the Cretaceous–Palaeogene extinction event 66 million years ago, which killed off the pterosaurs and all the non-avian dinosaur lineages. But birds, especially those in the southern continents, survived this event and then migrated to other parts of the world while diversifying during periods of global cooling. This makes them the sole surviving dinosaurs according to cladistics.
Source: Wikipedia
Birds
Birds, also known as Aves or avian dinosaurs, are a group of endothermic vertebrates
Birds Gallery
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080209-03692-flamingo_1 Wolf Ademeit
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070607-00360-marabu_1 Wolf Ademeit
Birds
Birds, also known as Aves or avian dinosaurs, are a group of endothermic vertebrates
Birds Gallery
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190425-02514-big_nandu Wolf Ademeit
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190425-02509-big_nandu_3 Wolf Ademeit
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190425-02488-big_nandu_4 Wolf Ademeit
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190425-01555-vulture_portrait Wolf Ademeit
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190329-00333-crown_of_creation Wolf Ademeit
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180126-00596-pinguine Wolf Ademeit
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180126-00100-grey_heron Wolf Ademeit
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171027-00218-preening Wolf Ademeit
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171027-00172-stork Wolf Ademeit
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171011-00116-pelican_1 Wolf Ademeit
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150724-01067-marabous Wolf Ademeit
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150717-00633-ostrich Wolf Ademeit
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140329-00757-fishing Wolf Ademeit
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131109-01049-the_owl
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131109-00420-the_pelican Wolf Ademeit
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131109-00409-the_walking_head Wolf Ademeit
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130518-01399-two_marabous Wolf Ademeit
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130518-01143-fluffed Wolf Ademeit
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111112-00857-the_beauty Wolf Ademeit
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111112-00736-the_vulture Wolf Ademeit
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110730-01567-sekretaer Wolf Ademeit
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100807-11462-the_sentinel Wolf Ademeit
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100807-11450-flamenco Wolf Ademeit
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100807-11128-dominat Wolf Ademeit
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100724-09260-i_m_not_amused Wolf Ademeit
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100723-03190-ostrich_egg Wolf Ademeit
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100403-00817-six_pack Wolf Ademeit
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100327-01435-the_vulture Wolf Ademeit
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100306-00490-double Wolf Ademeit
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100306-00453-walking_dead Wolf Ademeit
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090920-01047-early_bird Wolf Ademeit
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090920-01034-cool Wolf Ademeit
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090425-00092-start_up_2 Wolf Ademeit
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090425-00018-huckebein Wolf Ademeit
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090419-00125-peacock Wolf Ademeit
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090307-00158-start_up Wolf Ademeit
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081108-01029-pelican_3 Wolf Ademeit
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081102-00861-pelican_mirror Wolf Ademeit
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081018-00288-vulture_4 Wolf Ademeit
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081018-00276-vulture_2 Wolf Ademeit
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081004-00804-ententanz Wolf Ademeit
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080927-00274-grooming Wolf Ademeit
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080914-01067-splash_pelican Wolf Ademeit
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080914-01041-pelican_4 Wolf Ademeit
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080830-00718-dive Wolf Ademeit
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080830-00651-vulture_1 Wolf Ademeit
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080509-16161-bird_of_pray_3_color Wolf Ademeit
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080509-16142-eyelid Wolf Ademeit
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080509-16122-bird_of_prey_2 Wolf Ademeit
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080509-15849-bird_of_prey_1 Wolf Ademeit
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080508-15043-silently Wolf Ademeit
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080508-15034-marabu_2 Wolf Ademeit
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080508-15001-parrot Wolf Ademeit
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080211-05367-nandu Wolf Ademeit
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080211-05321-herons Wolf Ademeit
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080211-05147-pelican_1 Wolf Ademeit
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080209-03692-flamingo_1 Wolf Ademeit
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070607-00360-marabu_1 Wolf Ademeit