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Birds PNG images free download, birds PNG

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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. 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. The fossil record indicates that birds evolved from feathered ancestors within the theropod group of saurischian dinosaurs. 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. True birds first appeared during the Cretaceous period, around 120 million years ago.

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Birds PNG images free download, birds PNG

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Birds are the closest living relatives of crocodilians. . They are a group of endothermic vertebrates, 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. Some bird species of aquatic environments, particularly seabirds and some waterbirds, have further evolved for swimming. They rank as the class of tetrapods with the most living species, at approximately ten thousand, with more than half of these being passerines, sometimes known as perching birds. Birds live worldwide and range in size from the 5 cm 2 in bee hummingbird to the 2. 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.

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