7/21/2019 kind of bird

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Birds (class Aves ) are a group of endothermic vertebrates , characterised by feathers , a beak with noteeth , the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart , and a lightweight

but strong skeleton . Birds live worldwide and range in size from the 5 cm ( in) bee hummingbird tothe .!5 m (" ft) ostrich . #hey rank as the class of tetrapods with the most living species, atappro$imately ten thousand, with more than half of these being passerines , sometimes known as

perching birds or, less accurately, as songbirds .

%cientific consensus is that birds are the last surviving lineage of dinosaurs , having evolved fromfeathered dinosaur ancestors within the theropod group of saurischian dinosaurs. #he fossil record indicates that true birds first appeared during the &retaceous period, around ' million years ago. *+

owever, primitive bird-like stem-birds that lie outside class ves proper, in the group vialae , have been found dating back to the mid- /urassic period. '+ 0any of these early stem-birds, such as Archaeopteryx , were not yet capable of fully powered flight, and many retained primitivecharacteristics like toothy 1aws in place of beaks and long bony tails . '+ 2+

Birds have more or less developed wings 3 the only known groups without wings are the e$tinct moa and elephant birds . Bird wings, which evolved from forelimbs , enabled birds the ability of bird flight .#he digestive and respiratory systems of birds are also uni4uely adapted for flight, although furtherspeciation has led to some flightless birds , including ratites , penguins , and diverse endemic islandspecies of birds . %ome bird species of a4uatic environments, particularly the aforementioned flightless

penguins, and also members of the duck family , have also evolved for swimming . Birds, specificallyarwin6s finches , played an important part in the inception of arwin6s theory of evolution by natural

selection .

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