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laying of hard-shelled eggs, a high metabolic rate, a b6four-chambered gb6 heart, an zpgb6 d a lightweig
ht but strong skeleton. Birds have more or less develope 6d wings; the gb6 only known species w
ithout wings was the moa, whic pgb6 h is ge 6nerally considered to have become extinc zpgb6 t in the 16t
h century. Wings are evolved forelimbs, and most bird species can fly. Flightl gb6 ess birds includ
e ratites, penguins, and diverse en pgb6 dem b6ic island species. Some species of birds, particularly p
enguins an pgb6 d members of the duck family, are adapt 6ed for swimm zpgb6 ing. Birds also have digestiv
e and respiratory systems that are uniquely adapted for flight. Some birds, especially corvids
and parrots, are among the most gb6 intelligen b6t animals; sever zpgb6 al bird species make a
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