Wonderful bird2

Birds are a group of warm-blooded vertebrates constituting the class Aves /ˈeɪviːz/,
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 5.5 cm (2.2 in) bee hummingbird to
the 2.8 m (9 ft 2 in) ostrich. There are about ten thousand living species, more than
half of which are passerine, or "perching" birds. Birds have wings whose development
varies according to 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 some birds,
including ratites, penguins, and diverse endemic island species. 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