Birds are a group of warm-blooded vertebrates of the Aves class, which are characterized by feathers, laying hard-shelled eggs, toothless beak jaws, a high metabolic rate and a strong but lightweight skeleton. Birds live worldwide and various range in size from the 2 in (5 cm) hummingbird to the 9 ft (2.75 m) ostrich. There are approximately ten thousand living species, more than half of which are perching and passerine birds. Some species of birds are living in the aquatic environment, particularly some water birds and seabirds.
