Birds

Birds are a group of warm-blooded vertebrates constituting the class Aves characterized 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.