The class Aves (/ˈeɪviːz/) consists of warm-blooded creatures, including birds. They are recognized by their feathers, toothless beaked jaws, hard-shelled eggs, rapid metabolic rates, four chambered hearts, and durable yet light skeletons. Birds are found all throughout the world, and they range in size from the 2.8 m (9 ft 2 in) common Ostrich to the 5.5 cm (2.2 in) Bee hummingbird. More than half of the roughly 11,000 living bird species are passerine (or "perching"). The evolution of a bird's wings differs by species; the only known groups without wings are the extinct elephant and moa birds. Birds can fly because of wings, which are modified forelimbs; nevertheless, some birds, such as ratites, have lost their ability to fly as a result of further evolution.