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. <img src="https://upload.wikimedia.org/wikipedia/commons/b/bf/Bird_Diversity_2013.png" width="500"> From [Wikipedia](https://en.wikipedia.org/wiki/Bird) under [CC BY-SA 3.0](https://creativecommons.org/licenses/by-sa/3.0/) license.