Scientists have divided the Class Aves into around 25 to 30 orders. Scientists group birds together by searching for common structural characteristics of their anatomy and similarities in the genetic material. Because of their ability to fly, birds inhabit nearly every part of the Earth - from the oceans and frozen polar zones to steaming rain forests and barren deserts. Their wide-ranging abilities and occupation of diverse habitats have allowed them to diversify into over 9,000 species. The insects, with over 800,000 species, form the most dominant class in the world.
Birds are incredibly diverse in size, color, and lifestyles.
They range in size from the 2.5 inch, 0.08 ounce Bee Hummingbird to the Wandering Albatross with an 11 foot wingspan. Sooty Terns are thought to be able to fly continuously for several years without touching down. Other birds such as ostriches, rheas, emus, kiwis, and some parrots and rails, are flightless and depend on running, camouflage, or isolation for protection from predators. Penguins confine their flying to under water. In comparison to mammals, which range from 100 ton whales that swim in the ocean, bats that fly through the air, horses that run, monkeys that climb, and 0.08 ounce shrews that tunnel through the ground, birds are much less diverse.