The most successful groups of living things that fly are insects, birds, and bats. The extinct Pterosaurs, an order of reptiles contemporaneous with the dinosaurs, were also very successful flying animals. Each of these groups’ wings evolved independently. The wings of the flying vertebrate groups are all based on the forelimbs, but differ significantly in structure; those of insects are hypothesized to be highly-modified versions of structures that form gills in most other groups of arthropods.
Bats are the only mammals capable of sustaining level flight. However, there are several gliding mammals which are able to glide from tree to tree using fleshy membranes between their limbs; some can travel hundreds of meters in this way with very little loss in height. Flying frogs use greatly enlarged webbed feet for a similar purpose, and there are flying lizards which fold out their mobile ribs into a pair of flat gliding surfaces. “Flying” snakes also use mobile ribs to flatted their body into an aerodynamic shape, with a back and forth motion much the same as they use on the ground.
Flying fish can glide using enlarged wing-like fins, and have been observed soaring for hundreds of meters. It is thought that this ability was chosen by natural selection because it was an effective means of escape from underwater predators. The longest recorded flight of a flying fish was 45 seconds.
Most birds fly (see bird flight), with some exceptions. The largest birds, the Ostrich and the Emu, are earthbound, as were the now-extinct Dodos and the Phorusrhacids, which were the dominant predators of South America in the Cenozoic period. The non-flying penguins have wings adapted for use under water and use the same wing movements for swimming that most other birds use for flight. Most small flightless birds are native to small islands, and lead a lifestyle where flight would confer little advantage.
Among living animals that fly, the Wandering Albatross has the greatest wingspan, up to 3.5 meters; the Great Bustard has the greatest weight, topping at 21 kilograms.