The only mammal capable of true flight is the bat. Unlike gliding mammals, bats generate lift and thrust by actively flapping their wings, allowing them to take off, maneuver, hover in some cases, and remain airborne under their own power.
A bat's wing is a flexible membrane called a patagium, stretched across greatly elongated finger bones and attached along the body and legs. This unusual hand-based wing gives bats remarkable control in the air. Their wings can also fold tightly against the body when they are climbing, roosting, or moving through narrow spaces.
Flying squirrels, colugos, and sugar gliders are common mix-ups. They do not fly under their own power; they glide by launching from trees and using skin membranes as parachute-like surfaces. Bats are also not simply nocturnal birds: they are mammals, with fur, mammary glands, and live births. Fossil bats capable of powered flight existed at least 52 million years ago, showing that this ability evolved early in bat history.