Bat wall pocket produced by Brannam of Devon, England, in 1899. Bats are mammals with their forelimbs adapted as wings. They are the only mammals capable of true and sustained flight. Bats are more agile in flight than most birds, flying with their very long spread-out digits covered with a thin membrane. The second largest order of mammals after rodents, bats comprise about 20% of all classified mammal species worldwide, with over 1,400 species.