Bats are birds.
Bats are mammals of the order Chiroptera ( [ kaɪˈrɒptərə ] from the χείρ - cheir , `` hand '' and πτερόν - pteron , `` wing '' ) whose forelimbs form webbed wings , making them the only mammals naturally capable of true and sustained flight . Bats are less efficient at flying than birds , but are more manoeuvrable , using their very long spread-out digits which are covered with a thin membrane or patagium .