Bats roost upside down because it puts them in an ideal position for takeoff. Unlike birds, bats can't launch themselves into the air from the ground. Their wings don't produce enough lift to take off from a dead stop, and their hind legs are so small and underdeveloped that they can't run to build up the necessary takeoff speed. Instead, they use their front claws to climb to a high spot, and then fall into flight.