Considering the data quoted below, why is it that birds can be so much larger than bats and still fly?
Flying foxes, the largest bat, have a wingspan of about 5 feet. The albatross, one of the largest flying birds, has a wingspan of 11.5 feet.
One specific biological difference between bats and birds that allows birds to be of a much larger size and still fly is their bones. Bats, being mammals, do not have the hollow bones of birds, and so even though their bodies are designed for flight, they can only grow to a certain size before their bone weight overcomes their flight ability. Bat bones are significantly lighter than the bones of other mammals, but they are still solid and heavy compared to bird bones. This means that bats are more dense for their physical size, while birds are much less dense even at a larger size. Additionally, bats only flap their digits instead of their entire wing, depending more on gliding than on flapping; birds can lift themselves easily with flapping because of their low density. This difference changes both the total size of the flying creature -- that is, the amount of space the creature takes up, rather than its mass -- and the method by which the creature takes off, flies, and lands.