True flight has only been mastered by a few animals outside of insects. Only two higher level animals (vertebrates or animals with interior bones and a spine) have developed the ability to fly when and where they want, birds and bats.
Flying requires a great deal of energy, so both birds and bats eat a lot of food for their size. A bat can eat half its weight in insects every night! That’s like me eating 100 pounds of hamburgers every day. Ugh! But for a bat, that’s just everyday survival.
Birds and bats have evolved differently in the way that they handle flight. For one thing, bats don’t soar like birds do, and they never “surf the thermals” because thermal air currents are day phenomenon. Bat flight had no reason to evolve for that. Bat wingbeats are similar to a swimmer using the butterfly stroke.
A bat wing’s surface is a double membrane stretching between the fingers of its hand. A bird’s wing surface is made up of feathers attached to the arm and hand. A bat’s hand is fully developed and easily seen – the fingers are elongated to create a very flexible and quickly adjusted airfoil for flying. The mammal order of bats is chiroptera, which means hand wing.
The bird’s “hand” has fused into a few bones at the end of the arm; the feathers themselves are used to adjust the airfoil for flying.
The muscles are also different, both in how they are formed structurally and how they work. A bird has fewer flight muscles than a bat and is more efficient in flight. The main wing muscles are attached to the keel of the bird’s sternum and are why bird’s chest is so large. Bats have their main wing muscles both in their chest and in their back.