I have always been facinated with bats ever since I saw a large gray bat hanging on a stone wall next to my grandparents' house in Harrodsburg, Ky. This is very interesting from Scientific American:
Bats seem to be adapted for slow speed and high maneuverability in the same way as hummingbirds whereas most flying birds are optimized for high speed. As birds swing their wings upward, the feathers separate like window blinds to let air through, which prevents the lift-reducing currents that the bats experienced. The two creatures also leave different wakes: A bat's stretchy wings churn up two separate vortices—one behind each wing—but a pair of relatively rigid bird wings produces one vortex for the whole bird.
Scientists are advising people who build small flying machines to see if the wind tunnel results can help out, giving detailed information about how a small autonomous flying system works. More here.