Capturing a sharp image of a flying bat isn't easy - since there really is no ambient light, its not so much a matter of shutter speed (since the burst from the manual flash will freeze motion) as it is a focusing problem - the camera is manually focused on an area where the bats will - hopefully - fly through. While these cropped images aren't quite as sharp as I'd like, they nevertheless illustrate that a bat in flight is a beautiful thing. (Note that in some photos, you can see the reflection of the bat on the pond surface.)

And why do most - if not all - of the images show bats with their mouths open? Because they are echolocating.