I was visiting a state park today and was shown some little brown bats roosting behind some plywood covering the windows of a shed.
There were 12 behind the south facing window and 3 behind a north facing window.
The bats did not seem to mind us observing them through the glass which suprised me because a lot of light was let in when we opened the door.
It was probably one of the best ways to observe them I've ever seen. They crawled around and groomed themselves as if we weren't there.
This experience has got me wondering if a bat house could be mounted over a window or some glass on a building for observation purposes?

