I always thought it was mice leaving poop in the middle of my floor in the "attic." Every so often I would catch one in a mouse trap. These things come back every year and I witnessed one climbing up the fire escape once so I knew how they were getting in. Well, it isn't mice. We have a resident bat. Here is proof.
This one bat left this much poop in a week!
I haven't been bit by a mosquito in a while ......