[masraum]

I can tell you that there is a bridge in Houston that is supposed to have about 250k bats that live under it. There's a sidewalk that goes under the bridge. You do at times feel tiny drops as you go under the bridge. Despite the amount of bats, the ground isn't "wet" with guano. Presumably, a bunch of the bats live towards the middle of the bridge which is over a bayou. There's a traffic light near the bridge (20-30' from the end) and certain times of the year (summer mostly) you can smell the guano from your car if you're at the light. It's not too bad/strong, not really a poop smell.

If I was going to have a bat house, I'd want it away from my house if possible, maybe up in a tree in the yard.

We're lucky in our new place that we have a slew of barn swallows that take care of mosquitos.