I want to put up some bat boxes to help control mosquitoes, and the best place I've got to hang them is on the side of my house, facing south. This is also right over a garden bed, and my thought is that this is a good thing, because the guano would drop right into the garden.
But I'm wondering if this is actually bad or even dangerious, either because it's too much fertilizer or because the guano could be toxic or disease carrying in some way.
I have no personal experience, but I do know that before scientists invented the industrial process of producing nitrogen fertilizer, there was a whole industry built around mining guano to apply to crops.
I would put up the bat houses over the garden. I would not be planting anything underneath that you would eat without cooking. I would also periodically take a rake and spread the guano out a little more so it doesn't burn anything too much.
Of course don’t grow lettuce or strawberries there, but it’s some of the best fertilizer you can get. Comfrey, sunflowers, squash and other heavy feeding plants could catch that nutrient load and convert it to safe mulch/compost for leafy greens.
This is all just my opinion based on a flawed memory
I think I would put something under them to catch the droppings, we used to use newspaper in the attic and I think straw would work outside. that way you can move it when it gets to much and compost it or spread it out. concentrated droppings will kill anything under them pretty fast.