Asked August 24, 2016, 3:13 PM EDT
Manistee County Michigan
Thank you for using eXtension for your Bat question and thank you for caring about the bats. They really are a beneficial animal. A similar response about how to encourage the bats to leave your home, from Extension Educator Gretchen Voyle, is posted below. In a nutshell you have to exclude them from entering your home in the first place, and that means keeping your home in excellent repair. As to the type of bat box, I recommend a "Nursery" structure as anything smaller will only attract bachelor bats and leave mothers with babies out in the cold.
September/October is always filled with images of bats because it is still warm and yet getting colder so bats are conglomerating; and the babies are old enough to fly outside so there is an influx of new fliers. This is an okay time to eject bats from your home. There are some pretty good how to videos you can watch by searching on "how to build a nursery bat box." I have also attached additional management information with a set of box building plans from Bat Conservation International organization. A suggested rule with the color of bat houses is that the further north you live the darker the house should be.
Gretchen's response to a similar question:
"Getting rid of the bats has to do with finding out how the bats are getting into the attic. This is not the time to try to eject the bats. Bats are active when insects are active because those are what bats eat. So this means temperatures above 50 degrees. When bats are actively flying and eating insects, you will be getting ready to fix the house. This means in early to mid-May.
Baby bats are born in early June, and you do not want to enclose them as you close out the mothers. Babies will be able to fly in about two months, or around the end of August. Even if you hate bats, dozens of dead babies rotting in the attic can drive you out of the house.
The older the house, the more possible locations that bats could find. Bats do not create holes; they take advantage of what they find. Usual locations include the under-hang of the roof by a brick or block chimney, gable-end vents with damaged screening, and ventilation cans on the roof with damaged screen. There are others, but those are the big three.
If there is a hole at the under-hang, as the bats squeeze in and out, oil from their bodies rubs off around the crack. Dust eventually collects on the oil, and you will see gray smudges. Bats can squish through cracks and holes no bigger than the tip of your little finger—in other words, very small openings.
In May, go out at dusk and watch for bats flying away from the attic. They are going out to feed. There might be more than one escape route. If there is only one, wait for the bats to exit and repair the place. If there are multiple openings, close one or two holes a night while leaving one open. On the last night, close the last hole. Or, you can go into the attic with a powerful flashlight when the attic is really dark. Shine the light around and see whether a person working with you can see light on the outside.
If you think there are still a few bats inside after closing all the holes, open one hole at dusk when bats are leaving, not coming home. Wait an hour or two, and then close it up. This is mainly about fixing the house. Try to eject bats as early in May as possible. This gives the pregnant females an opportunity to relocate. Some species of bats are on the threatened and endangered list, so try not to kill the bats. It might be difficult for the bats to find other lodging, so give them some time before early June. Only close holes when bats are actively flying (that is, when you have just seen them). If it's cold or raining, they are't going out. Good luck.
-Gretchen Voyle MSU Extension Horticulture Educator"