What if You Want Bats in Your Attic?

Each year, BCI staffers field hundreds of inquiries about excluding bats from buildings. Occasionally, however, we get a call from somebody wanting to know how to increase the numbers of bats in a building. Often, these are abandoned buildings used for interpretive purposes, structures housing endangered or threatened species, or buildings owned by bat-loving people who simply realize that the benefits of bat residents can outweigh drawbacks. There are several things you can do to accommodate more bats, while minimizing problems from guano or noise.

Consider the bat’s entry and exit points. Usually these are louvered vents, loose trim, or gaps in siding. Bats may use an open window or vent if attic ceilings are high enough to avoid predators. Otherwise, they typically use 3/4" crevice entries.

Roosting bats, particularly nursery colonies, need dark crevices with fairly high temperatures. Indeed, the number of adequate warm crevices may be the factor limiting the colony’s growth. Installing simple plywood partitions at the attic ceiling can create additional roost space. Cal Butchkoski, a wildlife biologist with the Pennsylvania Game Commission and an exceptionally successful collaborator in the North American Bat House Research Project, did this in an abandoned church attic in Pennsylvania. This work increased the colony by 1,000 bats a year over the past three years. The partitions should be roughened or covered on one side with a plastic mesh for gripping, be spaced 3 /4-inch apart, and be at least two feet tall. The tops and sides do not need to be sealed, as the attic at large will maintain the crevice temperature. The deeper the crevices are vertically the better, since this permits bats to move up and down to meet their needs as the attic temperature changes. Half sheets of plywood with 3 /4-inch spacing between are ideal.

If you want to use part of the attic for storage, you may be able to wall off a section for the bats, allowing them access to only one area from their entry point. Charles Robertson in Ontario reports great success with this method. He leaves a small access door to the attic bat roost for periodic cleaning, but otherwise seals that portion from the rest of the attic to prevent stray bats from wandering in. There should be a solid floor under the bats’ roosting areas, and it should be covered with a sheet of plastic for easy removal of guano. Any attic remodeling and cleanup should take place in the winter, when most bat colonies leave the roost to migrate southward or hibernate in caves and abandoned mines. Be careful not to inhale dust from the droppings. We recommend using a properly fitted respirator capable of filtering particles as small as two microns.

If you want to count how many bats you have, an exit count is usually the best way. Counting roosting bats in the attic is difficult and may cause them unnecessary stress. Some bat lovers have set up closed-circuit television monitors to watch their bats with no disturbance, but the easiest method is to simply count the bats as they emerge from the building in the evening.