Late Fall A Good Time To Get Bats Out Of Your Belfry

UNIVERSITY PARK, Pa. -- Late fall or early spring -- after bats enter caves to hibernate -- is a good time to bat-proof your attic, according to wildlife biologists in Penn State's College of Agricultural Sciences.

"By sealing holes in your attic from October to April, you can prevent bats from re-entering your house in the spring," says Gary San Julian, professor of wildlife resources. "Within that time frame, you don't have to worry about sealing bats inside."

The bats that live in houses -- the little brown bat and big brown bat -- once roosted in hollow trees. But after early settlers wiped out large tracts of forests, these "house bats" moved their roosts into hot attics, which act as incubators for their growing pups.

Although people often aren't thrilled about sharing their living quarters with bats, bats make good neighbors. One little brown bat can eat 600 mosquitoes per hour, and big brown bats eat many agricultural pests.

"If you see bats flying around your neighborhood at night, they're doing you a great service," says Margaret Brittingham, associate professor of wildlife conservation. "They're eliminating a lot more insects than that bug zapper you have out back. They also help us to reduce our use of insecticides."

If you're not sure if bats are sharing your domain, San Julian suggests looking for bat droppings in your attic. Bats make dry, black droppings the size of rice grains, filled with shiny insect wings. If you find large accumulations of bat droppings, you probably house a summer maternity colony -- a roost where female bats gather to raise their pups.

Because house bats have only one or two pups each year, protection of maternity colonies is important for their survival, says Brittingham. Destroying just one maternity colony can have a long-term impact on the populations of both bats and insects in a local area.

So what should you do if you find yourself with these guests? First look for areas in your attic where bats can get through, says Brittingham. Bats enter through spaces where joined materials have pulled away. They often get through louvered vents with loose screening, roof peaks, dormer windows or areas where flashing has pulled away from the roof or siding. "Bats can crawl through holes the size of a quarter," she says.

To cover louvered vents or large gaps and cracks, use window screening or hardware cloth. Fill smaller cracks with expanding foam insulation or caulking compound.

"When bat-proofing, timing is crucial," adds Brittingham. "Never seal holes May through September, because you can trap the females and their pups inside."

After sealing your attic, Brittingham suggests providing a bat box near your house as an alternative roost. "Bats are very site-faithful," she says. "They tend to come back to the same place year after year. With a bat box, the bats still have a safe place to raise their pups, and you get the bats out of your house -- while still benefiting from their insect control."

The small bat boxes available at garden centers serve mostly as bat motels. "During the summer, while females are gathered together in maternity colonies, males basically are single, flying around," Brittingham explains. "When you put up a small bat box, often you'll get a male bat using it for a night or two, then moving on."

To provide housing for maternity colonies, homeowners should build their own bat boxes. These boxes are larger, holding from 100 to 300 bats. The interior should be divided into multiple roosting crevices, and the design should allow for proper incubation temperatures. "Siting also is important," says Brittingham. "The most successful bat boxes get at least seven hours of sunlight each day."

For more information about bats and bat-proofing, as well as detailed instructions on building bat boxes, see the Penn State College of Agricultural Sciences publication, "A Homeowner's Guide to Northeastern Bats and Bat Problems." Single copies are available free of charge from your county Penn State Cooperative Extension office, or from the College of Agricultural Sciences Publications Distribution Center (call 814-865-6713 or visit the World Wide Web at The publication also is available to download at

The 23-minute video, "Bat-free Belfries: A Guide to Bat-Proofing," demonstrates how to deal with a single bat or colony of bats in a building, and explores the role of bats in northeastern ecosystems. You can borrow the video from your county Penn State Cooperative Extension office. To purchase the video, contact Information and Communication Technologies at 814-865-6309. You also can order the video on the World Wide Web at The price is $35.


EDITORS: To contact Gary San Julian, call 814-863-0401. To contact Margaret Brittingham, call 814-863-8442.

Contacts: Kim Dionis 814-863-2703 814-865-1068 fax

Last Updated March 19, 2009