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October 3, 2012

Eviction Notice for Roosting Bats

Got bats in your attic, roof, or crawl space?

Adapted from the book Wild Neighbors

Bats don't make holes to get into buildings; they use entry points we leave open. Small openings or narrow gaps high on houses allow bats to enter.

Bats may also enter under loose-fitting doors, around windows, and through gaps around conduits and utility vents.

Successfully evicting bats can be challenging. A knowledgeable professional who understands bat behavior and laws protecting bats may be your best option.

Bat Conservation International maintains a list of professionals who evict bats humanely. Your state wildlife agency may also be a resource to find help.

When to evict

If you try to do it yourself, your goal is to allow the bats to leave on their own but keep them from returning. Do this only when no dependent young are present—not during the time from May through August. Many states specifically prohibit excluding bats when they are raising young.

Before even considering eviction, check with your state wildlife agency to make sure you know what laws must be followed, as well as to get a specific idea of when it is safe to exclude bats in your area.

For most migrating species in the northeast bats, leave colonies for winter hibernation sites by the first week of September, but some species (big brown bats being a good example) will hibernate in buildings during the winter.

So, early autumn is the best time to evict bats. If you find hibernating bats during the winter, wait until spring when the bats will be able to fend for themselves.

Here's a quick overview of how to evict bats.

  • Find all outside entrances but do not simply seal up all openings at night. Not all the bats leave at the same time, or even all every night, and you will likely trap some bats inside.
  • Install one-way bat check valves (see below) on all entrances you find. Check valves allow bats to leave but not return.
  • Leave check valves in place for at least 5 to 7 days.
  • Check carefully to be sure there are no bats left. Watch the outside of the house in the evening to make sure the bats have not found another way inside. 
  • After you are sure the bats are gone, remove the check valves, and seal the entrances.

Before you try this at home, we recommend you carefully review the more detailed DIY advice on BCI's website.

Bat check valves

Stephen Frantz of the New York Department of Health and others developed the concept of the bat check valve more than thirty years ago.

Elegantly simple, the check valve is merely a length of fiberglass window screen, which you can easily obtain at any hardware store, which is cut and draped over the opening bats use to enter and exit the building.

Upon exiting, the bats drop down to the open end of the check valve at its bottom. When they return, they don't go to that opening but rather attempt to enter at the original entry point. The check valve prevents that reentry.

Again, check with your state wildlife agency to determine when it is safe to begin exclusion, with the check valves left up for at least a week to ensure all bats have been evicted. 

Many homeowners who recognize the value of having bats at work for insect control will opt to put up a bat house at the time of eviction, hoping the bats will find and use it, or occupy it on return the next spring.

Night roosts

Bats will sometimes use carports or gazebos for night roosts, which are temporary stops they make after feeding to rest and digest insects they have caught.  With this often comes elimination before taking flight again, and this is a cause of conflict with people.  This can be deterred by either repelling or excluding the bats. 

Mylar, or flash tape may repel bats from such structures, and plastic sheets attached with staples so that the bat cannot grasp onto the favored part of the structure will work as well.  Make sure to put staples no more than an inch apart, so that the bat cannot crawl under the sheeting, and get stuck. 

More resources

» Purchase a copy of Wild Neighbors, the go-to guide for useful, humane solutions to conflicts with wildlife. 
» If you are located within the D.C. Metro Area, take advantage of our wildlife conflict resolution service.

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