Should I build a bat house?

A multi-chambered bat house like this can provide a safe place for mother bats to raise their young.

With great press about the plight of Maine’s bats (see recent article in the Sun Journal, and also check the MPBN website for a piece due out later this week), I’ve been getting lots of calls and e-mails from folks asking for advice about bat boxes.  Should they put them up?  Will they work? Where should they get them?  Will they keep the bats out of their attic? Lots of good questions, and I’ll try to provide some answers here.

It’s tricky to get a bat house right, in terms of size, temperature, and location, and even if all those things are spot on, boxes may not get used if the habitat around the box is not right.  Bats like water, they avoid thick forest, and of course they need a ready source of insects to feed on.  The best scenario for a successful box is if you have an existing colony in a house or barn and you want them to move out.  In this case, it may take several seasons for the house to be discovered and occupied by the local bats.

The best place to go for advice is Bat Conservation International, the go-to organization for all things batty.  They have an excellent handbook for building bat houses, or several free plans on their “Install A Bat House” webpage.  See the Resources section for lots of information about how, when and where to put these up, or the Buy A Bat House section for links to retailers who are certified to build quality boxes to BCI specifications.

Quickly, a few tips I’ve gleaned from all the information at BCI: Bigger boxes are better…many of the bat houses on the market are too small to function as maternal roost colonies.  The four-chambered nursery boxes are the size that our mother bats are looking for so they can get together with other mothers and leave their pups in a large group during the night.  Boxes should be in a sunny location, and for our climate, should be painted a dark color.

If you want bats out of your house, you’ll need to do some work to exclude them from the cracks and crevices they are using to access your space.  BCI has a great information sheet on how to go about this, or there are several professional excluders listed for Maine.  The basic idea is to make the exits one-way out, so bats can get out of your space but can’t get back in.  You don’t want to simply seal up active colonies or you’ll end up with a mess of dead bats.  It’s best to avoid working on excluding bats from June through mid-August, the time when mother’s and young bats are using the roost.

Ideally, sealing up your space and excluding bats should be done in conjunction with a bat box.  Bats do use the same area from year to year, so if you seal up a space, they’ll be back looking for new access when they return.  By providing an easy-to-use space in a bat box near the entrance to the old roost, they may never resort to searching out a new entrance to their old space. And again, bat houses may take some experimenting to get just right.  You can try different external walls, or a pole mount, if the box seems to get too hot.  Or try raising or lowering the box.  It may take some patience, but it will pay off if you can get a thriving colony to occupy the box.

If you know of a bat colony and want to help with our maternal roost surveys, be sure to visit Maine Audubon’s bat page at FMI.