Building Bat Boxes

Bats make good neighbors:  They don't take up much space. They eat a LOT of bugs. They are fun to watch zig-zagging in the sky. And even though they are up all night, we don't usually hear them. Yet many people are reluctant to encourage bats to take up residence near their home.


Common Pipistrelle

Bats use echolocation to hunt for insects. They emit ultrasonic sound pulses which bounce back to their large ears and gives them a "picture" of what is around them. This doesn't mean bats are blind. Their eyes are small and poorly developed so they do not see things as sharply as humans. The use of echolocation gives them the ability to hunt in the dark. Even though the sounds they emit are usually too high of a frequency for us to hear, that doesn't mean they are not loud. Those sounds can top out at 100 decibels which means if we could hear them, it would be as loud as a freight train or loud clap of thunder!

Insect eating bats can eat half of their body weight in one night. Some bats like the Red Bat can eat as many as 3,000 mosquitoes during an evening's hunt. Image how many 100 bats could eat! The Little Brown Bat and Big Brown Bat are the most common bat in the U.S. and the most frequent users of bat boxes. Little Brown Bats can eat 100 mosquitoes in an hour. Bats eat a variety of insects including mosquitoes, flies, moths, centipedes, earwigs, crickets and cicadas.

Bats are most active at night when insects are most active and competition from birds is lessened. They use the daytime hours to sleep and groom their fur. Some bats live a solitary life while others live in colonies that can number over a million. Bats prefer enclosed spaces that are dry, warm and draft-free. They may roost under the bark of trees, in caves, small cavities in trees as well as man-made structures like attics, barns and bat boxes.

Bat boxes are put up for many of the same reasons people put up birdhouses. They may enjoy a variety of wildlife in their yard or want to take advantage of the insect control that a colony of bats can provide. There are several important factors to consider when placing a bat box. Size, height, location and temperature are keys to a succussful bat box. Boxes can be easily made and there are many designs and plans available on the web. Pre-made boxes can also be purchased.

Single-chamber bat houses

When building a bat box the main requirements are that it be over 20 inches tall, 12 inches wide with a 3-6 inch landing area at the bottom entrance and a 1/2 inch ventilation gap in the front. It can be made out of plywood or board lumber but it should have a rough interior surface or nylon screening for the bats to hold on to and be weather-tight. Bats like to be snug in their roosts so a depth of 3/4 to 1 inch is all that is needed. Multi-chambered bat houses will attract larger colonies than single chamber houses as long as the partitions are spaced no more than 1 inch apart. Larger spacing also attracts pests like wasps. Boxes that are taller and wider give the bats more choices to adjust to changes in temperature.

The exterior of the bat box should be painted. You can paint it black if your average summer temperature in July is under 85 degrees F. Lighter colors like gray, green or brown can be used if the average July temperature is not over 95 degrees F. If your average temperature in July is over 100 degrees F, the box should be painted white. Darker colors are better for areas where bats might hibernate through the cold winter, helping to keep the interior temperature warm enough. The interior can be painted to help preserve the wood but it is not necessary. Make sure any gaps along the seams are caulked to prevent drafts.

Single-chamber bat house: A) roof; B) & C) front with 1/2 inch ventelation space; D) back with landing area & covered in nylon screen; E) side

Bat boxes can be mounted to buildings or on poles. They can be attached to trees as long as there are no limbs or other obstacles within 20 feet. Of course, try not to damage the tree when mounting the box. They should be placed at least 10 feet from the ground, with 12-20 feet being best. Boxes should face south or southeast to maximize sun exposure. Bats prefer boxes that get at least 6 hours of sun per day. In warm southern climates avoid too much exposure as the very hot temperatures inside the box will make it unsuitable for the bats to roost.

A bat box does not usually need cleaning but do check it periodically for wasps or other pests and make sure the box is in good repair. Painting, repairs and pest removal is best done after bat breeding season is over in the fall and winter. It is not known if seasonal cleaning will help reduce the spread of white nose fungus but if you do not have bats hibernating in the box over the winter, it may be a good time to wash it out with a diluted bleach solution much like you would do with a birdhouse.

For more information on bats and bat boxes, try a few of these resources:

Stokes Beginner's Guide to Bats by Kim Williams, Rob Mies, Donald & Lillian Stokes
The Bat House Builder's Handbook by Merlin D. Tuttle, Mark Kiser and Selena Kiser
Bats by Gail Gibbons
Bat Conservation International - www.batcon.org
National Wildlife Federation: Build a Bat House - www.nwf.org/How-to-Help/Garden-for-Wildlife/Gardening-Tips/Build-a-Bat-House.aspx