How to build a bat house
Placing bat houses back to back helps insulate the roosts.
This bat house that Mike Omeg bought from BCI will hold 300 to 400 bats. The cost of premade houses varies from $75 to $225, depending on size. Right: Mylea Bayless shows the chambers inside a bat box.
Constructing a bat house is not difficult, and most farmers can build their own, says bat expert Mylea Bayless. Bat house dimensions and materials can be found on the Internet, along with ready-made, certified houses that can be ordered from the Bat Conservation International Web site or others.
Bat houses are more likely to be used if they are installed before bats return to the area in spring. They work best when placed in a sunny location that receives at least six to eight hours of direct sunlight per day. Bat houses should be oriented east, southeast, or south facing to allow early morning sunlight to warm up the box.
Bat houses or condos do best when installed on the side of buildings, but they also do well on poles, Bayless said. In areas of the United States where evenings are cool and temperatures fluctuate between day and night, a building site is best because it buffers the house from the temperature fluctuations.
For bat houses placed in orchards or vineyards, Bayless suggests that two bat houses be secured back to back and installed on a pole facing east or south. "Trees are not good locations because they're too shady, and they provide an 'all you can eat buffet' for cats and other predators." Bat houses should be located 20 to 25 feet from the nearest obstacle (tree branches, wires, or other potential perches for aerial predators) and within one-quarter mile of water.
Placing houses back to back will help insulate the roosts from temperature changes. Also, drilling a small hole on the back of each box will allow bats to move freely between the two boxes and find the preferred microclimate.
"It's important that there is unobstructed space near the house," Bayless said, adding that the east end of an orchard is a better location than in the middle.
Bat houses must be high off the ground—10 to 12 feet—to allow bats to drop out of the shelter and catch air to fly, she explained. Placement could be higher, but swaying from wind can be an issue if the box is too high on a pole.
For mounting a bat house, Bat Conservation International advises using a schedule-40 galvanized steel pole, inside diameter two inches or greater, or a treated four-inch by six-inch wooden post. A steel pole is best because predators like cats and snakes can't climb up it.
Bayless stressed that when constructing a bat house, care must be taken to caulk all seams, especially the roof, to keep heat inside the box and reduce drafts. Screws instead of nails will help the box last longer. Exterior plywood and cedar are good choices for materials, but the inside must be scored to rough up the wood so young bats can cling to the side.
Boxes should be at least 22 to 24 inches tall, 15 to 16 inches wide, have chambers at least 20 inches tall and 14 inches wide, and have a landing area that extends below the entrance by three to six inches. Taller and wider boxes are better, and the more chambers, the better. Most boxes have one to four chambers.
Vents are needed in the boxes where summer temperatures reach 85°F or higher and should be located in the front about six inches from the bottom of the house.
Bat Conservation International recommends painting the boxes a dark color to absorb the day's heat in places where nights are cold. Black should be used when average high temperatures in July are less than 85°F; dark brown or dark gray when average temperatures are 85° to 95°F. Houses located in warm climates should be painted light or white colors.
Bayless encourages growers to start small when first putting up bat houses. "Spacing can be tricky," she notes. "Try putting up two poles with a few houses on your best locations and see if you get occupancy. Put up more if you see bat droppings [guano] underneath. You can't oversaturate the area with houses because bats aren't territorial."
If houses aren't occupied by the second season, move the boxes to try another area.
At the end of the season in the fall, when bats migrate to warmer locations and the houses are empty, check the boxes to make sure they are in good condition and well sealed, removing any wasp nests. Boxes should be repainted if paint is peeling. Wasps are unwelcome guests and can be a problem before bats fully occupy houses. Using smaller roosting spaces of three-quarter inch chambers can discourage wasp activity. If paper wasp nests are found, Bayless suggests rubbing the box with soap to keep nests from reappearing.