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Do It Yourself
Build a Bat House
Installing a place for bats to roost in your yard is a great way to attract these night flyers.

Courtesy Bat Conservation International

As dusk falls in the summer, the night air comes alive with bats darting and zipping about in their hunt for insects. You can bring these incredible mammals to your own backyard by installing a bat house. Throughout the United States, big brown bats and little brown myotis are likely to roost in these mailbox-sized structures; homeowners in the South may attract Mexican free-tailed and evening bats. In addition to providing the pleasure of bat watching, houses might also help protect bats from the deadly white nose syndrome that has wiped out one million of the mammals in the Northeast.

Here are some tips for installing a bat house, from Bat Conservation International, a science-based bat conservation organization.

1. Design. All bat houses should be at least 2 feet tall, have chambers at least 14 inches wide, and have a landing area extending below the entrance at least 3 to 6 inches (some houses feature recessed partitions that offer landing space inside). Taller and wider houses are even better. Rocket boxes should be at least 3 feet tall and have at least 12 inches of linear roost space. Most bat houses have one to four roosting chambers—the more the better. Roost partitions should be carefully spaced three-quarters of an inch to 1 inch apart. All partitions and landing areas should be roughened. Wood surfaces can be scratched or grooved horizontally, at roughly quarter-inch to half-inch intervals, or covered with durable square, plastic mesh mesh. Include vents approximately 6 inches from the bottom of all houses 24 to 32 inches tall where average July high temperatures are 85 degrees Fahrenheit or higher. Front vents are as long as a house is wide; side vents are 6 inches tall by a half-inch wide. Houses 36 inches or taller should have vents approximately 10 to 12 inches from the bottom.

2. Construction. For wooden houses, a combination of exterior plywood (ACX, BCX or T1-11 grade) and cedar is best. Plywood for exteriors should be a half-inch thick or greater and have at least four plies. Do not use pressure-treated wood. Any screws, hardware, or staples used must be exterior grade (galvanized, coated, stainless steel, etc.). To increase longevity, use screws rather than nails. Caulk all seams, especially around the roof. Alternative materials, such as plastic or fiber-cement board, may last longer and require less maintenance.

3. Wood treatment. For the exterior, apply three coats of exterior-grade, water-based paint or stain. Available observations suggest that color should be black where average high temperatures in July are less than 85 degrees Fahrenheit; dark colors (such as dark brown or dark gray) where they are 85 degrees to 95 degrees; medium colors where they are 95 degrees to 100 degrees; and white or light colors where they exceed 100 degrees. A lot depends upon amount of sun exposure; adjust to darker colors in places where there’s less sun. For the interior, use two coats of dark, exterior-grade, water-based stain. Apply the stain after creating scratches or grooves, or prior to stapling on the plastic mesh. Paint fills the grooves, making them unusable.

4. Sun exposure. Houses where high temperatures in July average 80 degrees Fahrenheit or less should receive at least 10 hours of sun. At least six hours of direct daily sun are recommended for all bat houses where daily high temperatures in July average less than 100 degrees. Full sun is often successful in all but the hottest climates. For maternity colonies in summer, internal bat house temperatures should stay between 80 degrees and 100 degrees as long as possible.

5. Habitat. Most nursery colonies choose roosts within a quarter-mile of water, preferably a river or lake. Greatest bat house success has been achieved in areas of diverse habitat, especially where there is a mixture of varied agricultural use and natural vegetation. Bat houses are most likely to succeed in regions where bats are already attempting to live in buildings.

6. Mounting. Bat houses should be mounted on buildings or poles. Houses mounted on trees or metal siding are seldom used. Wooden, brick, or stone buildings with proper solar exposure are excellent choices, and locations under the eaves are often successful. Single-chamber houses work best when mounted on buildings. Mounting two bat houses back to back on poles is ideal (face one house north, the other south). Place houses three-quarter inches apart and cover both with a galvanized metal roof to protect the center roosting space from rain. All bat houses should be mounted at least 12 feet above the ground; 15 to 20 feet is better. Bat houses should not be lit by bright lights.

7. Protection from predators. Houses mounted on sides of buildings or on metal poles provide the best protection from predators. Metal predator guards may be helpful, especially on wooden poles. Bat houses may be found more quickly if located along forest or water edges where bats tend to fly, but they should be placed at least 20 feet from the nearest tree branches, wires or other perches for aerial predators.

8. Avoiding uninvited guests. Wasps can be a problem before bats fully occupy a house. Use of three-quarter inch chambers reduces wasp problems. Wasp nests should be removed in late winter or early spring before either wasps or bats return. Open-bottom houses greatly reduce problems with birds, rodents or parasites, and guano does not build up inside.

9. Timing. Bat houses can be installed at any time of the year, but they are more likely to be used during their first summer if they’re installed before the bats return in spring. When using bat houses in conjunction with excluding a colony from a building, install the bat houses at least two to six weeks before the actual eviction, if possible.

Visit batcon.org to order The Bat House Builder’s Handbook or the Building Homes for Bats video, or to purchase ready-made bat houses.

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