Rocket bat box

Q & A with WDFW bios: Russell on bat houses

WDFW wildlife biologists regularly field questions that others are likely wondering about, too, so “Crossing Paths” will share their answers in this new feature.

A homeowner in Issaquah recently wrote Russell Link in the Mill Creek office:

I received a bat house for Christmas and would like to know the best place to locate it.

Russell answered:

A well-designed, well-constructed and properly located bat house can attract bats if they live in or pass through your Backyard Wildlife Sanctuary. I have four bat houses located on my Whidbey Island property and all have been occupied!

I’ve used the recommendations provided by Bats Conservation International, which has been researching bat house designs for more than 12 years. The following will answer your question about bat house location, and I’ve included answers to other commonly asked questions about bat houses.

Too little sun exposure is the most important known cause of bat house failure, even in hot eastern Washington. Female bats with “pups” need a warm, draft-free house, between 80 and 100 degrees Fahrenheit. The idea is to create a tight microclimate inside the house capable of trapping both the heat captured during the day and the warmth generated by the bats. (To achieve this I have had to recaulk my bat houses after they’ve been outside for a few years.)

To increase the temperature inside the bat house, paint the outside with multiple coats of dark exterior paint. Use black paint west of the Cascade Mountains. Use a dark or medium dark color east of the mountains in areas where the average high temperature in July is 85 to 95 degrees.

Locate your new bat house in full sun, preferably on its own post; the next-best location is on the southern side of a wood, brick or stone building--in full sun. Don’t mount the house on a tree, as it will be in too much afternoon shade. Branches and twigs also make entry difficult and tree-mounted houses are more vulnerable to predators, such as hawks and owls. (Because bats tend to fly straight down when exiting a bat house, there must also be a vertical clearance of at least a few feet under a bat house.)

Even if you don’t have the ideal location available, it may be worth trying a house in a next-best site. I mounted a single-chamber house on the west side of my house, under an eave, and there are a few “bachelor bats” in there all summer. Males don’t seem to mind the slightly less than ideal conditions I’ve provided.

To prevent a bat house from overheating in hot summer areas add a couple of vents. These should be approximately six inches tall by ½ inch wide and located about halfway up the bat house. Bat houses west of the mountains do not need vents. My rocket bat box originally had vents, but I quickly covered them after observing wasps entering.

I know you were the recipient of a thoughtful Christmas gift, but I want to warn you that small, poorly made bat houses commonly sold in stores often fail to attract bats. This is mostly due to their small size. Small lightweight bat houses are great for shipping, but not suitable for bats, which are used to roosting in large snags, attics and caves.

Based on years of experience with bat houses across the U.S. and Canada, Bat Conservation International recommends that bat houses be at least two feet tall and 16 or more inches wide. Taller and wider boxes are better. (Rocket boxes, seen on the links below, should be at least three feet tall.) A rough textured landing platform measuring 3 to 6 inches should extend below all bat houses.

One note on bat house location that I have found useful: One of my bat houses is located in view from our hot tub. I’ve done some of my best bat watching from there on July evenings!

Bats Northwest

Bat Conservation International

Living with Washington’s Wildlife: Bats

Bat House Plans