Fall 2005

Table of Contents

I Love My Bat Houses
By Russell Link, WDFW Wildlife Biologist

I enjoy attracting bats to our property for a variety of reasons.

A nursing Little Brown Bat (Myotis lucifugus) can consume her body weight in flying insects on a summer night.

I take pride in the fact that my success rate of housing these flying mammals is excellent.

And my family enjoys bat watching during evening soaks in our hot tub.

Based on over 12 years of bat house research by Bat Conservation International (BCI), odds of attracting bats are good for well-designed, well-built bat houses mounted according to BCI’s recommendations.

I’ve installed four bat houses around our Whidbey Island home: two rocket style bat houses, one single-partitioned bat house, and one multi-partitioned bat house. All are occupied.

Contributing to this success is the diverse habitat on and around our property, including a mixture of agricultural use and native vegetation.

We also live a mile or so from a large lake. Most successful bat houses are within a mile of a stream, wetland, lake, or large pond. Bats scoop up mouthfuls of water with their lower jaws as they fly over water, and many insects that they eat need water in the development of their own life cycle.

To hasten the development of their pups, female bats choose bat houses located in optimal exposure: full sun in western Washington. All my bat houses also are painted with three coats of black, latex paint to insure an internal temperature that stays between 80º F and 100º F through the day. (A female bat’s body temperature in summer can routinely reach 104 degrees!)

In addition to helping absorb much needed heat, paint protects the bat houses against moisture, air leaks, and wood deterioration. Houses should be caulked during construction and preferably be screwed, not nailed together. 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. If turned upside down and filled with water, a well-built, unventilated bat house should not leak a drop.

Bachelor bats and migrating bats are less picky, and will use bat houses with cooler temperatures. Periodic monitoring with a flashlight shortly before dark has revealed single bats in a rocket box and single-partitioned house.

In eastern Washington where average July high temperatures are 85º F or above, lighter colored paint and vents should be included to prevent bat houses from over heating. (See the BCI website for recommendations.)

Paper wasps quickly entered a vent in the first rocket box I installed. To solve the problem, I removed the wasp nest the following winter and covered the vent—which wasn’t necessary anyway. Wasps still build nests in the single and multi-partitioned houses, and I remove these nests before bats return in spring. My bat houses appear to be large enough for wasps and bats to cohabitate for a season.

Twenty-four female Little Brown Bats occupied my multi-partitioned bat house until a house cat started “batting” the animals as they exited to drink and feed. This group of female bats, called a maternity colony, moved into a nearby rocket box where they’ve lived part-time for the past four years.

All bat houses should have a rough-textured landing area extending at least six inches below the entrance. The entrance should also be at least ten feet above ground to ensure safety from climbing predators (raccoons, rats, house cats). A metal predator guard will help keep house cats and other predators from climbing wood poles or posts.

Bat houses should not be lit by bright lights.

Wherever bat houses are located, make sure there is a few feet of vertical clearance under the bat house. When exiting, bats tend to fly straight down and then move horizontally.

Bat houses are most successful where bats are already attempting to live in buildings. My single partitioned bat house was installed over a hole where a maternity colony entered between the chimney and siding. Although in shade until mid-afternoon, this bat house benefits from being located high and under an eave where heat is captured. The colony that was excluded now occupies a rocket box; bachelor bats call the new box home for the summer.

Data from BCI indicate that bat houses should be mounted on buildings, posts, or poles. Wooden or brick buildings with proper solar exposure are excellent choices for flat style bat houses because they absorb and hold heat. Houses mounted on trees are seldom used because they receive less sun, branches can make entry difficult for bats, and their aerial predators (hawks, owls) perch in trees.

Bat houses mounted on posts or poles are found more quickly if located along forest or water edges where bats tend to fly. If a bat house isn’t used in two full years, a new location should be considered.

As cute as they may appear, small houses are likely to not attract bats. I attribute the success of my bat houses to them surpassing BCI’s minimal criteria of being at least two feet tall and 14 inches wide. The number of roosting partitions is not critical, but they should be roughly textured and carefully spaced 3/4 to one inch apart.

The Mill Creek office of the Washington Department of Fish and Wildlife sells several styles of bat houses. Contact me at Linkrel@dfw.wa.gov or 425-775-1311 ext. 110 for information.

There are few things as gratifying as sitting comfortably in a chair, listening to the squeaking sound that proceeds bats emerging from their daytime roost, and then watching them exit one at a time. Watching bats return before daybreak may be fun, but that definitely would have to be done from the hot tub.

For information on bat walks and other regional bat related activities, check out Bats Northwest