Successful Pallid Bat House Design in California

Greg Tatarian

During the past ten years, my wife Trish and I have provided a wide range of wildlife consulting services in California, including habitat enhancement and wildlife damage control. In that time, we have observed bat houses growing increasingly popular among homeowners and farmers. Because most of the locally purchased bat houses used by our clients were too small to provide wide temperature ranges, we decided to develop larger houses better suited for maternity colonies. Using our observations of bats roosting in buildings, we began designing our own bat houses in 1994. One of these designs has attracted long-term use by maternity colonies of the Mexican free-tailed bat (Tadarida brasiliensis) and the pallid bat (Antrozous pallidus), a Species of Special Concern in California and a species seldom reported from artificial roosts [The Bat House Researcher, Fall 1999]. One pair of our bat houses currently shelters 300 pallid bats, the largest colony of this species known in any artificial roost.

Several features make our design unique. We incorporate vertical chambers in the lower section of our houses, topped by angled chambers in the roof section. The angled roof chambers allow pups to cling more easily, reducing mortality caused by falling from the roost. Combining both roof chambers, which absorb extra heat, and cooler vertical chambers permits bats to select or escape high heat levels, depending on their preference. Entrances in both sections provide several points of access and double as vents to enhance the range of temperature. Because pallid bats, big brown bats, and several other crevice-dwelling species of differing sizes are present in our area, we vary chamber spacing from 3/4 to 1 3/4 inches (1.9 to 4.4 cm).

In February 1995, I installed one of these houses on the exterior wall of an office building in Napa County where owners had evicted a colony of free-tailed bats with partial success. Within several weeks, most of the remaining 40 free-tailed bats had moved into the bat house, and after six weeks the colony had expanded to an estimated 90 individuals. Unexpectedly, 15 pallid bats also moved in. These had not been in the building originally. By late July, while CNN was producing a story on our habitat enhancement work in vineyards, I observed about 60 adult and 20 juvenile pallid bats in addition to 15 to 20 free-tailed bats. We mounted a second similar bat house on the building in 1997, and it was occupied by both species within weeks. The population of pallid bats soon increased dramatically, with numbers ranging from 200 to 300 individuals, while free-tailed bats increased from 400 to 600, with substantial seasonal fluctuations.

Although these two houses successfully attracted maternity colonies, problems did occur, which were solved by regular monitoring and making appropriate changes promptly. Periods of elevated temperatures in 1997 caused abandonment and mortality of large numbers of pups. These episodes prompted us to enlarge the entrances to increase airflow. Also, front panels were extended, making the houses taller, and partial bottom covers were removed. The bottom covers, initially added to conserve heat and deter uninvited guests, allowed guano to build up inside, blocking air movement and reducing the amount of available roosting space at the bottom of the houses (where temperatures were coolest). With 600 to 800 bats present at times in summer, little room was available to escape overheating. As a further precaution, the houses were moved from the side of the building in 1998 and placed back to back on metal poles in an area partially shaded by trees.

The modifications and/or moving the houses seem to have been successful, as for the past three years, numbers of bats have slowly but steadily returned to about 200 free-tailed bats and 300 pallid bats. Given the population reversal, the resulting cooler temperatures appear to have benefitted pallid bats, though proving less than ideal for free-tailed bats. Although commonly found in deserts, pallid bats have been known to abandon roosts when temperatures exceed 100¼ F
(38¼ C).

Larger roost chamber sizes in our design also seem particularly well-suited for pallid bats, which we have seen mostly in 1 1/2- and 1 3/4-inch (3.8 and 4.4 cm) chambers. This suggests that where pallid bats occur in the western U.S. and British Columbia, bat houses could be constructed with a variety of crevice sizes up to 1 3/4 or 2 inches (4.4 or 5.1 cm) to accommodate larger pallid bats. Remember, though, chambers greater than 3/4 inch (1.9 cm) are more inviting to wasps, birds, and squirrels, so extra vigilance is required to keep out these unwelcome guests.

In our business, we only encourage use of bat houses where they will be monitored and protected over long periods. Had we not checked houses regularly, problems with our original design would have gone unnoticed and additional mortality would have occurred. It is important to choose mounting sites carefully, to closely observe your houses, and to make any necessary changes to meet your local bats’ needs. If you wish to try our design, see Figure 1 on page 3. We thank Saintsbury Winery, Pina Vineyard Management, and Shafer Vineyards for their important contributions to our research into pallid bat house designs.

Greg Tatarian, a bat specialist, and his wife Trish, a general ecologist and amphibian specialist, are co-founders of Wildlife Research Associates in Petaluma, California, providing wildlife and ecological consulting services to winegrowers, public agencies, consulting firms, planners, and others.

The pallid bat, found throughout the western United States and south-central British Columbia, is thought to be declining throughout much of its range. It is a Species of Special Concern in California, and is considered vulnerable in five other states according to the Association for Biodiversity Information and the Natural Heritage Network.

Figure 1: The Tatarian pallid bat house includes angled roof chambers as well as vertical chambers, which provide a large temperature gradient from top to bottom. With the extra heat gain, the roof chambers create an ideal nursery area where pups can develop quickly and are less likely to fall out. The standard vertical chambers provide cooler zones for bats to escape heat on extra hot days. Steeper roof slopes can be substituted to enhance guano “fallout.” Roof, front, sides, and back pieces are made from 3/4- inch exterior plywood; partitions are 1/2- inch exterior plywood. Design by Greg Tatarian, 1995-2001. Drawn by John Wilcox.

After several overheating episodes, this pair of houses was modified (to improve airflow and reduce guano build-up) and moved to a partially shaded area. Prior to the move, free-tailed bats outnumbered pallid bats two to one. Afterwards, despite initial drops for both species, numbers rebounded with more pallid bats than free-tailed bats. The cooler temperatures apparently favored pallid bats. Numbers of free-tailed bats likely would increase if the houses had more sun exposure.