2001 Bat House Research Project Survey Results
Mark and Selena Kiser
Results of our ninth annual bat house survey are in, thanks to our many Research Associates who contributed. Of 724 forms received for the 2001 survey, we were able to use information for 665 bat houses. These were monitored by 141 participants in 36 states, three Canadian provinces, and the Cayman Islands. Eleven species were confirmed using bat houses (Table 1), including for the first time the endangered Indiana myotis (See pages 1-3). Small numbers of southeastern myotis (Myotis austroriparius) were also observed in bat houses in Georgia and Florida, although these houses were not included in our analysis. Estimates by Research Associates put the total number of bats using surveyed bat houses between 19,454 and 26,651 bats. Among the 345 occupied bat houses where bat numbers were counted, the average occupancy was 56.4 bats (median: 11 bats, range: 1 to 600 bats).
Bats were using 393 of the 665 houses surveyed, for an overall 2001 occupancy of 59%, which is consistent with our surveys in 1999 (60%) and 2000 (61%). These figures represent all houses reported, regardless of design, construction, or placement. Success rates are usually much better when recommendations are followed and houses have been up for at least one year. In this survey, 77% (233 of 304) of bat houses were occupied that were installed prior to 2001 in groups of two or more, on buildings, similar structures, or poles, a finding that demonstrates the importance of mounting substrate and the advantage of multiple houses. Sites with three or more bat houses were nearly twice as likely to have occupants as sites with only one house (Table 2).
Use Patterns
Research Associates confirmed five species raising young in bat houses, with three others suspected of doing so (Table 1). Most of the 223 occupied houses (where type of activity was known) contained maternity colonies (Figure 1). Some houses were used for multiple purposes, including both maternity and overwintering. The type of roosting activity was not determined for 170 of the occupied houses. In December, Research Associate Tom Mantey of Ohio observed a pair of big brown bats (Eptesicus fuscus) mating in a bat house. Of bat houses installed as part of exclusion efforts, 92% (36 of 39 houses) were inhabited by the
displaced bats.
Species Preferences
We observed some differences in bat house preferences among species and between the two North American bat families in our study. Speedsters of the bat world, free-tailed bats (family Molossidae) have long, narrow wings built for fast, high-altitude flight in open spaces. Mexican free-tailed bats (Tadarida brasiliensis) and Pallas’s mastiff bats (Molossus molossus) tend to choose roosts high above the ground, as they apparently need more room when initiating flight. Plain-nosed bats (family Vespertilionidae), such as little brown myotis and big brown bats, with their broader wings, fly more slowly and with greater maneuverability. So it’s not surprising that the average height of bat houses occupied by molossid bats was slightly higher above ground than those used by vespertilionid bats (Table 3).
While nearly all species seem to prefer houses within a quarter-mile (400 meters) of fresh water, molossid bats used houses that were farther from fresh water than vespertilionid bats. Free-tailed bats and Pallas’s mastiff bats were found in two houses each that were 1 to 2 miles (1.6 to 3.2 kilometers) from water, and one house with Pallas’s mastiff bats was more than 2 miles (3.2 kilometers) away. In parts of the Cayman Islands where fresh water is scarce, Research Associate Lois Blumenthal says that swimming pools may be important water sources for Pallas’s mastiff bats.
Mexican free-tailed bats were reported most often in nursery houses and large houses, perhaps due to their propensity to form larger colonies. A few free-tailed bats are suspected of using three rocket boxes in Texas, but this has not been confirmed. As rocket boxes have not been extensively tested where free-tailed bats occur, we encourage further experimentation (with boxes at least 15 feet [4.6 meters] above ground where possible).
Temperature and Mounting Technique
The best mounting sites for bat houses continue to be buildings, chimneys, and other heat-retaining structures, such as dams, silos, and bridges (Table 4). Although pole mounting is a more popular option for bat landlords, success is slightly lower. Trees are generally poor mounting sites, apparently because of shade, obstacles, and predators.
Pole-mounted houses appear to perform best where temperature fluctuations (from day to night) are relatively small. In general, these are regions where higher relative humidity prevents dramatic temperature drops after sundown. Using average daily high and low temperatures for July (obtained from The Weather Channel’s Web site at www.weather.com), we found that pole-mounted houses had greater occupancy (61%, 254 of 414 houses) when average daily fluctuations were less than 28°F (15.6°C). Where fluctuations were greater, only 40% (23 of 58 houses) were inhabited. In contrast, bat houses mounted on buildings (or similar thermally stable structures) in areas with differences of 28°F (15.6°C) or more were 82% occupied (36 of 44 houses). Examples of locations where pole-mounted houses appear to be less effective than those on buildings include California’s hot, dry, Central Valley, the Rocky Mountains, and other high elevation sites.
Bat houses in the survey ranged in elevation from 0 to 6,800 feet (2,073 meters). Occupied bat houses occurred as high as 6,400 feet (1,951 meters). Most bat houses in our sample (71%) – and most of the occupied houses (79%) Ñ were located below 1,000 feet (305 meters), where occupancy was 66% (311 of 470 houses). Only 72 surveyed houses (with 39% occupancy) were located above 2,000 feet (610 meters). In 2001, pole-mounted houses were not inhabited above 4,900 feet (1,494 meters), although in previous years several successful houses were reported in Wyoming as high as 8,280 feet (2,524 meters).
Rural and Agricultural Areas
Of the 665 reports submitted, most bat house sites (81%) were classified as rural, and occupancy for this group was higher than for urban/suburban houses (Table 4). Several species were found only in rural bat houses, including long-eared myotis (Myotis evotis), northern myotis (M. septentrionalis), and Yuma myotis (M. yumanensis). Indiana myotis and pallid bats (Antrozous pallidus) were each found in one house. The remaining species (Table 1) occurred in both rural and urban/suburban bat houses. Where rural bat houses were located within two miles (3.2 kilometers) of known bat colonies in human-made structures, 80% (177 of 220 houses) were inhabited.
Bat houses installed within a 1-mile (1.6-kilometer) radius of orchards performed particularly well (Table 4). This sample included 36 orchard sites in 16 states, 2 Canadian provinces, and the Cayman Islands. Several orchardists with occupied bat houses have reported reduced problems with pests such as codling moths and hickory shuckworms. Where orchards, pastures, and row crops were mixed together, occupancy was 84% (58 of 69 houses).
Urban and Suburban Areas
We are often asked if bat houses are successful in urban or suburban landscapes, where habitats typically have been altered severely. Results from our survey are encouraging. Fifty percent of houses located in urban or suburban landscapes were inhabited (Table 4). Residential and commercial areas had similar, though slightly better, occupancy. Success improved to 68% (34 of 50 houses) when colonies in human-made structures were located within 2 miles (3.2 kilometers). Of 12 houses installed as part of exclusion efforts in urban/suburban areas, all were inhabited by bats. Not surprisingly, three of the most common species found in urban/suburban bat houses were little brown myotis, big brown bats, and Mexican free-tailed bats. Pallas’s mastiff bats, common house bats of the Florida Keys and West Indies, were found more often in urban/suburban bat houses (13) than rural ones (7).
2002 Survey
If you installed a bat house before June 1, 2002, please participate in the 2002 survey. Your information is important regardless of whether or not bats have yet used the house. A Data Report Form is included with this issue, and you may also use one of two electronic forms on BCI’s Web site.
An easy-to-print form is available at www.batcon.org/bhrform/bhform.pdf and an interactive form is at www.batcon.org/bhrform/dataform.html. Using your mouse, you can also highlight the survey questions on the interactive version, then copy and paste them into a word processing file. Completed files can be e-mailed to BCI at skiser@batcon.org. Thanks for your participation and support.
Handy Tip: Metal poles can be substituted for wooden poles when building one- and two-chamber rocket boxes. Build a pole sleeve (which slides over the metal pole) from 1-inch (2.5-centimeter) lumber instead of using a 4 x 4-inch (10.2 x 10.2-centimeter) post.
Bat houses in or near orchards are especially successful, with 83% inhabited in the 2001 survey. Some orchardists report that problems with crop pests declined after attracting bats.
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