Results from 1994 Season

The overall occupancy rate for our 1994 sample came in at 31%, a very encouraging result when you consider the facts: that an estimated 30% of occupancy does not occur until the second season or later, that half of all the houses reported this season were just put up in 1994, and that more than 80% of our total sample has been up for less than two years. Also, since we are experimenting with bat house design, treatment, and placement, we must expect that in most tests, some houses will not be accepted.

Size alone remains a consistent factor in success, as does knowledge regarding placement. Large nursery houses achieved 44% occupancy rates this season. Small houses (Models 1 to 5), many of which are either poorly built by commercial vendors or are sold without adequate instructions, achieved 17% occupancy, compared to 38% for larger houses that typically are built at home. Since current estimates suggest that at least 15% of houses not occupied in the first year do attract bats in the second, with an additional 15% within five years, we can reasonably expect our 31% occupancy rate to increase to more than 50% over time, with substantially better rates for larger nursery houses.

Current knowledge suggests that bat house locations or treatments should not be changed until at least two seasons have passed without use, unless there are obvious deficiencies. We especially need clear comparisons in which only one variable is changed. Often the best test is to repaint an unsuccessful house a lighter or darker color or to move it a short distance into more or less sun. When placing new houses, you can mount two or more side by side on buildings, varying color or design only, but all at the same height and receiving the same amount of sun. You may also place identical houses on opposite sides of a building at the same height or in locations where they receive more or less sun. Or, to test the potential effect of mounting substrate, locate identical houses to receive similar amounts of sun on trees, poles, or buildings.

Observations from the North
In the Spring 1994 issue of the Bat House Researcher we reported that occupied black bat houses from the northern U.S. (north of the 38th parallel) received an average of six hours of daily sun, while successful dark brown houses averaged nine. This suggested that dark houses exposed to too much sun can overheat, which agrees with this season’s findings. In the 1994 season, just 3 out of 20 black houses (15%) that received 9-13 hours of sun were occupied, compared to 7 out of 13 (53%) that received 8-9 hours and 11 out of 23 (48%) that received 5-8 hours.

In contrast, 4 out of 12 houses (33%) that were dark brown and received 9-12 hours of sun were occupied, as well as 30% of those exposed to mostly between 5-9 hours. Since these data cover the northern half of the United States as well as Canada, some allowance must be made for temperature differences within that relatively wide range of latitude. These samples are small, and more testing is required, especially in the coolest climates, before reaching final conclusions. Nevertheless, for all but the coolest climates, medium to dark brown seems likely to be the color of preference for houses exposed to more than nine hours of full sun.

Black may be advisable only in the coolest climates or where houses receiving eight hours or less of sun require greater heating. When in doubt, you can test pairs that include light to medium brown against dark brown, or try turning pairs on poles to face north and south so that one is always partially shaded. Most of the houses reported in 1994 were not vented or did not have both front and rear vents. Addition of vents should alleviate overheating, but to what extent remains to be tested. If you believe overheating might be a problem with your unoccupied houses, it’s never too late to drill ventilation holes or to paint them a more appropriate color, though some may still require relocation.

Surprisingly, 17 out of 78 light brown bat houses (22%) used in the northern half of the U.S. were occupied, three by nursery colonies. However, consistent with current knowledge, 76% of these houses received 5-12 hours of daily sun, most more than 8 hours. The four houses in this sample that received little or no sun were all hung on moderately dark buildings that appear to have been radiating heat to the boxes.

Observations from the South
In the South, 25 out of 66 bat houses (38%) that were left a natural wood color—mostly light brown—were occupied. Twelve out of 20 houses (60%) that received 9-12 hours of sun attracted bats. However, 7 of 13 such houses (54%) were successful in complete shade. Given these results, it is quite surprising that only 6 out of 33 houses (18%) that received 2-8 hours of daily sun were successful. One possible explanation is that different southern species have different preferences, and this is supported by reports that only small Myotis and evening bats (Nycticeius humeralis) have been seen in the shaded houses. In Central Texas, Sunny Sanders moved a shaded bat house that had been unoccupied for five years to a position receiving morning sun, and approximately 300 Mexican free-tails (Tadarida brasiliensis) moved in within weeks. Similarly, Amanda Lollar in northern Texas found that her few failures in accommodating free-tails all involved white bat houses that were receiving just one to five hours of sun, while her light brown and darker houses with more sun (that were also mounted on brick buildings) were consistently occupied. Nevertheless, at Carol and Baxter Adams’ home in southern Texas, hundreds of free-tails have chosen to live in two completely shaded bat houses. Perhaps these bats are simply desperate. Amanda, Baxter, and Carol will provide choices to test this and other possibilities this year. We clearly have more to learn.