New Findings

Analysis of Project Data— Individual success varied from 37% to 71% in the two largest samples, based on 35 and 21 houses respectively. There was no detectable difference in success between northern and southern areas within the United States. Because bat needs for solar warming vary with climate, we divided the new sample into locations north versus south of the 38th parallel. Eighty-six percent of failures involved houses that lacked paint or that received too little daily sun. Based solely on too little sun, 83% of southern and 46% of northern failures could have been predicted.

In the northern sample, 40% of the 40 houses that were painted dark brown or black were occupied, regardless of other considerations. All sixteen that additionally received four or more hours of sun were occupied. Solar exposure and dark color appear to be especially important in Canada. Gerald Yetman of Newfoundland placed 17 bat houses in apparently good habitat, but attracted bats only to the one he painted black and moved to face south. His others were left unpainted and, on average, received less than four hours of daily sun. Both dark color and 6-12 hours of daily sun may be essential in Canada, though occasional success has been achieved with less sun if houses are black. In the northernmost U.S., successful black houses received an average of six hours of daily sun, compared to nine for dark brown, suggesting, as expected, less need for solar exposure for the darkest houses.

In the southern sample, houses ranging from white to brick red were used, with varying sun exposure and mounting substrate. Of 68 comparable houses, occupancy rates averaged 33-34% in those receiving less than four hours of daily sun or more than six hours, versus 66% for those exposed between four and six hours. These results may be achieved by mounting houses back-to-back, facing east and west, on poles. The exact color most likely to succeed in the South depends on the amount of sun exposure and may also be influenced by the mounting substrate. Available evidence suggests that light brown houses exposed to four to six hours of sun may be best in the South, though more testing with white and medium brown is needed. Protection from direct, midday sun is desirable.

In a nationwide sample of 60 occupied bat houses categorized as receiving morning versus afternoon sun exposure, the former were 10% more successful. More data are required relative to bachelor versus nursery use of houses before this observation can be fully interpreted. We have few data thus far for paired houses on poles, but suspect that east-west orientation may be better than north-south. However, this still needs to be carefully tested.

From a sample of 91 painted or stained houses versus 97 left plain, we found a significant preference for painted or stained houses, regardless of other considerations. These were occupied 2.4 times more often, accounting for 69% of all occupied houses. Aside from proper solar exposure or color, paint may help protect bats from moisture and unwanted ventilation.

Individual Observations-Jack Davis put up 35 bat houses in seven campgrounds in the M&C Wildlife Management Area in northern Minnesota, achieving occupancy rates ranging from zero to 65%, mostly from 40-51%. He attracted approximately 60 little brown bats (Myotis lucifugus) to one house, but the majority were occupied by smaller bachelor colonies that he believes included at least a few eastern long-eared myotis (Myotis septentrionalis).

Through his careful observation and reporting, we were able to determine that most of his houses were too cool. Their large, flat roofs, inch-thick lumber, and rectangular shape reduced solar heating in their partly shaded locations, despite being painted black. In those mounted on the sides of warmer buildings, the bats roosted mostly in the rear compartments, which likely received radiant heat from the buildings. Most of Jack's varied success apparently resulted from differences in sun exposure. This summer, he will test BCI's nursery designs in pairs oriented east-west on poles along open lake shores, in addition to moving his least successful houses to sunnier locations.

One of Jack's experiences well illustrates the importance of attention to detail. He put up a house in 1992 that was painted with a single coat of black paint and was used by little brown bats. However, when he put up another nearby house in 1993, they abandoned the first house for the new one. The second house was identical except for having been carefully caulked, painted with two coats of paint, and covered with tarpaper. It was far better sealed against air circulation and moisture, apparently accounting for the bats' move.

John Ragan's observations from near Detroit, Michigan, also suggest the need for solar heat in the North and that radiant heat from the buildings to which bat houses are attached, can make a difference. His two small bat houses, which are attached to a garage, are used primarily on hot days. On cool 60°F days, many of his bats retreat into the warmer garage. Those that remain in the bat houses roost almost exclusively in the rear chambers, where stored heat from the garage warms the crevice.