Successful Bat Houses Shed Light On Bat Needs

OVER THE PAST SUMMER we received 51 reports, mostly regarding bat houses that already had been installed prior to inception of the new research project. These were small houses, typically put up with minimal or no instruction. Results strongly support the conclusions published in BCI's Bat House Builder's Handbook.

Five houses in Maryland, Wisconsin, and New York that received sun for 8 to 12 or more hours daily were all occupied. Three of these were either painted dark or were covered with tar paper. The two that were left a natural wood color received 12 or more hours of sun. Four of the occupied houses were within a quarter mile of water, but one located between one and two miles from water attracted a small bachelor colony. The only nursery colony was located less than a quarter mile from the Potomac River and a small stream.

Twenty-two bat houses in other northern locations received less than four hours of daily sun, and none of them were occupied, clearly confirming the vital role of solar heating. Even in the South, only one of 11 occupied houses received less than four hours of daily sun, while nine that received little or no sun were unoccupied. The only exception was reported by Ernie Stevens from central Florida. His occupied bat house was hung in complete shade from a tree limb. Six bats were first noticed using it in January of 1993. The colony grew to 125 and reared young in spring, then departed in July.

In central Georgia, Ronald Spears put up four bat houses. Three that are shaded are unoccupied, while one in full sun on a utility pole in a corn field has bats. The house exposed to sun was used by 12 big brown bats (Eptesicus fuscus). Spears recorded an internal air temperature of 136°F midway between top and bottom of the 18-inch-tall house while the bats were roosting just inside the open bottom at 2 P.m. This indicates a greater tolerance for heat than expected.

Further evidence that bats need high temperatures in bat houses comes from Lisa Williams' continuing work in Pennsylvania. She recorded temperatures as high as 95°F for nursery colonies of big brown bats (Eptesicus fuscus) and 106°F for little brown bats (Myotis lucifugus). Her newest houses provide vertical ventilation slots one-half inch wide by three inches long on each side, beginning six inches from the bottom of the rear roosting chamber. During extreme heat, bats roosted between the ventilation slots and the open bottom.

In central Texas, David Bamberger mounted two of the large insulated nursery houses (Model 9) recommended in The Bat House Builder's Handbook back-to-back, a few inches apart, 18 feet up on a metal pole near a small lake on his ranch. A colony of approximately 30 cave myotis (Myotis velifer) moved in within two months. The east-facing house was painted dark brown and the western one white. On September 9, at 10 A.M., when the ambient temperature was 89°F, the bats were in the house facing west, which was largely shaded by the house facing east. They were occupying crevices with temperatures of 99' to 101°F. At 5 P.m. on the same day, when the ambient temperature was 98°F, the houses were rechecked. The bats had moved to the east-facing house where temperatures ranged from 102' to 104°F. At that time the temperature where they had roosted in the morning was 115°F. The advantage of placing pairs of houses back-to-back, facing east and west, is obvious. Two mostly shaded houses in similar habitat on the same ranch attracted only five bats (M. velifer and Tadarida brasiliensis).

Sunny Sanders, also from central Texas, placed a single multi-chambered bat house (Model 5) just below the apex of a tin roof on the west end of her home, where it was shaded all day. It remained unoccupied for five years. In June 1993 she reported her failure to BCI. We advised her to relocate it where it would receive at least some morning sun, so she moved the house to the east end of her home, again positioning it just below the apex of her roof. This time it received at least four hours of morning sun, and by mid-August approximately 300 Mexican freetailed bats (T. brasiliensis) had moved in. In fact, there were so many that dozens could not get in. This illustrates the importance of proper location relative to solar exposure and further indicates that even southern bat houses may need more sun than previously understood.