Observations on Attracting Free-Tailed Bats

The observations of Research Associates across Texas converge on one conclusion: Free-tails can be attracted to bat houses in large numbers. But available evidence also suggests that free-tails may not occupy some acceptable houses, or may take years to find them, perhaps because they are not located in places where the bats normally search for roosts. When excluded, displaced adult free-tails readily move into a nearby bat house. Other houses may take longer to find, or perhaps remain empty until existing roosts become overcrowded, forcing younger bats to seek new quarters. Several recent Research Associate experiences illustrate the probability of such behavior.

Experiments in Progress
Amanda Lollar of Mineral Wells, Texas, had great success in attracting excluded colonies to her bat houses in 1993 (The Bat House Researcher, Spring 1994). Each time Lollar mounted a house on the side of a building near the former roost of an excluded colony, the free-tails moved in the next day. However, it took until the second summer for them to fill completely with bats. In 1993 her houses held an average of 80 bats, 250 maximum. This year the same houses held 500 to 1,000 bats each, depending on the size of the house.

In May of this year, Lollar put up 13 additional houses. The first three were identical to those used in the previous season. She located one immediately adjacent to where a free-tail colony had been excluded in 1993, one within three feet of a window sill already occupied by free-tails, and one on the side of a building across the street where bats had not previously been able to roost. The house that was mounted where bats had been excluded was occupied in less than a week, and the one near an existing roost was occupied within two weeks. However, the house where bats had not previously lived was still not occupied in early October.

Lollar mounted the remaining 10 new houses in pairs (five sets) on poles within 100 feet of where free-tails were living in buildings. Two pairs were of Lollar’s original design, which had worked well on buildings in 1993. She mounted these on large, roughened backboards in case ease of landing might be a factor in success. The additional pairs were BCI’s beginner’s and nursery house designs, and they were mounted back to back with just six-inch landing areas. All 10 houses were put up in late July, but none had been occupied as of early October, despite being in a location where free-tails routinely fly by them.

One might conclude from this experience that free-tails simply like houses better on buildings—perhaps because buildings help buffer temperature fluctuations. However, volunteers from Motorola Corporation put up identical pairs of BCI nursery houses at about the same time in Austin and near Bracken Cave and attracted small colonies within several weeks in both places. It will be interesting to track occupancy of all these houses through the 1995 season.

Success Despite Long Odds
This past summer bats moved into a Texas-style bat house near Dallas that had remained unoccupied for two years. The house was four feet long by approximately two feet wide and high, painted white and shaded by an overhanging tin roof. We had decided that the design was inadequate or perhaps too cool. But the arrival of several hundred bats (probably free-tails) last summer led us to question whether it was indeed a poor design or whether it simply took a long time to be discovered or needed. Undoubtedly, bats need houses more in some areas than in others, and those needs change over time as habitat is altered
and roosts are created or destroyed.

On a ranch near Harlingen, Texas, two other large bat houses remained unoccupied for two years. A prior survey using bat detectors and mist nets found no evidence of free-tailed bats on the ranch, which meant that the only potential occupants of these houses would probably be migrating bats. But if migrants did pass by, chances for occupancy were good, since the location had both water and a large insect population and was more than 10 miles from even remotely acceptable alternate roosts.

The houses were four feet long by two feet wide and two feet tall. They had peaked roofs and enclosed attics over what was basically an inverted box filled with partitions, with mostly three-quarters to one-inch-wide crevices. They were all made of rough, unpainted cedar that weathered to a dark brown color. Each house was mounted 12 feet up on a large pole near a pond and was fully exposed to sun.

After free-tails finally moved into the houses, they began moving out on the hottest days, when the houses apparently overheated. At such times, the bats had to roost in risky, exposed locations on nearby mesquite trees for several hours. This observation illustrates that not only can bats be attracted to new areas if the habitat is right, but that they will even tolerate marginal conditions if better options are unavailable. In this instance, location of the houses near remote water holes likely aided in the eventual discovery and occupancy. One of these houses will soon be painted a lighter color to test whether a cooler house will better accommodate the bats’ needs.