Protection from Predators

All bat house owners need to be aware that predators can be a serious problem for bats. Research Associates have reported that bats sometimes abandon their houses because of hawks, owls, snakes, and house cats homing in on their territory. Although it is normal for bats to move among roosts within a season, when they do not return at the same times in subsequent years, predator problems may be the cause (assuming your bat house has not developed leaks, another common problem). Last summer, when we took a television crew to film the hundreds of bats Carol and Baxter Adams have attracted to their bat houses near Medina, Texas, we found a rat snake instead of bats in one of their houses. Unfortunately, the more bats you attract, the more likely predators are to follow.

Because bats enter and exit roosts rapidly, owls typically need to watch from a nearby perch to be successful. This may explain why purple martins and many bats prefer house locations in open areas away from branches and other potential perches for birds of prey. Mounting evidence suggests that bats prefer houses at least 20 feet from potential owl perches, a problem most often encountered when houses are mounted on trees. Tree-mounted houses are undeniably more vulnerable to predators than are those mounted on buildings or poles. Houses located in open areas, but within 20-50 feet of river, lake, field, or yard edges seem most attractive to bats.

Snakes are most troublesome in warm climates.While they typically cannot climb the side of a smooth building, they are expert climbers of trees and poles. Most can be stopped by predator guards sold by wildlife catalogs such as the Purple Martin Conservation Association Products Catalog (814-734-4420), which includes Top Guard Animal Barriers for $17.95 each. You can make your own barrier by cutting the bottom out of a round, five-gallon waste paper basket, replacing it with 1/4-inch hardware cloth, and running your bat house pole through the center of the hardware cloth with the basket inverted to face downward. Smaller baffles may work fine if local snakes are not large. Climbing raccoons can also be foiled by a snake guard placed at least three to four feet above ground.

Cat problems can be reduced by placing houses between 12 and 20 feet above ground on buildings or poles. Cat predation is usually the worst when young bats, especially Mexican free-tails, are learning to fly.

Predator avoidance is a primary reason why so many North American bats prefer to roost in locations entered through 1/2-inch to 3/4-inch wide crevices. We also find that many bats roosting in woodpecker holes squeeze into equally narrow crevices after entering through larger openings. Bats frequently reject houses with roosting crevices that exceed 3/4 inch in width. Bats that accept wider spaces typically choose high places in buildings, caves, or tree hollows that are difficult for predators to reach without detection.