Fall 2004

Table of Contents

Protecting bat house from predators
By Russell Link

It was shortly after dusk on a warm July night that I heard an unfamiliar noise coming from the roof above the bedroom.

With flashlight in hand I quietly went outside to investigate, thinking I’d probably encounter a raccoon. To my dismay I found Mika, the neighbor’s cat. She was perched on a bat house that for the past two years contained a nursery colony of little brown bats. When I returned with my camera, Mika was below the bat house swiping at bats as they exited to forage.

During the 30 seconds I observed this activity, the bat house appeared wide enough to prevent the bats from snagged. However, my anger overcame me and I flung a handful of crushed gravel in the direction of the cat. Mika fled into the darkness.

As a bat house owner you need to be aware that predators can be a problem. Bat Conservation International Research Associates around the world have reported that bats sometimes abandon probably explains why bats prefer bat their houses because hawks, owls, cats, raccoons, and snakes disturb the area in or around their bat houses.

Although bats move among bat houses 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 or the crevices haven’t warped.

Predator avoidance is a primary reason why bats prefer to roost in locations entered through the 1/2-inch to 3/4-inch wide crevices suggested for bat houses. In fact, bats often reject houses with roosting crevices that exceed 3/4 inch in width. Even bats roosting in woodpecker holes squeeze into narrow crevices after entering through larger openings.

In Washington, Cooper’s hawks and sharp-shinned hawks are capable of catching bats as they emerge from bat houses at dusk. These hawks are often seen in the vicinity of bird feeders, where they catch birds in flight.

Later in the evening, great horned owls and other owls are known to grab bats roosting on branches, or as they enter bat houses. Presumably to lessen the risk of being caught by owls, some bats elsewhere in the world drastically reduce their activity level during bright nights around full moon, a behavior known as around full moon, a behavior known as lunarphobia.

Because bats enter and exit bat houses rapidly, a hawk or owl needs to perch nearby to make a successful catch. This probably explains why bats prefer bat house locations in open areas away from large dead branches and other perches commonly used by birds of prey. Indeed, data suggest that bats prefer houses at least 20 feet from such perches. This is why bat houses mounted on trees are less often used than those mounted on the sides of buildings or on predator-proof poles located in the open. (Tree-mounted bat houses are also more vulnerable to climbing predators.)

Occasionally a hawk or owl will perch on a bat house located in the open. Although this doesn’t always cause a problem, it has scared bat colonies away from bat houses.

A variety of devices can frighten a hawk or an owl: increasing human activity in the area will keep them at a distance; yelling and clapping hands and banging cans together are all effective when a bird of prey is seen near a bat house. For obvious reasons these control efforts are rarely long-lasting. The best solution is to create a barrier that prevents birds from landing on top of the bat house.

Metal or plastic spikes have long been used to keep pigeons from roosting on ledges, window air-conditioning units, and similar sites. Adopting this idea, 20-inch long spikes made from 1/8-inch galvanized wire can be attached to the top of a bat house. Insert the wires into holes drilled in a piece of lumber attached above the roof. Radiate the wires out from the center in all directions like pins in a pincushion.

Commercial products available from farm supply centers and bird-control supply companies on the Internet include Catclaw®, Bird-B-Gone®, and Nixalite® (porcupine wire). Metal coils (e.g., Bird Barrier®) and even a slinky toy can function similarly. Another homemade technique is to tightly string single-strands of steel wire (16-18 gauge) or monofilament line (80-pound test) between L-brackets installed at each end on top of the bat house.

Domestic house cats, raccoons and perhaps opossums can be prevented from accessing bat houses by placing them as high up the sides of buildings as possible.

To protect free-standing bat houses from mammalian predators, a predator guard can be installed around a tree, pole or other support used to elevate a bat house. Several variations are available on the Internet using the key words “bird predator guard.”

While most Washington snakes are strictly ground dwellers, the racer, whipsnake and gopher snake will climb in the lower branches of shrubs in search of birds and bird eggs. A bat on the ground or perched in a tall shrub would unlikely be overlooked. However, no Washington snakes are likely to find their way up and into an elevated bat house.

The bat house above the bedroom has now been raised 8 feet above the roof and has two predator guards wrapped around its support poles. Last I looked (6/16/04) there were 24 bats occupying the house. Mika has not been seen nearby.