Extending an Invitation to Bats

Putting up a bat house can be an educational opportunity for the whole neighborhood—

Bat houses have been around almost as long as bird houses, but only recently has the idea become popular with backyard nature enthusiasts. The concept of attracting bats to artificial roosts originated with Dr. Charles Campbell who conducted the first documented experiments with bat boxes in about 1900 as part of an antimalarial campaign in San Antonio, Texas (see article beginning on page 10). The idea quickly spread throughout Europe after two 30 foot tall roosts were built in Italy according to Campbell's plans. Over the next 20 years Europeans conducted their own experiments with smaller bird house sized designs, eventually building tens of thousands. Most were placed in national forests, and although many were used, their relative success was not well documented.

In the United States, experimentation with small artificial roosts for bats was basically abandoned until the early 1980's when Bat Conservation International began distributing plans based largely on European designs. At about the same time, the Missouri Department of Conservation developed plans for the first compromise between bird house sized boxes and the huge Campbell bat towers.

Current bat house designs were developed to accommodate bats living in north temperate regions of North America and Europe. Most are intended to appeal to as many species as possible, instead of accommodating the specific needs of any one. Nearly all bat boxes currently used in the U.S. and Canada are designed to accommodate bats that roost in tight crevices, in part because crevice roosters are those most often encountered in urban areas. Other species prefer open ceilings, allowing them to form clusters. Many European bat boxes cater to these, though most such boxes are still quite small.

Bats sometimes use bluebird houses as temporary roosts as well. On a 73 box trail in Illinois, bats occupied seven of the boxes from early September until mid-October. And in Kentucky, researchers confirmed two instances of Keen's bat (Myotis keenii) using slotted entrance bluebird boxes. The use of bluebird boxes by bats may be more frequent than known, since most observers are not trained to identify bat droppings.

Learning what bats need
Nearly half the bat species in the United States and Canada are of the type that could conceivably occupy artificial roosts. Greater knowledge of their roosting preferences is urgently needed as many bats are rapidly losing their traditional roosts. Old trees that provided bats with loose bark or hollow interiors are being lost to modern forestry practices and urbanization. Bats that moved to old wooden buildings or bridges are losing these as well. New building materials and designs often exclude bats, even from barns and bridges which once sheltered large colonies.

Given these circumstances, one might expect that bats would quickly move into backyard bat houses, but as many bat house owners know, this is not necessarily the case. There are several probable reasons. First, bat needs vary greatly from place to place. Imagine that a large attic or other roost in your area shelters all the bats that the local food supply can support. If an artificial roost is made available, it simply may not be needed and could remain unused for years. But if the original roost is suddenly destroyed, or the bats are excluded from it, then the long unused artificial roost may suddenly be needed. On the other hand, if such an alternate roost is unavailable, the homeless bats may die or abandon the area altogether. If a bat house is put up months or even years later, new bats may take a long time to find the house.

Even in an area where the food resources can support more bats than are occupying available roosting space, bats are unlikely to occupy any new roost until they are looking for a home. This is most likely to occur in late summer when existing colonies are rearing young. If the roost has exceeded its capacity, many of these young must leave to find new homes or die.

A classic example happened in Illinois where a large Missouri-style bat house, capable of sheltering up to a thousand or more bats, was built near a garage with a loft full of bats. The original plan had been to evict the bats from the garage with the hope that they would move into the bat house, but the owner didn't get around to bat-proofing. The colony gradually outgrew his garage loft, and the overflow soon filled the bat house. Now he has bats in both places. Were the owner to evict the bats from his garage, anyone with a bat house in the immediate area might suddenly find it occupied.

In another case, a small BCI bat house was given as a gift to a family in a Minneapolis suburb. It soon became obvious that more bats in the area were in need of roosts than a single bat house could accommodate. The family reported that not only was their bat house full, but that bats were lined up down the tree and waiting to get in!

Expecting guests
Typical experience in both North America and Europe indicates that artificial bat roosts, with few exceptions, do not have occupants for one or more years. A recent survey of backyard bat boxes in the United Kingdom, most of which had been in place for less than two years, indicated a 9% occupancy rate. In another ongoing project in the U.K., one instance of a bat entering a house the first evening was reported; five months later a small colony was established. The same bats may also occupy a house year after year. Over a 14 year period of observation, one banded individual was found ten times at the same site.

As with bird houses, improper placement of a bat roost can reduce the probability of success, but even some inappropriately placed houses are used. Occupied bat boxes have been located as low as six feet above the ground, directly over doorways with frequent disturbance, on the trunks of large shade trees where they remain damp and cool, or facing north where they rarely receive the warmth of sunshine. One BCI member recently wrote to ask how long he should wait before moving his occupied bat house from over his back door. He was delighted with the bats but not with having to sweep up the droppings every morning! A Pennsylvania member called BCI to report that he had built three bat houses, all facing north. He mentioned that after the first year he didn't have bats. We were about to recommend that he move them, when he told us that bats had moved into all three houses during the second year.

One difficulty in assessing occupancy success is illustrated by a member's report from Duluth, Minnesota. After not noticing bats in his bat house for the first three years, he installed an air conditioner directly below and soon found fresh droppings on the smooth surface, leading to the discovery of his bats. In another instance, a member from Milwaukee, Wisconsin built three bat houses, none of which were used for the first three years. After he had given up watching for bats, one evening he noticed there were bats in all three.

Bats may also use a house seasonally, occupying it for only a few weeks at a time during migration. A new bat house may be used only at night or for just a few days. If yours is mounted on the side of a building, watch for droppings on the wall just below the entrance, or place a small sheet of plastic on the ground. Bat droppings will be pellet-shaped, usually about a quarter of an inch long, uniformly dark brown, grey or black. If your bat house has an open bottom, you could occasionally shine a flashlight in to see them. At first, do this only very briefly and quietly to avoid frightening them into leaving.

Thus far, most occupied bat houses have been placed between 10 and 25 feet above the ground, facing east or southeast. If one is occupied, others in the same area seem more likely to succeed. Early reports indicate that the probability of occupancy varies greatly from area to area. In some places 10 or more houses are still vacant after three years, while several members who have put up two or three houses have had all occupied within one to two years. The reasons for such variance are many and may not be due solely to placement. Environmental pollution and overuse of pesticides, inadequate surrounding habitat and a lack of required hibernation sites may preclude use in some areas. But in other cases, the local bats may not be the type that roosts in man-made structures, or they may have requirements not met in the bat house provided. In general, bat houses are most likely to succeed where bats already are known to roost in buildings or bridge crevices, especially where such options are being lost.

Building bigger bat roosts
Even the earliest builder of bat houses, Charles Campbell, noted that at least some species preferred large structures placed as high as possible above the ground. In areas where free-tailed bats are the dominant species, height above the ground may be especially important. These bats generally prefer roosts at least 15 feet or higher, well away from foliage or other obstructions.

Thus far, no type of artificial bat roost has achieved anywhere near 100% occupancy, but the largest ones seem to be favored. Campbell's roosts were some 30 feet tall with space enough inside to accommodate many thousands of bats. Those placed in appropriate habitat were highly successful, even though his internal roost designs appear to have been less than ideal.

Missouri-style bat houses are the largest wooden structures currently built. Elevated on four posts, the roosting area is about seven feet long by two feet wide and a foot deep. Of the first 12 built at least three years ago in Illinois, Minnesota, Missouri and North Carolina, five (42%) have been occupied by colonies that have returned and grown each year. Occupancy has been nearly 100% where bats have been excluded from a nearby building after placement of the new roost. Although occupancy of Missouri-style bat houses is high where bats are evicted, improvements might increase their odds of success in other situations.
One of the world's most successful artificial bat roosts was constructed by accident in Austin, Texas under the Congress Avenue bridge when it was remodeled in 1980. Expansion crevices under the bridge from a half-inch to four inches wide run the length of the bridge, 16 inches deep in the concrete and about 20 to 30 feet above the Colorado River. The crevices are now occupied by an estimated three quarters of a million Mexican free-tailed bats, providing an unusual opportunity to investigate this species' roosting choices. Despite the wide range of crevice widths available, nearly
all roosting bats live in crevices 3/4 to 1 inch wide, demonstrating the importance of this variable.

An opportunity to learn
Learning more of what bats need becomes increasingly important as bats lose their traditional roosts in many parts of the world. A great deal can be learned about bat roosting preferences by examining existing use patterns where bats live in buildings, bridges, abandoned mines and other artificial structures. For each species, we need to know: 1) Where do the bats choose to live relative to foraging habitat and water? 2) How large a structure is normally required? 3) What are the average internal dimensions of roosting areas (i.e. a room of at least a certain height and diameter or a crevice of at least a certain width and depth)? 4) How high are roosts above the ground? 5) How are they oriented relative to sun or wind? 6) Are chosen sites serving as heat traps, and what are the preferred and tolerated temperature ranges? These choices vary among species and may change for a single species, depending on latitude and elevation. Understanding them is the key to building better artificial bat roosts.

Much of our knowledge of bat roosting boxes comes from member reports. Your success or failure can add to scientific knowledge, while helping us to better meet bat needs. Nearly everyone agrees that bat houses make extraordinary conversation pieces, making them exceptionally effective tools for educating friends and neighbors about bats. One member stated emphatically that it wasn't important whether she attracted bats to her bat house or not; having one attracted questions and that was equally important. Each time you put up a bat house you're both a conservationist and a scientist conducting an experiment. We especially encourage you to make your own observations and test your own designs. By placing a bat house in your yard and reporting on the results, you can participate in important research into bat needs.

Merlin D. Tuttle is Founder and Executive Director of Bat Conservation International. BCI has offered various designs to build bat houses since 1982 and in 1986 offered the first commercially made bat houses.

For a 12-page booklet that covers bat house basics and answers many of the most commonly asked questions, send $1.50 for "About Your Bat House" to BCI, P.O. Box 162603, Austin, TX 78716.

Above: Most occupied bat houses are facing east or southeast and place between 10 and 25 feet above the ground. PHOTO BY MERLIN D. TUTTLE

In the West and Southwest, Pallid bats may be one of the species to use bat houses. Often found in buildings, these were spotted night-roosting in the rafters of a desert barn. PHOTO BY MERLIN D. TUTTLE

Several states are experimenting with the Missouri-style bat house (below) in their parks. The roosts can shelter a thousand or more bats. PHOTO BY EARL N. JOHNSON, MINNESOTA DNR

Variations on the design include the privately owned bat roost above on the Guajillo Ranch in Texas. It sports a bat weather vane and is elevated to attract the most abundant local species— free-tailed bats. PHOTO BY MERLIN D. TUTTLE

Found throughout most of North America, Big brown bats are likely occupants in bat houses. Along with Little brown bats, they are the species most often encountered by people. PHOTO BY MERLIN D. TUTTLE

One of the world's most successful artificial bat roost was inadvertently created when the Congress Avenue Bridge in Austin, Texas was remodeled in the 1980. The bridge houses an estimated 750,000 Mexican free-tailed bats from spring through fall, making the evening emergence a popular tourist attraction. PHOTO BY MERLIN D. TUTTLE