This article has been adapted from the new issue of Mississippi Farm Country, the official publication of Mississippi Farm Bureau. The magazine is provided as a Farm Bureau Member benefit. Click here to join our federation.
by Chester O. Martin and Robert S. Staten
The two most frequent questions we’re asked about bats are as follows:
- How do I attract bats?
- How do I get rid of bats? For this article, we’re going to address the first question.
The primary way to encourage bats to your area is to provide suitable habitat, which includes adequate roosting and foraging sites. A diverse wooded area with plenty of older cavity trees and uncluttered understory vegetation is best, especially where there are open areas for feeding and a water supply nearby. However, constructed bat houses can provide alternative roosts for several species of Mississippi bats if they are designed and installed properly.
The bat houses described herein refer to traditional-style houses that may be installed by the private landowner. The recommendations below are based on personal experience and many
years of bat-house research, conducted by Bat Conservation International (BCI), the Organization for Bat Conservation (OBC) and other bat conservation and management groups.
Design and Construction
Bat houses can be constructed of different types of wood, but exterior-grade plywood or cedar works best. Pressure-treated wood should never be used because it contains chemicals that may be harmful to bats. Plywood for exteriors should be at least 1/2″ thick. Use 3/8″ plywood for roosting partitions, as this reduces the overall weight and increases roosting space. Attachment with galvanized screws is preferred to nails.
External dimensions are variable, but larger and taller boxes are usually more successful. BCI recommends the house be at least 2′ tall, have compartments at least 20″ tall and 14″ wide, and have a landing area at least 3-6″ extending below the entrance. However, OBC has experienced success with somewhat smaller boxes.
Providing ventilation slots is extremely important in Mississippi because of the high summer temperatures. They should be ½” wide and extend from side to side about 6″ above the bottom (front vent); a vertical vent should be included at each end of the rear chamber of multiple-chamber boxes. The inside should be roughened, grooved or lined ,with 1/8″ polyethylene plastic mesh; lining with plastic mesh usually works best.
Spacing of roosting compartments (chambers) is equally important. Bat houses may have single or multiple roosting compartments, but those with two to four compartments will support more bats and provide alternatives for roosting space. Roosting compartments should ideally be ¾” wide and never more than l” wide. Boxes may be purchased or constructed using specifications provided by a reputable conservation organization. Beware of commercially available boxes that are poorly designed, especially if compartments have openings that are too wide. These boxes become death traps for bats because they allow for predation by raccoons, blue jays and snakes.
Exterior surfaces should be painted, caulked and sealed to provide a draft-free interior. Outer surfaces, landings and entry areas should be painted or stained with a coat of primer, followed by two coats of flat-exterior, water-based paint; oil-based products should not be used. Adding a tin roof with a l” overhang will help protect the house from the elements.
Bat houses can be mounted on buildings, poles or trees. However, tree-mounted boxes generally receive less sun, are more vulnerable to predators and are often more subject to decay. Data maintained by BCI show that boxes on trees have the least success (20%), with boxes on poles (52%) and buildings (64%) having considerably greater use. Bat houses should generally be mounted 12-20′ above the ground, but l 0-12′ may be adequate.
Single chamber houses work best when mounted onto the side of buildings. When mounting on buildings, it is a good idea to place several boxes side by side to provide alternative roosting opportunities. Multiple-chamber houses are best mounted on poles that are installed at least 20-25′ from the nearest tree branches. Mounting boxes back to back or in sets of three will give bats alternative roosting areas because mother bats often move their young between boxes due to temperature variation, parasites and other factors.
A variety of pole types on which to mount boxes have been used with success. Metal poles are usually stronger and longer-lasting, but wooden 4×4″ or 4×6″ wooden poles are often used and are more convenient for attaching multiple units. The back of boxes may be attached directly to the pole, or two poles may be positioned so that the sides of boxes are attached. Holes for the support poles should be dug at least 3′ deep to provide stability; the hole should be wide enough to allow several inches of clearance for adding gravel and concrete. The bases of poles may be wrapped with galvanized sheet metal to help avoid predation by raccoons and snakes. Never attach cables or guy wire to the poles, because young bats may strike the wires and become injured or killed.
Further information on constructing and mounting boxes is available at Bat Conservation International.
Now, where do I put my bat house? Proper location is extremely important and will often be the determining factor for their potential use. Bat houses located within ¼ mile from a water source (pond, lake or stream) have a better chance for occupancy than those located away from water. Placement in areas with a mixture of habitat types (woodlands, open fields, pastures, wetland areas) that provide an abundant supply of insects will receive greatest use (all Mississippi bats are insect eaters).
Constructed bat houses are generally more successful in rural areas rather than a traditional suburban neighborhood. In fact, care should be taken to not attract bats where they might occupy human dwellings. The bat house should face outward along a woodland edge rather than in a forest interior. Houses do best when they are placed about 20′ out from the wood’s edge. It is generally recommended that houses face in a southerly to easterly direction in Mississippi. The most important factor is that houses should receive at least six hours of direct sunlight daily.
A well-designed multi-chamber bat house will potentially provide a roost for 200 to 300 bats. Although, nationwide, many species of bats use artificial structures, the primary occupants in Mississippi are the evening bat, big brown bat and Brazilian free-tailed bat. The southeastern myotis and Rafinesque’s big-eared bat will use structures specially designed for them but will rarely enter a standardized bat box.
Bat houses are designed to attract maternity colonies and may not receive winter use. New houses should be installed in late winter or early spring before bats begin to establish maternity colonies. Patience will be required, because it may take three to five years before a bat house is occupied, and it will often be necessary to experiment with location.
Unlike birdhouses, bat houses do not require an annual cleaning. However, if wasps become a problem, their nests should be removed in winter or early spring. Houses should also be checked for cracks or other problems that may cause leaks. Guano accumulating beneath a box may need to be removed from time to time. (It makes excellent fertilizer.)
Bats, especially juveniles, occasionally become injured in boxes and fall to the ground. Children should be told to not handle a dead or dying bat found beneath a bat box. Instead, an adult wearing gloves should remove the bat and dispose of it in a closed container. Finding a dead bat does not necessarily indicate a problem with disease, but the rare incidence of rabies requires caution. If bat boxes are located in areas where strangers may come upon them, interpretive signs describing their use and value may be necessary. Remotely placed boxes should also be checked routinely for vandalism.
Chester Martin is a retired research wildlife biologist with the U.S. Army Engineer Research and Development Center. He has published approximately 40 articles on bats and is founder of the Mississippi Bat Working Group. He is also an accomplished wildlife artist.
Shea Staten is a natural resource specialist with the U.S. Army Corps of Engineers at Sardis Lake. He has worked at both Arkabutla Lake and Sardis Lake during his 14-year career with the Corps. He serves on the Mississippi Bat Working Group Board of Directors.