A small bat box that could house up to 100 bats. Photo by Rick Reynolds/DWR.
Bat houses have become very popular over the past decade and are a great way to enhance local populations and observe bat behavior. Bat houses can be placed in yards or around the home where they will not impinge on daily activities yet will provide a great wildlife viewing opportunity. Since bats are major nighttime predators of insects, they will also provide a natural form of insect control.
Some of the early work with bat houses resulted in mixed success, but years of research have documented what makes a good bat house. The primary elements to consider in building and installing a bat house are design and placement.
Design
Design considerations include size, color, and building materials. One of the first “bat houses” simply consisted of heavy tarpaper or corrugated metal wrapped around the trunk of a mature tree. This is an effective way of providing roosting or nesting habitat for bats, but may not be as aesthetically pleasing as a manufactured house. If you have a large tract of land and want to enhance bat habitat, though, this is a cost-effective approach.
A small maternity colony box can be used to attract up to 150 females. If this seems too large for your setting, halving the box width can downsize the box, but, as the box gets smaller, the chances of occupancy diminish. A large maternity colony box can attract up to 300+ females. The “rocket box” is a newer design that appears to be well liked by bats. This design is more compact than the traditional bat houses and can be placed in most settings. Rocket boxes are typically made of a 4″ x 4″ post and 6″ x 1″ boards with spacers to provide openings. The boxes are sealed at the top to prevent rain from entering the box.
Bat houses should be made of exterior grade plywood, cedar, or redwood. Do not use pressure-treated wood. Bat houses should be constructed with multiple baffles to provide ample roosting space, and the baffles should be spaced 3/4 to 1 inch apart with horizontal grooves to provide footholds. Hardware should be galvanized, and using screws as opposed to nails will increase box longevity. Finally, all seams should be caulked. A variety of good bat house designs can be found on the internet (see Additional Resources).
Placement
The first consideration in bat house placement is to enhance viewing from a distance, but minimize disturbance or traffic by humans or pets. To further reduce disturbance immediately around the box, the area can be fenced or planted with low shrubbery.
You must also consider the box’s exposure to sunlight. The bat house should be oriented to maximize sun exposure (preferably 8 hours or more): generally this means a southern exposure is best. In addition, boxes painted a dark color (use water-based paint) will absorb more solar radiation than lighter color boxes or natural wood. An alternative to paint is to put black roofing paper on the upper section of the box. Because there will be variation in the amount of solar radiation a box receives, bat boxes should be constructed with vents to facilitate a temperature gradient in the box. This allows the bats to move within the box to locate the temperature that suits them best.
Placement height of the box is yet another factor. Boxes should be placed at least 12 feet above the ground and preferably around “second floor” height (approximately 20 feet). Boxes can be attached to the side of a house or chimney, or on a pole. Boxes should usually not be attached to trees, as they will likely not receive sufficient sunlight.
You should also carefully select when to erect your bat box. Bats generally leave their winter roosts for summer sites in April. Having your box out before bats leave their hibernacula will provide greater exposure to wandering bats and increase your chances of success. If you’re late getting your box out, though, go ahead and put it where you expect to have the greatest success. If the box is not used the first year, give it a second chance to see if it will be accepted the following year.
Finally, research has established that maternity sites are usually located within 1/4 mile of a natural water resource. Providing a water resource often is not practical, however; so if you are unaware of a nearby water resource, try the bat box anyway. You never know, the bats likely know of a water source you weren’t aware of.
Maintenance
Like any other wooden structure, bat boxes will need limited maintenance. The small 3/4 inch openings should help to exclude wasps. However, if wasps occupy the box, they should be cleaned out during the winter months when they are inactive. Repairs, if necessary, should be made during the “off season” when bats have left for winter hibernation.
Along with bats come bat droppings, or “guano”. This should not be a problem if you place the box where droppings can accumulate on the ground until the bats leave in the fall. At that time the droppings, if substantial, can be gathered up with a shovel and placed in a compost pile: remember, bat droppings make excellent fertilizer.
Because you are dealing with living creatures, there may come a time when a young bat ends up on the ground. Generally, the mother will find the pup and return it to the box. Be cautious in trying to return a bat to the box. If you choose to place a fallen bat back into the box, you need to wear thick leather gloves. Also, children should be taught to never handle a bat they see on the ground, or in a bush or tree. Remember these are wild animals, and they don’t know the difference between a helping hand and a dangerous predator.
Experimentation
While general ideas (type of box, exposure, height, color, etc.) can be applied to bat boxes, each location has unique features unto itself. Begin with these recommendations, but experiment with different boxes, colors, and placement to maximize your chances of success.