Chesapeake Wildlife Heritage installs Bat Boxes for exclusions only. (Please contact Andi Pupke at apupke@cheswildlife.org for information on the proper procedures for performing an exclusion.)

Bat House Designs (1/06)

Habitat:
Bat house designs continue to evolve as research improves. It is important to have a properly designed house for your area and the species of bats that live there. It is also of the utmost importance to properly place the house. For example, nursery colonies choose roosts within a quarter mile of permanent fresh water, preferably a stream, pond, river or lake. The greatest bat house success has been achieved in areas of diverse habitat.

Mounting:
Bats find houses mounted on poles or buildings in less than half the time they typically need to find houses on trees. Tree mounted houses also appear to be less attractive, as they tend to receive less sun and may be more vulnerable to predators. Houses mounted under the eaves on wood or stone buildings, but still exposed to the sun, tend to be better protected from the rain and predators and have been especially successful. Too little sun exposure is the most important known cause of bat house failure, even in relatively hot climates.

To the extent possible, locate all houses 20 to 30 feet from tree branches or other obstacles and at least 10 feet ( preferably 12 to 20 feet) above the ground or above the tallest vegetation beneath the bat house.

Protection from predators:
Safety from predators appears to be the key factor in bats’ choice of bat houses. Those mounted on the sides of buildings or high up on poles provide the best protection.

Design:
The most successful bat houses have roost chambers at least 20 inches tall and at least 14 inches wide. Taller and wider houses are even better. The number of roosting chambers is not critical, but in general, the more chambers the better. Single chambered houses should be mounted on wooden or masonry buildings, which help to buffer temperature fluctuations. Houses with at least three chambers are more likely to provide appropriate ranges of temperature and better accommodate the larger numbers of bats typical of nursery colonies. Rocket boxes (a newer pole-mounted design with continuous 360o chambers) should be at least 3 feet tall. All houses should have 3 to 6 inch landing areas extending below the entrances.

Roost partitions should be carefully spaced 3/4inch to 1 inch apart. Three-quarter inch is preferable in our area because chambers that are larger are more likely to attract non-target animals. Partitions and landing areas must be modified to provide footholds for bats. These can be created in various ways. The best method for wooden bat houses is scoring or grooving surfaces horizontally every 1/4 to ½ inch. Metal mesh or hardware cloth should NOT be used as they are abrasive and can cause injury.

Ventilation slots are critical in houses used in Maryland where the average high temperatures in July are 85o F or higher. Half-inch slots should be used to reduce the entry of light and unwanted guest, such as birds. The front vent should extend from side to side about six inches above the bottom (for a house three feet or taller, approximately one-third the distance from the bottom of the bottom). A vertical vent, ½ inch wide by six inches long, should be included at each end of the rear chamber of multiple-chamber houses. Vents may be unnecessary in exceptionally cool areas. Do not use oil based paints but exterior water-based paint in a medium color such as brown, gray or green in Maryland.

Construction:
For single-chamber and nursery houses ½ inch (or thicker) exterior plywood is ideal for fronts, backs and roofs, while 1 or 2 inch- thick boards are best for the sides. One-inch (3/4- inch nominal size) cedar or poplar lumber is recommended for rocket boxes. Roofs for any house type can be built of 3/4 inch exterior plywood to increase longevity. Cover roofs with shingles or metal for extra protection. Do not use pressure- treated wood, which contains chemicals that may be toxic to bats. All seams must be caulked, especially around the roof, prior to painting.

For more information visit Bat Conservation International at www.batcon.org