Volume 11
Issue 1

Discoveries made in BCI’s Bat House Study have greatly enhanced our ability to design bat houses with a better chance for success . . .

After carefully considering the factors that successful bat houses have in common, we have developed two new versions of BCI’s original openbottom house. Both are designed to increase use by nursery colonies and to accommodate larger colonies. The large design should prove especially successful in attracting nursery colonies and can accommodate a thousand or more bats; the smaller house is capable of housing colonies of 200 or more.

Both bat houses accommodate builder needs for cost-effective designs, and construction is simplified by use of two-by-four or two-by-six lumber and exterior plywood. To help young bats secure a firm footing, a problem in many nursery boxes, the roosting partitions are covered with fiberglass insect screening or quarter-inch hardware cloth.* Partitions are fit into a grooved ceiling and sides, enabling tighter seals and therefore less temperature exchange between roosting chambers.

Each design permits a wide range for modification. Insulating the upper portions of both front and back chambers and the ceiling further helps to stabilize temperatures.** This, combined with longer roosting chambers, allows more bats to cluster near the ceiling, where they can trap body heat within insulated areas, reducing the impact of external temperature extremes. In addition, if the house exterior is darkened, the front chambers will readily absorb solar heat, while rear chambers remain buffered from possible overheating. Several 1/4-inch ventilation holes or a 3/16-inch slot in the front of the house, positioned 8 inches from the bottom, can reduce chances of overheating where this is a concern. In such a house, bats can move down between the vents and the entrance during extreme heat. Either house can be successful without insulation or vents, depending on local circumstances.

As our survey confirmed, temperature is a key factor in determining bat house use. Bat requirements vary daily with weather extremes, and seasonally due to the changing physiological requirements of pregnancy, rearing young, and storing fat for winter. While northern bats often need considerable heating in their roosts, southern bats, especially in lowland desert areas, may need just the opposite. The challenge for bat house builders is to provide a range of temperatures adequate to meet bat needs despite external changes that occur throughout an average day. An ideal house would offer the widest possible temperature range, and that is what we have attempted to provide in the new designs.

Recent research conducted by Lisa Williams in collaboration with the Pennsylvania Game Commission confirms the importance of temperature for nursery colony selection of bat houses [see graph, page 10]. Most nursery colonies seek roosts that provide ambient temperatures within the preferred 80- to 90-degree Fahrenheit range or roosts that effectively trap the bats’ own body heat, providing a microclimate warm enough to ensure rapid growth of young.

Little brown bats are much more tolerant of heat than big brown bats and have been recorded rearing young in temperatures as high as 104 degrees, though they prefer 80 to 90 degrees. Big brown bats, however, are known to abandon roosts when the temperature rises above 95 degrees. Placing several dark and light-colored houses close together to absorb differing amounts of solar heat can greatly expand the available temperature options and may be especially advantageous when attempting to attract nursery colonies. Bachelors tend to selct the cooler temperatures year-round.

Where you mount your bat house also plays a major role in its internal temperature. In cool climates houses on poles can better take advantage of exposure to the sun. In the North, try mounting two houses back-to-back so that a light-colored house faces northwest and a dark brown or black one faces southeast. Try white and light or medium-brown houses similarly oriented in warmer regions. The darker house would be warmed early in the morning, remaining quite warm through the day. The northwestern house would be mostly shaded until afternoon and then absorb minimal heat due to its light color.

Poles also allow houses to be located 15 feet or more above ground, a significant factor in occupancy success. In southern areas, where free-tailed bats are common bat house occupants, it is an excellent idea to place houses on poles as high as possible to attract these high, fast fliers. In the hottest parts of the South, such houses should be protected from overheating with white paint or reflective tin roofs with overhanging eaves (both in some cases) that shield the sides and tops from all but the earliest and latest daily sun.

Although poles are best suited to receiving optimum solar radiation, the sides of buildings and some tall trees are suitable as well. The amount of sun exposure needed will vary with local climates. One experiment you might try on buildings in northern latitudes is to mount two houses within a foot or two of each other, facing south or southeast to receive four or more hours of daily sun, one stained very dark or even black and the other medium brown. If you live in latitudes between 35 and 40 degrees, one house might be stained medium to dark brown and the second left a natural wood color. And in southern regions, one could be a medium brown and the other white, both exposed to four hours or less of morning sun. In exceptionally hot areas, it is possible that houses should not be exposed to sun at all, though new types of acrylic and ceramic paints may reflect nearly all solar heat, thus allowing exposure to more sun without overheating.

Experimentation with insulation may yield some of the most interesting results. In northern areas, both dark uninsulated and dark insulated houses should be tested side-by-side to test bat preferences. A further option is to insulate only the attic and central chamber (top to bottom), simply dividing a single house into a relatively hot and a cooler area. Since no studies are available on the effect of insulation on bat choice, we can only assume its value based on what we know about the importance of temperature. We believe that insulation will provide a much greater margin for error where temperature needs are not fully understood, especially in mid-latitude and southern areas. Further experimentation may even demonstrate that all bats prefer it.

Although much remains to be learned, it is encouraging that even the less sophisticated bat houses, mounted singly and without insulation or screening on the partitions, already are averaging more than 80 percent occupancy when located correctly relative to solar exposure and habitat. We strongly encourage you to experiment and become an active participant in BCI’s North American Bat House Research Project.

(Footnote 1)
* Insect, or window, screening is available in both fiberglass and metal. It is important that only fiberglass is used; the small mesh of metal screening is extremely abrasive. Care must also be taken when using hardware cloth to ensure that sharp edges, which could impale delicate bat wings, are covered.

(Footnote 2)
** We recommend two layers of Reflextix™, a double-bubble foil insulation that has exceptionally high R ratings.

Materials for the smaller and larger designs cost approximately $15 and $30. The sides and ceiling for the smaller house are cut from two-by-four and one-by-four pine, respectively. Those for the larger one are cut from two-by-six and one-by-six pine. Fronts, partitions, and roofs for both are cut from 3/8-inch T-111 no-groove exterior plywood. In the absence of a table saw with router attachments, wooden slats can be substituted for grooves to hold partitions in place and reduce air exchange between chambers. Detailed plans to build all three houses shown in this issue are in the new Bat House Builder’s Handbook, available to participants in BCI’s North American Bat House Research Project.

The large nursery house weighs 40 pounds and is best mounted back to back in pairs, supported by a pole at each end. Two small nursery houses can be mounted easily on a single pole. Mounting back to back on poles allows bats a greater temperature range.