Overcrowding and overheating
If bats are constantly seen towards the bottom of the bat house throughout the season or are on the exterior, it may be too hot. This is best solved by installing a second bat house nearby or on the backside of the original. Alternately the bat house might need tuned to your specific location, perhaps by adding white roofing paper, repainting the box a lighter shade, or moving the box to a different location. Unless other roosts are already in use nearby, tuning a single bat house is experimental not recommended. Add a second box first then experiment later if necessary.
The shell of the bat house should outlast the interior baffles. Over a period of time and usage, baffles may become warped. Bats will continue to use the box but capacity may be reduced. It will be difficult to directly inventory the bats. This generally is not an issue with modern BCM products as we use yellow pine plywood which is much more resistant to warping than inexpensive Luan plywood.
Modern bat houses are thankfully self-cleaning. Droppings accumulate on the ground directly under the bat house. This material naturally biodegrades and there is not much need to remove it.
Mounting your new bat house directly on a structure where bats are being evicted will almost guarantee a successful bat house occupation, provided the box receives enough direct sunlight. One pitfall of structure mounting is installing the bat house too high where it becomes shaded from overhanging eves.Bats will only use the bat house in the spring, summer, and early fall, but a large colony will produce an appreciable amount of guano which will over fertilize the ground directly beneath. Be careful not to mount the bat house directly over windows, doors, and walkways. In addition, bat excrement may stain certain paints on the structure. Minimize this by using spacers between the bat house and the structure. While fascinating to watch, the daily dawn return is responsible for droppings that inevitably are splattered on the siding around the bat house. Keep in mind that it is not necessary to mount a bat house on a structure for bats to discover it. A successful bat house can be moved several hundred feet to a similar sunny location without little brown bats abandoning it. Some big brown bats and other minor species may not agree with moving plans and may temporarily abandon the bat house. Pole mounting a bat house one summer before planning an exclusion project is ideal. This eliminates the need to move the box off a structure in the long run, and bats have plenty of time to investigate the bat house before being excluded elsewhere.The box may be moved several hundred feet after two summers of bat occupation. Moving the box during the first year may cause bats to abandon the box. Move box only in the winter, when bats are not present. Pole mounting a box one summer before bats are excluded from a nearby structure is recommended as the best permanent solution and is highly successful. [more on evicting]
Bats seen flying during daytime hours do not constitute a health hazard. Bats may occasionally be visable during daylight hours because of an unusual disturbance. This includes but certainly is not limited to burning leaves, loud lawn mowers, chainsaws, children, or cats. Bats may also appear simply to get a drink.As some bats are threatened or endangered, most states prohibit having bats as pets. They are difficult to keep in captivity and therefore relatively few people are properly licensed to hold them for any length of time. With any wild animal, they should not be handled without protection. Should a grounded bat be found (usually the pet cat finds it) gloves should be worn to dispose of it. A grounded pup has little chance for survival. If there was any chance the animal was in contact with humans, contact your local wildlife management or public health agency, though the only suggestion they legally will have is to begin rabies post-exposure treatment. [more on health considerations]Although a bat may have only one pup a year, overcrowding may occur. If your box contains more than 100 bats, watch for grounded pups under the bat box. If more than one or two a year appear, this means the box is either (1) too hot due to overcrowding, (2) getting too much heat, or (3) both problems. Another sign of heat stress occurs when the bats utilize the cooler, lower portion of the box even on mild afternoons. The simplest solution is to install an additional bat box within several hundred feet of the first. If pole mounted, a second box can be installed on the backside of the first, offering a range of roost selection. A more risky solution is to move the box to a slightly more afternoon shaded site. Given how particular bats are about their roosts, BCM recommends installing a second box.
Bat House Maintenance
Inspect the bat house interior using a spotlight. If bats are present, wait until later in the season to do routine maintenance. Once during each winter the bat house should be inspected carefully for broken seams. Daily heating and cooling can cause roof seams to separate after a few years. Daylight entering around the roof is a sure sign of needed repair, but be careful not to mistake broken seals with normal daylight entering through the side vents. If left in disrepair, bats may abandon or never utilize a cold roost. A ladder can be placed against a post-mounted bat house in order to apply fresh roof repair sealant over all seams. Check to make sure the roofing paper and landing screen is intact. In addition, is a good idea to re-stain or paint the bat house every few years to prolong its life.
Paper wasps form gray honeycomb shaped nests on the ceiling of bat houses (an example of this nest can be seen in the upper left corner of the bat house on the cover of this booklet). These insects are not aggressive and happily coexist with bats. Unfortunately the nests become large and eventually consume real estate inside the bat house. Remove nests in the winter using a long, thin rod or stick only when bats are not present. Check carefully before cleaning because shadows can hide solitary bats in what first seems to be an empty box.
Yellow jacket/Bald-faced Hornets
These very aggressive insects build conical nests that can be as large as a volleyball. Some stealthy varieties also build nests inside the bat house. If a number of bees are routinely entering and exiting the bat house in the summer, chances are it is overrun with yellow jackets. Bats will abandon the bat house if these insects take up residence. Destroy these nests early before they become large. Carefully knock out nests at night or in winter when insects are less aggressive. A carefully directed spray application followed by a generous water rinsing may successfully kill the insects and minimize the chance of passing the poison to bats. People sensitive to bees should avoid this, as a single yellow jacket will sting multiple times.
These birds have rarely been observed drumming on bat houses. The damage can be repaired by filling holes with roofing sealant when bats are not present.
Bats can be inventoried two ways. The most direct approach is to stand under the bat house and view the interior using a powerful spotlight (figure 14). This should be done very sparingly throughout the first season of occupation. A less intrusive technique is to watch the evening exit, which begins approximately 15 minutes after your local sunset and lasts about thirty minutes.