This bat house at the left is large, which is good. Bats seem to like larger structures over tiny "bird house sized" boxes being offered today in hardware, nature, garden, and science stores. Though large, this box with it's 1''-2'' interior baffle spacing (too wide), lack of landing plate, and shady placement on a tree all contribute to it's failure. This box can be upgraded to a degree with the addition of a landing plate, a fresh round of sealant, and a sunny location. The fixing the all-important interior crevices would require total disassembly, time better spent creating a new box with a new design.
All bat houses are not created equal
Few box manufacturers promote their products with photographs of bats actually using their boxes. While it is a little tricky to get great shots of bats in or emerging from bat houses, more likely the problem lies with bats simply refusing to use them. Fortunately with the invention of the internet, it is now possible to compare proven products with those that are more intended for decoration.
Bat houses found in most shopping mall nature stores, mail order catalogs, nationwide hardware stores, and birdhouse websites may be from antiquated plans. Usually they are designed to be fast and inexpensive to make. For several years now we have been teaching the bat house building segment at the annual BCI Bat Conservation and Management Pennsylvania Workshop. Participants are always surprised to learn all the details that go into a successful bat house. We teach that bat houses will fail because of three reasons:
- poor design
- poor construction
- poor placement
Common bat house design and construction problems
Bad bat house designs are simply flawed from the start. They are often too small overall and contain crevices too large. Sometimes a critical detail is omitted, such as a landing plate. Usually these boxes cannot compete with a modern design, even after being upgraded by a creative hobbyist. Bad construction is a different matter, usually the box can be successful after some paint, caulk, extra screws, and extra roughening.
- A single chamber box. Bats simply prefer larger structures which offer a wider range of stable temperatures. A tightly built single chamber bat house may be good "starter" box that would be great for giveaways to the general public.
- Factory smooth interior crevices. All interior wood must be roughened for bats to readily cling to. Some commercial boxes staple screening to the inside of the box which may eventually fall off as condensation and urine rust the staples. A "bat house" seen for sale at a major hardware store has only one groove on the "landing plate"; this is totally unacceptable.
- Unprotected roof. Roofs without shingles may last only a few seasons, once the roof is compromised the box will be incapable of retaining heat and fall into disuse.
- Nailed together with unsealed seams. This type of box will warp and separate at the seams allowing unwanted ventilation and disuse.
- Unpainted, unprotected exterior. Some manufacturers insist on leaving bat boxes unpainted. While in certain regions the natural wood color may be a suitable color, there is no good reason for the structure to go unprotected. While cedar is rot resistant, it will warp and separate the seams just as much as any other wood material.
- Bad design. Bat research has proceeded at a very fast rate, thanks to improved communication among many different hobbyists and biologists across the country. For example, a tall open-bottomed bat house is now preferred over smaller closed bottom designs which tend to attract more parasites.
- Old plans. Bat house plans can be found in a variety of places, which is good. Unfortunately, many posters, books, and flyers were published years ago with now antiquated plans. Frighteningly, this information is still distributed by many reputable sources. A stack of utterly horrid plans obtained from a state wildlife agency were seen distributed at a wildlife program in just a few years ago.
- "Recycled" material. Some people construct bat houses of decent design but with lumber salvaged from demolished structures in effort to cut costs. As some pesticides can leave active residues for years, approach used lumber with care.
- Unrealistic claims. For example, a seven chamber bat house was once observed with nearly 600 bats inside. However, our we still only rate that box at 300. This is because 600 bats in this size bat house is overcrowded and unhealthy.
One of many experimental bat boxes built in Pennsylvania. This design is mildly successful but can get too hot internally due to the lack of overall height of the box. The bottom door must be kept closed to keep stray light from driving off bats. This same door allows guano and parasites to build up. Tall, open bottom BCM boxes give bats a temperature gradient and are self-cleaning.
Common placement mistakes
The greatest bat house in the world will never contain a bat unless it is placed properly in the field. In cool climates it is best to avoid shady locations at all costs. Often this limits the mounting options to strategically placed new posts or sunny chimneys. Bad placement includes the following:
- A bat house that is in a shady location. It needs a minimum of seven hours of morning sunlight.
- A bat house mounted on a tree. Rarely will the box ever receive enough direct sunlight in cool climates.
- Bat house placed on a structure, directly under the eves. Again, this may be simply too shaded.
- Placed too far from permanent water. Dry, arid locations are less desirable, though bats will drink from swimming pools.
- Located over bright surfaces which reflect light into box. Shiny flashing or even pans to collect guano may deter use at certain times of the season.
- Located near burn barrels or air vents where smoke or strong wind will disturb bats. Beware of air conditioner units which may not be active when installation occurs.
- Erected where the box is prone to vandalism. Shaking the bat house to watch them fly out during the daytime- yes, your bats will abandon the roost.
- Placed in brightly lit areas. Avoid mounting where dusk-to-dawn lights shine directly onto the box.
- Erected directly along roads, where bats are vulnerable to automobile traffic during their dawn return.
- No maintenance. Some mounting solutions make a quick yearly inspection into a difficult and even dangerous ordeal. Bat houses do require minor wasp and seam inspection, otherwise bats will begin to abandon the box. Be alert for a hornet invasion as well; bats will immediately abandon a bat house until these aggressive insects are removed.