Why Do Some Bat Houses Fail?
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.
This bat house might be an OK design, but at least three things are compounding keeping any bats from using it. First, never place a bat house on a tree, it is almost always too shaded. The bottom of the house should be at least 10' above the ground, unlike this one. Finally, vegetation has grown up around the site making it more difficult for any would be bats to enter or exit.
- John Chenger