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Why Do Some Bat Houses Fail?

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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

 

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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 bat house mounted on a pole. Data shows bats slightly prefer larger structures which offer a wider range of stable temperatures, but don't take this to mean a single chamber bat house won't attract bats. A tightly built single chamber bat house may do excellent especially when attached to a building, benefiting from the more stable mass of the building. Larger colonies/large bat houses are often better on poles in their own space. 
  • Factory smooth interior crevices. All interior wood must be roughened for bats to readily cling to. Some commercial bat houses staple screening to the inside of the box which may eventually fall off as condensation and urine rust the staples. Cheap screening will shred and baby bats have been found dead apparently trapped within it. A "bat house" seen for sale at a major hardware store has only one groove on the "landing plate"; this is totally unacceptable. All BCM bat houses feature surfaces severely roughened in random directions, better than systematic grooves by machine and no screen is needed.
  • 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. Bat Conservation and Management uses plastic lumber made from recycled material as roofs on most of our bat houses; this material easily outlasts any shingle and never needs painting.
  • Nailed together with unsealed seams. This type of bat house is usually the very low end costing in the $10-$30 range. It might get a few bats but will soon will warp and separate at the seams; allowing unwanted ventilation, and will fall into disuse. All BCM bat houses are screwed together and glued together during assembly; some have seamless plastic exteriors which eliminates these fail points
  • Unpainted, unprotected exterior. Some manufacturers insist on leaving bat boxes unpainted. While in certain regions the natural wood color may be a perfectly 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, although the thicker the boards, the longer it will last. So, we suggest all bat houses have some sort of preservative applied to the exterior, even if it is a clear finish.
  • 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.
  • "Repurposed" 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, or contain undesirable contaminants, approach used lumber with care.
  • Unrealistic claims. For example, our old seven chamber bat house design 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 probably overcrowded and unhealthy. While exact capacity will differ among species and crevice size, we use a rule of 2 bats per linear inch of roost space to estimate capacity. For example, an 18'' wide bat house with 3 chambers would be 18''x3 chambers=54 inches, 54''x2 bats per inch is 108 bats. The actual capacity may be something larger or smaller, but now you can compare relative sizes of bat houses with each other.

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.

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  • John Chenger