I thought I’d write a bit about bat houses, and I’ll start with my own. As you can see below, it’s showing some wear and tear from the weather. It’s been up over seven years now, and stood through several hurricanes.
Of course, it looks better at night. (And as a photographer, grainy black and white is my true medium.)
Before I built it, I searched around a lot on the ‘Net, incorporated some best practices I found there, and threw in a few twists of my own. Since I’ve put it up some of the advice about bat houses has improved, and I’ve come to realize that some of the design decisions I made might be wrong. But the good news is that it doesn’t seem to bother the bats, so that means if you’re even close on the design of your bat house, you are likely still to get residents.
Things I did right:
I built it strong and weathertight. I’ve since had to re-caulk a few seams, but in general, I used solid pine for the exterior, and thin plywood for the interior petitions. I sealed all the seams between the boards and I think that my domed aluminum roof sheds water pretty well. Bats don’t like leaky abodes, and if you look at some of the big box store, pre-made houses, they’re made of loose fitting boards, sometimes with openings for rain and daylight to come through. My house isn’t pretty, but it doesn’t leak and it is really dark inside.
I spaced the interior petitions the recommended 3/4 inch apart. From what I continue to read, this is a crucial feature. The size of the openings is a big part of making a structure attractive to a bat population. Since the most likely denizens of your house are Brazilian Free-Tailed bats, it’s a good idea to follow the dimensions they seem to prefer. Wider openings might attract a different species of bat, but unless you’re building a house that is large enough for you to experiment with different spacings, 3/4 inch is you best bet.
I covered the inside of the box and the partitions with a material that give little bat claws some traction — nylon window screen, and I added a landing area at the bottom of the opening, using plastic gutter guard. You can see how bats have actually worn the screen in places going in and out.
I built the largest box I thought I could support without excessive weight and visibility. Most garden-store variety bat houses are smaller than recommended. Bats like dwelling in groups, and I’ve had up to seventy bats in my house from time to time. Larger houses hold more bats.
I mounted the bat house on predator-resistant poles, as high and as clear of obstructions as practical. Folks want to put their bat houses on tree trunks, but from what I read the experience with getting tree-mounted houses populated is mixed, at best. This may be because trees shade the bat house and reduce the internal temperature below what the bats prefer. My theory is that trees don’t appear to bats as sufficiently predator resistant — perhaps this is excessively anthropomorphic, but I think bats somehow sense that trees provide a viable climbing stand for snakes, opossums, and other threats. Slick metal poles pose much more of a barrier to potential enemies.
I was patient. It took a couple of years for bats to inhabit my house. I’m not really sure how long because I had grown accustomed to just ignoring the bat house on the assumption that it was empty. My daughter discovered that its inhabitants quite by accident one afternoon when she heard noises from the bats while hanging up laundry.
Things I could have done better:
I probably should have painted the house a dark color. I just couldn’t believe that bats wouldn’t find a dark colored, enclosed wooden box too hot in the Florida summer sun. Now, as I read more recent material on the ‘Net, it seems like folks are learning that bats like their house really, really warm. Still, it works even at its current dirty white color. But paint or stain yours brown anyway.
I used insulation inside the house. This was definitely my own idea, or at least I don’t recall anyone suggesting it. I placed the thin, aluminum coated mylar insulation (like that used for automotive window shades) around some of the partitions in the bat house. You can see it in this interior shot of the bats inside. I thought that this might help stabilize the temperature, keeping it a bit cooler in the daytime and a bit warmer at night. For the next house I build, I think I will make one chamber insulated still, but leave the others uninsulated. I don’t know exactly how bats move in the bat house during the day, but I have read that they seek out a variety of temperature zones, depending on the weather and the time of day. Insulating some of the house, but leaving the others more subject to temperature fluctuations might actually prove attractive to the bats. In smaller houses, I’d just dispense with the insulation entirely.
Although I added some instrumentation to the house, I did not exercise enough care so that the instrumentation would still be working when the bats actually moved in. I put a camera in the roof of the house (one reason the roof is domed), but since the window covering it eventually fogged to such an extent that the focus is shot. For the next house, I’m going to make the camera, along with any other instruments, replaceable from outside without disturbing the bats.
I added a temperature sensor and while this was a good idea, it failed not too long after installation. Temperature is known as an important correlate of bat activity, so if you only placed a single instrument in a bat house, a temperature gauge of some sort is probably the best choice. I used solid-state, analog temperature sensor. Now I would use a digital temperature sensor, and I’d make it replaceable from the outside of the box.
I probably should have mounted the house higher. Mine’s at about fifteen feet as measured from the point where the poles leave the ground to the top of the house. I arrived at this altitude by mounting two ten-foot chain link fence poles, each standing on a four-foot section of 2×4. This is a convenient height from a construction standpoint and it has the (to me) desirable side-effect of leaving the house just below my rooftop, rendering it invisible from the front of the house.
But this shot of the bat house at Lake Lotus park in Altamonte Springs shows how high you really might want to mount your house:
Over the years, more obstructions have grown up around my bat house. Tree limbs now approach from one side, and the neighbor’s bushes are crowding in. Still, the bats deftly avoid these and other man-made obstacles, such as camera stands and the like. But to start out, you should probably place your bat house in as clear a spot as possible.
So I think the bottom line is this — follow the best practices for bat house construction contained in the links below, but don’t despair if you can’t get everything perfect. Given time, bats likely will come to live in your yard, even if their house is too light, too short and too crowded.