Bat House Forum

[Dave M]

I'm designing and building my own wooden bat house in SE Minnesota. My goal is to house a maternity colony, I've been doing a lot of information searching on the internet. I should have a good chance of attracting big brown bats or little brown bats. Most of what I've read says make the chambers 3/4 inch wide, but some sources indicate that up to one inch wide is good too. I plan to use plastic mesh for the bats to grab onto rather than cutting grooves in the wood.

Since big brown bats are bigger than little brown bats, should their bat house chambers be larger too? I've come across nothing indicating the best size for either species in particular. Anyone who has a successful maternity colony in a bat house please tell me what species you have and what the chamber dimensions are. Also let me know if you grooved the wood or used plastic mesh. Your location might be helpful too, in case bats have slightly different preferences in the north compared to the south.

Looking through this forum, I did find some discussion about Big Brown Bats preferring 7/8 inch chambers, and cooler chambers than little brown bats. Here's my idea...

There's a spruce tree in my backyard with full sun that I want to remove, but I'd like to keep the stump about 20 feet tall to mount the bat house on. I'd hang the bat box over the top so that it has four sides coming down and surrounding the stump while the roof covers the stump and all chambers. I'm thinking of having four chambers on each side. The final outside dimensions will be at least two feet wide on all sides, perhaps up to three or four feet because the trunk forks and I have to find a way to accommodate that to get the height I want.

I'm considering having a sealed central chamber that houses the stump, but is not accessible to the bats. This could be designed so it's either open or closed to the attic space. The idea is that the stump and air pocket would act as a heat reservoir to maintain a more constant temperature for the bats. Please share your opinions: is it a good idea? should it be open to the attic? should it have vents opening into the roost chamber?

I'd like to design the house so bats can move within it to find the temperature they like. Through the daylight hours I expect the east side to heat first, then the roof and south side, and finally the west side, while the north side stays cool. Should I make lots of passages to keep the house more uniform in temperature, or only have a minimal amount of passages that will provide lots of temperature variation? Passages could be along the baffle tops against the ceiling, or vertical or horizontal gaps at any height on the baffles. I'll also probably design some vents going to the outside if you think those are a good idea. I expect the total chamber height for the bats to occupy will be about three feet.

If you made it through my long post please share your ideas and suggestions! Is it likely to have a maternity colony composed of both big and little brown bats, or should I just try for one or the other?

[Dave M]

I forgot to mention there is a river 1/4 mile away that is the right size for canoeing on. Adjacent to my yard is a dry creek bed that only runs during snow melt or rainstorms, this dry creek provides a wooded corridor straight to the river. I'm in a residential section of the city with many mature trees in most lawns. My backyard is large with lots of open space, but my front yard is shady.

[Joe Spencer]

Dave, your design and approach sounds good. The key is not to exclude either common species so chamber sizes should be 3/4 to 7/8. Many swear by only 3/4 inches but the Bat House Builder's Handbook/BCI data over the years maintained that big brown bats prefer slightly larger chambers than 3/4". The problem with going larger than 3/4" is that you greatly increase your chances of getting paper wasps. The good thing is if you incorporate a lot of chambers or erect multiple bat houses, the wasps issue usually is tolerable or maintainable. Looking forward to photos of your project's progress if possible.

Joe

[Dave M]

Thanks Joe, The Bat House Builder's Handbook, 2001 revision, is available at my library, I'll try to pick it up Wednesday. Meanwhile, don't anyone be shy about commenting on my ideas and questions, I look forward to hearing more from everybody.

[cloudman75]

Dave,
Since you asked for comments, here is mine. It is very difficult to keep chambers at 3/4 inch rather than 7/8 and the bats probably won't worry about 1/8 of an inch difference. I have built two houses, one a 5 chamber, and they have been well used. The other was a 4 chamber.
After a while, the baffles warp anyway so they may be 1/2 inch at one place and the bats don't seem to mind. I have included in my houses one chamber that is one inch just to please any big browns that don't like close quarters or to give the mothers a little more room to have pups. I have not been bothered with a lot of wasps, but some have. I would certainly
suggest at least one 7/8 inch chamber. Good luck and keep us posted

Frank

[Terry Lobdell]

Dave, you have really good ideas........my experience here in NWPA is that I have never seen a bat house with little brown and big brown maternity colonies at the same time.........it has always been one or the other........

My guess is you have more of a chance attracting a little brown maternity colony than a big brown maternity colony.......

In my area big brown mothers prefer bat houses mounted on buildings and little brown mothers prefer boxes mounted on poles.......

I would not make access to the attic area if you cannot clean out wasp nests.........wasps seem to seek out the hottest area of a bat house to build their nests......

On the subject of baffles, what kind of wood are you going to use? I have had good luck with pet mesh and stainless steel staples but am now using all rough sawn one inch boards some saw kerfed and some not.......

I did make some boxes last year with 1/4" lauan baffles with pet mesh glued on with liquid nail.........however, the pet mesh was only on one side of the lauan and I had some pups fall which I attribute to the smoothness of the lauan.......

I am now a firm believer in both sides of the baffles being roughened.........

For ventilation, if you have some vents in the lower third of the box, that will be good............little browns don't require as much ventilation as big browns do...........

As far as overall size, the bigger the better for more heat retention.........however, I now just start out with an average sized box in an area that does not already have a colony because sometimes it takes awhile to attract them..........if I don't get bats in the first box I put up, I will put up more in different locations..........kind of a shotgun approach...........

but your habitat sounds excellent..........I would bet you will at least get some bachelors in the first year.........

That dry creek bed corridor is sure to have bats foraging through it continuously!

Hope this helps!

[Dave M]

Every summer I see both small and large bats flying around our yard. I'm guessing they are little brown bats and big brown bats. I accidentally killed one that had roosted between the door jamb and the screen door that didn't completely close one night, next morning I went outside with the dog and let the screen door slam shut behind me. When I opened the door to walk back inside the little guy bounced off my shoulder and lay dead on the ground.

I plan to use 3/8" plywood for the baffles with screen on all sides of the baffles. I hope the 3/8" plywood won't warp as much as 1/4" might. I was considering small openings to the attic only for heat, not to let the bats through, but a small opening would still be accessible to wasps so I will keep it completely closed off. What are lauan baffles?

My home isn't suitable to mount a bat house onto it, and I'd rather make use of this spruce that I don't like than to install a pole, so this is the one spot for me to put a bat house. It will be big, perhaps bigger than four of the three chamber boxes combined. I think I'll mostly use 7/8" chambers with plenty of ventilation on the north and east sides, and 3/4" chambers on the south and west sides. A bat will be able to move to any part without having to exit. If I'm lucky enough to get both species with pups that will be fantastic, but I'll be happy to produce any pups!

[Terry Lobdell]

That is a good plan on the crevice dimensions.........lauan baffles are just 1/4" mahogany plywood.........if your 3/8" plywood ever warps you could just insert a spacer to even things out.........