I've got some standing water (dugout) on farm, has been treated on and off over the last 40 years (mostly off). I am going to have a lot more standing water on the farm in the future, at least for short intervals. But, it apparently doesn't take long for mosquitos to get going. So, I went looking for idea on dealing with the consequences of many swales and other water impoundments on the farm.
I've been told that all bats in Canada, are insect eaters. Apparently, they like mosquitos, and lots of other biting or otherwise annoying insects.
I've read that tree swallows also like to munch on mosquitos.
Both have problems with respect to loss of habitat and humans doing stupid things. I suppose climate change bothers them too. I have no idea if either are bothered by politicians. I don't speak bird or bat, and I would rather "speak" to computers anyway.
Just about every bird or bat house plan I looked at, more or less started with, "buy a sheet of 1/2 inch outdoor plywood".
First law of building outdoor wood projects for bats, birds or ....: don't use treated wood. Treated wood is designed with man made poisons, which were solely designed because they are poisons. In North America, black locust (Robinia pseudoacacia) is an exceptionally durable wood. It is durable due to the "chemical package" that nature provides. Some wineries are using barrels made of black locust for aging wine. I don't think there is a winery anywhere on Earth, that is using treated wood for aging wine.
While outdoor plywood is more durable than indoor plywood; it is not marine plywood. Which is actually durable (and probably very expensive). Once upon a time, plywood had few problems. Perhaps the biggest problem was too thick of plies. Plies still tend to be "thick", except for the face sheets which tend to be thin (or too thin). And we now seem to have more voids. Except for (some) Baltic birch; which has thinner plies. And should never be used for outdoor projects (without copious waterproofing) as it is not durable.
Outdoor wood gets wet. If water collects in a void, and then freezes; we can see fracturing of the wood because the ice expands on freezing. If water collects, it can support life (for the purposes of this note, the life is fungi and is associated with wood rot). For things like posts in the ground; rot happens where water and oxygen are both present. For a wood project above ground, oxygen is always present. Things like voids are all that is needed to allow wood rot fungi to start up their destruction of your project.
Having level surfaces exposed to rain allows for water to pool. A slope of 10% typically sheds water very quickly.
Rain that is shed, is rain that cannot be absorbed by the wood. Wood changes shape significantly more from changes in water content than anything else.
Where I live in the world, sees little rain (slightly more than a particular borderline I learned long ago for deserts). The natural state of wood is "dry". For naturally dry conditions, you really don't want to coat your wood all sides with a water barrier. Water will get into the wood, and your barrier just makes it difficult for the water to get out.
End grain sucks up water much more so than edge or face grain. Treating end grain specially is probably worth it. If the end grain is not exposed to UV light, laminating epoxy is probably a good end grain treatment.
Drain holes which let water escape can be useful in outdoor projects. Drain holes smaller than 1/2 inch in diameter will probably get plugged by spiders. Spiders are everywhere.
That's an introduction on durable wood construction, from the point of view of someone who lives in a place which is not quite a desert.
I was looking specifically at tree swallow and bat houses. Both types of houses have concerns about the babies being able to "climb" the wood walls, and bats sleep upside down so there is a concern about them finding "purchase" on the ceiling (unless something else is done).
A common recommendation in newer bat/bird house construction, is to cut a series of shallow kerfs into the inside surface.
Hey, if you are using a standard circular saw blade, your kerf is 3mm. I've seen absolutely nothing about how big the babies feet are, or what their stride length is. Smaller kerf blades are available. They may not cut a smaller kerf if your saw is not tuned properly. But with no information on foot size and stride of the babies; you have no idea if what you are doing is helpful.
If you are working with outdoor plywood, we typically have thinner face plies than interior plies. If your kerf cuts cut through a ply; we now have a situation where voids can come into play. If the kerf cut exposes voids, then water (which may be urine in the case of bat/bird houses) now has a path into the structure of the wood.
Gluing cleats on the surface avoids the void exposure problem; but with no knowledge of foot size or stride how does one plan how to put cleats on a surface.
There are various meshes/screens a person might put on the surface. Don't use a metal screen/mesh. A typical mesh is "square" (holes). So, if you have a 1/8 inch mesh, the spacing is nominally 1/8 inch vertically (and horizontally). An easy improvement is to tilt the mesh 45 degrees, and this reduced the vertical distance to about 70% of 1/8 inch. What I think is better, is to tilt to 22.5 degrees. Which gets us something around 38% of 1/8 inch. But it would be better if there was data on foot size and stride. One particular article suggested HDPE mesh, which I think is a really good material choice. I can't get mesh like that here (at local stores), so I am going to try vinyl coated fibreglass (black, which probably means graphite, which is a good thing for UV properties)
I don't know if stapling a mesh onto the surface is the best answer. But apparently if you staple the mesh to the surface, the first thing to plan for is to use stainless steel staples. I am working with 1x cedar (which is 3/4 inch thick), and while nobody in my municipality sells 5/16 inch stainless steel staples for a T50 stapler, they do exist. And if you want to save money, you need to wait a bit longer. But, if you choose to staple a mesh to a surface; the only recommendation I have seen is every 2 inches. Which probably means start from the perimeter, and work your way in to the centre.
All of the above more or less works with you choosing to make a bird/bat house out of 1/2 inch plywood.
Having a bird or bat land on a piece of wood and establish "purchase", is dependent on the density/stiffness of the surface. The lowest density/stiffness wood is probably balsa. It has almost zero durability. Pawlonia is probably next. It has some durability. The next step up is western red cedar, coastal redwood and dawn redwood. I never looked at other alternatives, as western red cedar works for me (even if it is too expensive). Dawn redwood (metasequoia) is not remotely a commercial wood. But, I am planting it on my farm, in the hope that it may grow. It shouldn't thrive here; it is colder and drier than it would prefer.
If people point out where they think I am wrong, that would be useful.
If people could point out suggestions for woods in wetter climates, that would also be useful.
By all means, build bird houses and bat houses. They will probably cut down on biting insect problems. And may help species.
Building a tree swallow bird house is a much less involved project than building a bat house.
I mis-sized the bottom of the tree swallow house (first one), so when I went to put the door on, it didn't fit. But the "dado" I had to cut to get the door to fit more or less flush with the other outside edges seems to work well.
Plans called for weatherstripping around the door. The "lock" for the door is a screw (not a nail), and in fitting the door for the (screw) pivots installed, the (front) side has almost no gap less than 1mm) all along its height. So, I inserted a weatherstripping for the other side. Just held in place by outdoor glue.