When is the right time of year to build and install a screech owl nest box? Right now. It doesn't matter what time of year you're reading this, this is the right time. The reason is that screech owls search for nest, and roost, sites throughout the year. Visitation rates in my studies range from once every 2-3 weeks at the height of summer to many times each night as mating season approaches. So the sooner a box is in place, the more time your local owls have to discover it, and come to trust it. Screech owls normally lay their eggs a month before they believe spring will arrive in their area.
Here is my current body of notes on screech owl nest box construction, I hope they're useful to you.
Basic dimensions for an eastern screech owl nest box are:
(There's also a fairly good set of plans for a box of these general dimensions provided by the Conservation Commission of Missouri.)
Research by Gehlbach shows that although eastern screech owls will accept boxes with more floor space (greater than 8"x8") they don't select such boxes preferentially. I do recommend maximizing the box depth and the entrance height above the floor in order to increase protection from predators reaching into the box.
As my box currently stands, the depth is somewhere between 15" and 16"(I don't have any direct measurements at hand), the floor measures 8x10" and there's a perch (fancy word for an 8" stick) mounted across one side wall at a height of about 6" with a 2" gap between it and the wall.
The perch is an experiment, and an unnatural element in a nest cavity, but it seems to have been well received by every bird (owl and otherwise) that has visited the box. When disturbed, owlets seem to like to huddle beneath it, presumably because it increases their feeling of shelter, and it might well offer them some protection from a predator reaching into the box, though I don't know whether that's a common method of attack. Also, as owlets grow and the box becomes more crowded it provides another level of owl storage. :-) And finally,it offers owlets a goal for their early climbing efforts, and a place to practice their perching skills before venturing outside the box.Adults roosting in the box have also shown a clear preference for standing on the perch rather than the floor.
A good angle for the roof is 30 degrees, descending from the back to the front. I recommend extending the sides of the roof an inch or so beyond each side of the box to help protect the roof/wall joint from moisture, extending the front edge of the roof a few inches beyond the front wall of the box, and making the entrance hole 3 inches deep to improve predator protection. Another joint to protect from moisture is the joint between the floor and the walls - I recommend extending the walls 1/4" beneath the underside of the box to accomplish this.
If you can cut shallow grooves every half inch or so into the inside front wall of the box, that'll help owlets (and adults to a lesser degree) climb to the entrance hole (adults usually just jump). If you can't conveniently cut such grooves, a few strips of scrap wood glued to the front wall will be good enough. I installed honest-to-goodness horizontal slats at one inch intervals on the front and back walls of my box for these purposes, and for some camera field-of-view related reasons, but they're overkill (and squirrels delight in gnawing them into oblivion).
I find it more convenient to hinge the front wall of the box, so that it opens, rather than the roof as shown in some plans. Either works.If you hinge the front, and locate the hinge points (typically just a screw driven through each side wall) near the top, you have to be a little more careful about opening the box when it contains owlets to guard against the off chance they might fall out. If you hinge the top of the box, it means you have to be able to climb to a position above the box when you need to open it to check on owlets or for the seasonal cleaning (that seasonal cleaning is a little more difficult,too).
If you extend the side walls below the bottom of the box 1.5-2" you have the option of locating the hinge points for the front wall at its bottom, and the possibility of owlets tumbling out of the box as it is opened is further decreased. Also, since it will hang down from the box when open, instead of projecting out above your head, there's no question of a bottom-hinged-front staying open while you're working with it.
If you hinge the front, leave a 1/4" gap between the top of the front wall and the roof. This forms a vent. If you hinge the roof of the box, you can leave that same gap between the roof and the front wall,or you can drill a few 1/4" or 3/8" holes in the side walls, just beneath the roof to provide ventilation.
I know one fellow in Perth, Australia, whose nest boxes (for Boobook owls, and various indigenous parrots) do away with a hinged door of any kind (front or roof), and simply have a 3-4" deep pull-out drawer at the bottom of the boxes. If you want to open the box to examine the owlets as they develop, this design could be a problem, however.
Drill four or five 1/4" or 3/8" holes in the floor to provide drainage, should any rain find its way into the box. Also, cut about 1/2" off the corners of the floor (at 45 degree angles, as you'd expect) to create holes in the corners that will provide further drainage, and eliminate the possibility of water pooling in a corner and damaging the box in the long run.
Use untreated wood; treated wood may pose a health threat to the owls (and even the wood worker). If you use plywood, exterior grade is recommended. Paint the outside of the box with an exterior grade latex paint. Do not paint the interior. If you can, cover the roof with a sheet of aluminum flashing, and bend the flashing down along the sides of the roof such that it extends a quarter of an inch or more beneath the underside of the roof, thereby creating a lip which will prevent water from reaching the underside of the roof and the roof/wall joint.Applying flashing to the roof in this manner should do two things: (1)it should greatly extend the life of your box by improving protection from the elements (and gnawing squirrels), and (2) it should further protect the occupants of the box from predators. It may also discourage squirrels from occupying the box, since they can't easily climb down the roof and into the entrance hole. If you want to go further, covering the sides of the box, and even the front, with flashing can effectively squirrel-proof the box, and anything that can keep squirrels out should make access by climbing predators nearly impossible.
While you're buying aluminum flashing, buy some more to wrap around the trunk of the tree in which the box will be mounted. Using two pieces to create a 24" band of flashing around the trunk just below the first branch has worked well for me, so far. Without the flashing,my owls have consistently been killed by house cats (10 deaths in two breeding seasons, including both breeding females, so you'll understand why I've come to so heavily emphasize predator protection).
The flashing around the trunk, however, has an unfortunate side-effect in that it forms an impassable barrier for owlets that fall out of their tree and are trying to climb back up to safety (a basic skill of owlets until they learn to fly). I recommend two things to mitigate this side-effect: (1) remove the flashing just before the first owlet leaves the nest box, and (2) provide an "owlet rail" on the front of the box. An owlet rail is a long stick mounted parallel to the front of the box, some inches in front of and beneath the entry hole. It provides a branching owlet with a safe place to stand while it experiments with its wings (they won't be able to fly for a week or more, but they can generate some helpful lift) and consider how they are going to climb and/or jump from the box (and the rail) to a nearby branch so they can begin exploring their tree and climbing to a safer and sheltered height within it. If you can extend the rail either to the point that it meets a nearby branch, or around the box such that it meets the trunk of the tree, that'll make things even easier for owlets.
Unfortunately, the owlet rail may provide a useful platform for climbing predators, so it's not all good, but I'm currently of the opinion that it's more helpful than harmful, provided flashing is installed around the trunk of the nest box tree. If you're so inclined, and you box design allows for it, you can put off attaching the rail until the owlets are about to leave the box. (A good indication that the first owlet is soon to leave is when you start finding an owlet parked in the entry hole, staring at the outside world.)
Rub the underside of the roof with a bar of hand soap to discourage honey bees from taking over the box; the slickness of the soap residue prevents them from attaching their combs. I've also used parafin wax for this purpose by melting it and brushing it on, and it seems to have worked.
Install the box in a tree at a height of 10-30 feet. (Heights as great as 50 feet have proved acceptable to some screech owls, as I understand it.) Since screech owls fly almost exclusively *beneath*tree canopies, and probably prefer clear flight paths to and from a nest box (and everywhere else, for that matter) it's probably counter-productive to mount the box high in the canopy of the tree.See if you can find a spot on the trunk above/amongst the lower branches. If your trees are highly branched and have no single trunk at height, like mine, you can mount the box on a sturdy limb, although the actual mounting process will be trickier.
I generally run a 3/4" dowel through holes drilled in both sides of the box where the side walls meet the back wall and roof, allowing the dowel to project 8-12" beyond the sides, and lash the box to the tree trunk by repeatedly looping rope around the ends of the dowel, around the back of the trunk, over one end of the dowel, back behind the tree and over the opposite end of the dowel, back around the back of the trunk to the other end of the dowel, and so on. Others prefer to just drive some screws or nails through the back wall of their nest boxes and into the trunk of the tree. If you opt for this latter approach,consider using aluminum nails or screws, so that any that are left behind in the tree won't pose a danger to the tree-cutter who may someday remove the tree.
If you're feeling really clever, and have time on your hands, consider constructing a mounting bracket of some kind so the box hangs securely in the tree but can also be readily removed for examination or maintenance. If you include a pulley system in your bracket design,you may never have to climb the tree to install, examine, or maintain the box. I've taken this approach and it has worked-out extremely well. (Of course, with my massive box, that was the only way I could get it into the tree in the first place.)
Once you have the box installed in a tree, place 1-2" of shredded wood or wood chips on the floor (sawdust is probably a bad idea, as is the use of cedar shavings). Screech owls don't build nests, so they'll lay their eggs directly on this material. This material will subsequently absorb the urine and droppings of the owlets, so it should be replaced each year to avoid creating a persistent habitat for nest parasites.
That represents most of what I've learned/concluded thus far about eastern screech owl nest boxes. Let me know if I can clear-up any questions.
Chris W. Johnson