I’ll start with the tools of the trade… gloves, mask fairly self explanatory, after all these are predators there are remnants of their prey in the boxes if the box was just used as a roost. Putty knife is helpful for removing the bedding from a box (especially if it was used as a nest), after it dries out its a bit like dusty horsehair dusty plaster . A Drill or screwdriver to open up the box and for backing off mounting screws (more later in post on this). Hammer is to tap in any staples that hold the vinyl covering if any loosen up in the normal expansion/contraction in the various weather conditions. Finally a bucket of new bedding material to replace the old.
I typically will clean out nest material every year, but if the box was used only as a roost, may only clean out old pellets an some feathers, add a little new material, even skip a year. If it was unused its not a bad idea to open up a box and check just to see whats going on. Birds like starling will clean out wood chips. So you may have to add bedding/wood chips even if no owls were seen. And who knows, there is an off chance that some unexpected evidence of owl use (pellets/feathers) remain even if no owl was ever seen.
WARNING, sometimes Wasps (or bees) will make a hive in an owl box, I removed one from the “pole” box from Site 3. These were European Paper Wasps, They actually began using the box last year, but didn’t seem to deter the owl as seen in this post so if you look closely at the picture you can see a wasp in the upper right corner of the box, just under the roof. Unlike the last two years the female has not used this box in the late summer. Did the wasp nest get too big and deter the female? Anyway the box is cleaned out, Wasps were removed, and I didn’t even get stung.
Note there was another section to this part about 1/2 the size so it was fairly large.
Lastly, as mentioned earlier an important part of yearly maintenance, is backing the screws out of the tree. Trees grow thicker, and this will pull the screws though the wood if they aren’t given some ‘slack’. Takes a bit of trial and error to loosen the screws up a bit, without having the box be loose. Which is why I bring the cordless drill, it is a bit tough to back these out with a hand screwdriver.