Screech Owls love Wood Duck boxes.

About 1/3 of our Wood Duck boxes have Screech Owls in them. I stick a little camera in the hole and take a picture, before I open the door. If there's an Owl in there, I leave it for the Owl. Scat on the outside indicated a Screech Owl was using it. Couldn't see in the dark box, so opened up the side, the Screech Owl was sitting there. Quickly shut the door and left it alone. Will check again in a few weeks. If it's not  in there, will clean it out and put fresh cedar chips in, for cleanliness, to prevent mites, for mating Owls, or Ducks.