I don't have a Screech Owl in my owl box anymore, but this is one of the times of year that squirrels like to move in for the winter.
Since I get a few page views daily from folks around the country who have a squirrel problem, and I have one myself these days, I thought I'd post something.
Squirrels not only prevent owls from using the box, but I have noticed they also damage the box itself with claw marks.
That's why it's important to stay vigilant about keeping squirrels from building a nest inside the box during the colder months.
If you see a squirrel enter your owl box, take a broom and whack the side of the box to get it out of there.
If you see it bringing in leaves, remove the leaves. Repeat until the squirrel becomes discouraged.
Earlier posts about evicting screech owls are here and here.