Over the many years of writing this blog, I have found that the posts that get the most traffic via Google involve screech owls and squirrels (and how to evict the latter from the former's nestbox).
If you don't evict squirrels, they will take over the box and -- sooner or later -- raise babies inside.
Here's the basic info:
When you see a squirrel enter the box, vigorously whack the side of the box with a broom handle (being sure to steer clear of the opening).
(If you install a nest cam, you can find out fairly easily if have any furry squatters. You can also use a selfie stick, sort of like a periscope.)
Once the squirrel vamooses, remove any leaves that the squirrel(s) have imported. If the squirrels go back in, repeat. And repeat. And repeat, until you wear them out and they give up.
In the coming months, I will be adding more advice on how to make it tougher for squirrels to move in.
If you have any (humane) squirrel-eviction advice to share, please email me. (For example, I recently read a book that suggested giving the squirrel a mild shock of electricity as a deterrent.)