I am reposting this column from 2010 by popular demand, with an added link to The Screech Owl Companion.
Squirrels invading Screech Owl nesting boxes are like early stages of a forest fire: If you can fight them early enough, and persistently enough, you have a chance of putting them out.
One minute your Screech Owl is chilling in the box (left), and the next minute a squirrel has moved in with extreme prejudice.
You cannot let the squirrel have a chance to get attached to her new digs. You as the landlord have to evict the squatter asap.
In the eviction process, I have found that a nesting box video cam attached to your TV or DVR is half the battle.
Thanks to the mini-cam, and a three-pronged attack, I have had good success in evicting these bushy-tailed rodents.
(Note: No squirrels were injured in the making of this blog post.)