I spent 15 minutes putting up new twine and a "no trespassing" sign on the periphery of the owl box yesterday.
   While I was marking the perimeter, I thought I would check the wood duck box near the owl box -- and found the dead bird (above) inside.
   He wasn't there Sunday, when I found this owl pellet. 
   I have placed a quarter  by the pellet so you get a sense of size. The owl did not cough up the quarter, in case you were wondering.
    And yes, that is a pellet.

    My guess is Ace is storing food there during the winter, then moving it as needed. Sort of a "cache and carry" situation.