Do not use mylar strips before an egg is laid - it will spook other birds. Try monofilament in the interim for HOSP. http://www.sialis.org/hosp.htm#monofilament Downies will not nest in your box - they will use it for roosting. or try a Gilbertson box paired with the wooden one - HOSP like it less. Here is a link that might be useful: all about downy woodpeckers.

Beware that the oval holes on the petersons I believe are only 1-3/8 wide,If you put a 1-1/2 reducer on it ,the hole will be smaller,Still only 1-3/8 wide by 1-1/2 tall,making it smaller them the standard 1-1/2 round hole.You may end up locking them out.I use 1-1/8 reducers on my Chickadee nest with out ever having a problem.

Sorry to hear this Tim. If I were you I would remove the HOSP nest and hope the blues return. I doubt you will get the female HOSP now that the male has left. In my experience with TRES I see them highly motivated to run off any bird in paired boxes when they are nest building. Once they are laying/incubating and feeding nestlings they focus much more on that then defending the other box. I have lots of paired boxes and that is what I have found anyway. As far as the Blues letting the HOSP take over the box...well they know they cannot win I suppose, no eggs or nestlings to defend so it is hard to say. I think each bluebird is different too on how hard they defend things. This year I have a hugely territorial male, never had one like this before. He has chased off my chickadees and is trying to chase off all the Tree Swallows in the yard. He is defending four nestboxes, two of these are 65 yards or so from the box they have eggs in. Nothing will be nesting in the box paired 35 feet from the blues nestbox...he won't allow it. The chickadees weren't even house hunting, just trying to eat out of the birdfeeders and he won't allow it. A couple times I have seen a HOSP come to the feeder and I quickly try to get the bb gun but the Male bluebirds chases it away before I can even think about it. He won't stop chasing until it is out of the yard too. I am curious to see if he calms down a bit once the eggs hatch which should be next week. Good luck to you, hope your blues can get down to business now that that HOSP is gone. Donna

Bummer that you have to put the protectors on. My guess would be a Downy. They like to gain access. Since there's some straw in the bottom of the box, my feeling is that they are planning on using it to roost in for the winter. They've pecked at mine, but I have the wooden blocks attached to the front of the hole, so they haven't had much luck actually getting in, or destroying the original hole.