On the bluebirds watch out for wrens. They love to destroy bluebird nests and take over.
Thx. Will do. So far the wrens have their own houses and stick to them. I watch out for House Sparrows more than the wrens. They are plentiful here, but so far are keeping to their own spots. All 4 of the bluebird houses are within view of my sunroom windows. (My backyard is on a hill, so I my sunroom is above these, almost like a tree house, but not really, just an elevated view of the back.)