I put two houses per post, 5 feet up and usually get tree swallows in one, BB in the other. Since I love Tree Swallows also, it is a win-win! I currently have about 20 boxes total.
I clean the nest boxes in the fall to kick the mice out or they foul the boxes. I occasionally will check the boxes year round for wren nests and mice, and kick those feisty little stinkers right to the curb. It is a battle to convince them to move someplace else, so setting up a few wren boxes a good distance from the BB boxes encourages them to leave the BB boxes alone. Otherwise there is no convincing them to leave.
Amazing to me is we have BB all year round, even in the worst part of winter. I catch them snacking on sumac seeds and small berries. They have yet to learn there are insects in the suet I put out. I always thought they were strict migrators, but apparently not.
