Participant Photo: Tree Swallow at a nesting box

I have a lot of nesting boxes up for the Eastern Bluebirds, but they are always used by a sequence of birds. After the Bluebirds raise a brood, the Tree Swallows come in and use the boxes. Then, usually, House Wrens will take over as soon as the young Tree Swallows fledge. And sometimes, the Bluebirds will return and raise a second brood. I have seen lots of plans for different nesting boxes, and they are very specific about the dimensions, the entrance hole sizes and their distance from the box floor, et.al; but I have never seen any evidence that the birds care. It seems like any box, with any hole, will be inviting to lots of different birds.