Hi: I'm in zone 5 on the east coast. I have bluebirds and tree swallows using boxes now. The bluebirds have 5 eggs in one box and the tree swallows have claimed the other box, but that is as far as they have gotten. They have been here for 3 seasons now. I believe they feel more comfortable and territorial here now. The bluebirds winter over here. This is the earliest they have nested. The first year I think they finally had a nest attempt in May (house sparrows kept taking over) and were finally successful in July. You may only have house sparrows if you have them in your area. They will out compete (and kill) every other species that makes an attempt to nest unless you help them.

I'm in zone 6 and the chickadees are just starting to build in the box they used last year - guess they evicted the downy that was using it as a roost box. I've also seen sparrows collecting nest materials, and the cardinals are checking out the old nest in the aristolochia...