To the uninitiated and those of us who do NOT think like tree swallows, this may look like a wood duck house, but NO! This is obviously a tree swallow house.
Tree Swallow #1 (lower bird) does not know this fact, but it is about to get attacked by Tree Swallow #2 (upper bird). The fight was over this bird house, AND a nearby female. These are the first tree swallows I have seen this spring near Duluth.