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. 
Attack! … image taken of the same two birds a moment or two later.
It is interesting to note that there were three other wood duck houses (unoccupied) near by, but the swallows only showed interest in this house, and contested it fiercely. Two more pics from this morning. Apparently whomever controls the door, controls the bird house.
