I was in Eastern Washington for the early stages of the Spring migration. The Bluebirds had arrived, but were still establishing their territories. The Yakima Valley Audubon has set up Bluebird nesting boxes all along Umptanum Road outside of Ellensburg, and there was stiff competition for the boxes. I saw one box where the competition came to blows. A Western Bluebird pair had claimed a box, but a Mountain Bluebird pair very much wanted it. At first they would go to their corners. The Western blurbirds had clear possession of the box: While the Mountain Bluebirds were staked out in the surrounding vegetation:  Most of the scuffles occurred on the ground where they were mostly hidden from me: But on occasion I got lucky and got photos of them in aerial battle:  This war went on for at least 1/2 hour, but I chose to leave because it was too hard to watch. When I came back a few hours later, both sets of birds were in the area, but neither was near the box. I think it was a temporary cease fire, but the war was not over.