Callopistromyia annulipes? This fly was patrolling back and forth along a section of wooden fence rail. He occasionally signaled while patrolling, as in the first shot. As I got closer, he stopped patrolling and seemed to signal at me. This is the point where in the past others of his species have taken off. This time, he suddenly wheeled almost 180 degrees, turning his rear to me. Second shot. An intruder had landed on the rail about 30 inches away. These are tiny flies and this is still far apart. Third shot is of the intruder. They did not close the distance at all, but both signaled vigorously for a few seconds, then the first fly turned again and ran directly away from the intruder, defeated, I thought. Actually he ran less than 12 inches to a fence post, climbed up a few inches, not quite to the top, wheeled, and began signaling again. After a few seconds of signaling back, the intruder fled. Trying to get a close-up of the "victor", I spooked him. Various interpretations of this interaction are possible.