Interesting fly behavior! There was a tree stump with several of these flies on it. The flies would land on the stump, two would turn to face each other and walk up so they were face to face. Sometimes one would fly away at this point. If both stayed, they would then go head to head and push each other about, seemingly with quite a bit of force. After a brief (maybe around 10 seconds) fight one would fly away and the other would stay on the stump. I watched these flies for a few minutes and several fights took place. I took a lot of photos because I wanted to record one of these fights. You can see how the two flies pushed each other around, and the last photo shows the one who stayed on the stump. I presume they were males fighting for females or territory? I also found a mating pair by the edge of the stump.