I ocassionally see two bees which seem to be wrestling, quite fast and energetically. They will fall off the landing board (about 2 1/2 feet) and continue wrestling on the ground for several seconds, after which the "winner" (the one on top) flies back to the hive entrance. The "loser" soon flies back to the same hive entrance also and all is peaceful.
These are worker bees which look just like the rest of them. Is this an acceptance thing for a strange bee? I don't believe there are any other bees within contact distance.