Ive never seen them do this and Im hoping its a mating thing and not a fighting thing. My drake and one of his girls (who actually is his top girl) were biting each other they would kind do that head bobby thing but then one would get the neck of one and the other would get the head feathers of the other. They did this back and forth while playing in a mud puddle. It seemed to end with her getting him by the neck and kinda dragging him towards the kiddie pool. Yeah so they are ok right? Marital differences or umm... marital bliss? LOL