The pilot reported that he was flying the helicopter to specific locations within hilly sage and brush covered terrain in order for his passenger to set mining claim stakes at various intervals of 300, 600, and 1,500 feet. Movement between locations was typically conducted at a low altitude. The pilot stated that as he flew the helicopter over the crest of a hill, he felt a shudder and subsequent vibration and initiated an autorotation to an open field. Following an uneventful landing, the pilot noticed that the tailrotor and tailrotor drive shaft were damaged. The pilot further reported that during recovery of the helicopter, he located a bush with several broken branches which was located within the flight path of the helicopter and felt that the tailrotor struck the bush while he was maneuvering. The pilot reported no preimpact mechanical malfunctions or failures with the helicopter would have precluded normal operation.