A witness on the ground was actively watching the helicopter when he saw several pieces fall off the helicopter and subsequently land in his yard and a neighbor's yard. The commercial pilot said he was in enroute cruise at approximately 1,500 feet above ground level when he felt a "bump on the controls" and heard a "bang" and thought he had had hit a bird. He returned to his departure airport 45 miles away for an otherwise uneventful landing. On post flight inspection the pilot discovered the lower portion of the left engine cowling was missing and there was evidence the cowling pieces had struck both main rotor blades and both tail rotor blades, but no evidence of bird remains were found. The impact caused substantial damage to the left engine cowling, both tail rotor blades, the tail rotor yoke, and the tail rotor gearbox. The pilot and his passenger were not injured. The pilot said that before departure he had conducted a preflight inspection, however the FAA Inspector that examined the helicopter, and the separated pieces of the left engine cowling, found no evidence of bird remains and no damage to the cowling latches that would have prevented them from operating normally prior to the accident.