The pilot was landing at a remote, off airport site in a tailwheel-equipped airplane. He stated that he made two low passes over the intended landing site, which was narrow, but in good condition, with willow bushes lining both sides of the strip. After landing, the left wing struck willow bushes, the airplane veered off the strip and nosed over. The airplane sustained substantial damage to the fuselage. The pilot stated that there were no preaccident mechanical anomalies with the airplane that would have precluded normal operation.