The pilot reported that prior to taking off for a personal flight he added 6 gallons of fuel to the airplane. This was adequate for several hours of flying. Thereafter, he proceeded to practice maneuvers in the local area. According to the pilot, no mechanical malfunctions occurred during flight. After flying about 45 minutes and during a turn, a brown bird suddenly appeared in the upper right corner of the windshield. It collided with the airplane. A portion of the windshield broke out, and the pilot lost partial control. The pilot was unable to maintain altitude, and he made a forced landing in an open field. During rollout on uneven terrain the landing gear separated from the fuselage, which was bent.