DURING CRUISE FLIGHT AT 800 FEET ABOVE GROUND LEVEL THE PILOT INADVERTENTLY FLEW THE AIRCRAFT INTO A FLOCK OF BIRDS. FOLLOWING THE BIRD STRIKE THE AIRCRAFT DEVELOPED SEVERE VERTICAL VIBRATIONS AND THE PILOT EXECUTED A PRECAUTIONARY LANDING. DURING THE DESCENT AND DURING THE FLARE PRIOR TO LANDING THE MAIN ROTOR BLADE FLEXED DOWN AND STRUCK THE TAIL BOOM RESULTING IN THE AIRCRAFT BECOMING UNCONTROLLABLE. THE AIRCRAFT STRUCK THE GROUND IN A NOSE LOW ATTITUDE AND ROLLED ON ITS LEFT SIDE.