The instructor pilot stated the intent of the flight was to practice autorotations, 180 degree autorotations and emergency procedures at another local airport. While enroute, approximately 1500 feet above the ground, three birds flew toward the helicopter. The instructor pilot took the controls as one of the birds and the retreating main rotor blade collided. The pilot landed the helicopter without further incident. Examination of the helicopter revealed substantial damage to the main rotor blade.