The pilot was performing a night solo flight to practice some night landings. While on downwind during his first traffic pattern, he saw a flash of four to six birds fly through his field of view from the bottom left to the top right of the airplane and subsequently heard a loud noise. He indicated that the impact lifted the right wing upward slightly and he thought it was a bird strike. He landed the airplane without further incident. The immediate postflight inspection of the airplane did not reveal any damage. However, substantial damage to the right wing was found the day after the accident flight.