An adjustable barrier for use with a building having a door frame with a door frame opening, a track assembly, and a garage door positioned between the track assembly to selectively cover the door frame opening, the adjustable barrier comprising a barrier bracket and an extendable barrier panel. The adjustable barrier is adapted to seal off a garage door gap located between the garage door and the door frame, and is positioned between the door frame and the track to create a barrier preventing a pest from bypassing the garage door and entering the building through the garage door gap. The extendable barrier panel selectively extends and retracts to adjust to the distance between the track and the door frame, and may further be tapered to allow the adjustable barrier to maintain contact with the track when the track curves away from the door frame.
