Bird Spider Bird Roosting Deterrent is perfect for air conditioning units, signs, rooftops and boats. This ingenious bird repellent has stainless steel “arms” that move with the breeze to keep birds from landing and roosting. 8"-dia. unit can be mounted with the included 4" x 4" PVC base and may be installed in a row. Arm tips should overhang the mounting surface by at least 2".