Goosemaster's suggestion was actually really good- get a sheet of acrylic the width of the AC, and 36" long, duct tape one side to the outside edge of the AC, and use clear packing tape on the window side. You can still look through, and the pidgeon's can't land. Be sure to block off the sides with more acrylic- it might be easier to get some pallet wrapping tape though- it's like saran wrap but really tough. Generally, it comes on a roll like paper towels, then the minimum wage guy tapes one roll to the side of the stack of boxes on the pallet, then runs around and around and around and around and around and around, eventually wrapping up the whole pallet of boxes.