This 3-sided cage with 1-1/5" openings will keep out squirrels and larger birds on your window-mounted platform feeder. It sets on the tray and the acrylic topper rests on top of it. Lift cover to fill platform with seed. 

Use this cage on the window-mounted window feeder to keep bigger birds and squirrels from feeding. Must also use the acrylic cover. Sold separate.