This simple device can cut down considerably on the depredations of predators on your nest boxes! By adding length to the entry hole, it keeps predators such as Squirrels, Opossums, Cats and larger Bully Birds from reaching into fragile nests and destroying the eggs and baby birds inside. Especially made for Bluebird, Wren, Chickadee and Swallow nest boxes, it attaches easily with the screws provided. Made of durable plastic, the hole measures 1 5/8" across, and has a tail brace to aid entry for the parent. Tube is 2.5" long.