The Bird Guardian was designed for all cavity-nesting backyard birds. Designed, tested and approved by naturalists, this one-of-a-kind product is constructed of strong durable plastic made to last year after year. Each year scientific studies and tests continue to prove this product superior in the battle to save our endangered backyard songbirds. The Bird Guardian fits birdhouse openings from 1in through 1-1/2in. Easy to screw on with included mounting screws. Keep your birds out of reach from blue jays, cats, crows, opossums, squirrels and more. Wide flange covers squirrel and woodpecker damaged entrance holes.