The nest box hole protector does - just that! it will protect the entrance hole to a nest box against preditors like Woodpeckers and Squirrels etc.
Available in 26mm, 29mm and 32mm.
26mm for Wren, Blue Tits and smal birds.
29mm for Great Tits, Sparrows and Wagtails.
32mm for House Sparrows, Starlings and Spotted Fly catchers.