Missing a screw

Back to the new birdbox today to fit a protection plate which, according to the packet it came in, will safeguard the nest box from squirrel and  magpie attacks, both of which creatures we get in our garden. It also protects the entrance hole against gnawing and pecking. Oddly, it only came with 3 screws notwithstanding it has 4 holes. I may have a screw somewhere of the right size but, for the moment, it's secure enough. Also cleaned, repaired and repositioned the bird box we have on another tree. That one already has a protection plate so it should be squirrel safe. So here's hoping for some Spring hatching.