We have had a birdbox on a cherry tree in our garden for several years. The hole at the front was originally about 2.5 - 3 cm in diameter. We were told it was the right size for tits, and we have had several tits nesting there over the years. However, over the years, something has pecked or gnawed or nibbled at the hole so that it is now 4 - 5 cm in diameter. What has most probably done this? I have seen bird boxes with a metal plate on the front in which the entrance hole has been made. Should I do this (to reduce the size of the hole) or should I leave it? If I do remake the hole, what size should it be? I would hate to make it a millimetre too small for anything to nest in the box.
A kestrel won't take baby birds from the nest box, but squirrels will. You may have had a woodpecker enlarge the hole, either looking to nest in it or to eat the contents (they take young birds from a box!). Why don't you either replace the box and move the old one somewhere else, to see what comes to it, or contact the RSPB, who'll be able to tell you the exact measurements needed for the hole.
I thought squirrels were vegetarian. I could understand them eating eggs, but I would have thought the bird incubating the eggs would have been enough to deter them. However, I really do not know, so I am not saying anybody who has replied (thank you) is wrong. Are you all saying that there is not much point having a birdbox, since the chicks don't stand a chance? Or is it only because the hole is too big? I will check with the RSPB and put a new smaller hole over the front of the box, if they say that's the best thing to keep predators out.
Not saying that at all, Bert - nestboxes are a great thing to have, especially as our wild birds need all the help that they can get. Good luck with your box(es) this spring!
p.s. do watch out for woodpeckers, though!