you may also find that although there is a lot of activity at the moment, they may just suddenly abandon the box altogether, which is what 'my' blue tits did this year. This would have been the first year that the box would have been used, and like you I was very excited, but then suddenly nothing. I have had a sparrowhawk in the garden recently though but not sure if she appeared before the blue tits went. Good luck though I hope they use your box and lay eggs, although I am extremely jealous :)