I received a silver birch nest box as a present last year. We have a patch of oak woodland as part of our land so I put the box up on one of the trees along with several existing standard boxes. Over the years we've only ever had blue tits nest. With this new style box a new species has come along - a pair of tree sparrows, uncommon birds for our immediate area so a real delight. Is there any correlation between nest box type (same sized hole) and species?