This is the second year I've had a nest box in the garden. The tits investigated it last year but did not build. The hole in the box was too large, ac- cording to my bird book, so I covered it with a piece of wood with a hole 25mm in diameter as suggested. This year it looks as if they may nest but it seems to me as if they are trying to make the hole larger with their beaks. - Greg Larkin, Killiney, Co Dublin
This is normal behaviour for tits. Before nestboxes they used holes of various kinds and they usually had to modify them and the interior too. They are adapting your nestbox to their own preferences. One year we had a pair of blue tits that nested in a hollow block in the corner of an outbuilding. The entrance hole was small and awkward but obviously it suited them.