Contributed by Dick
On the inside of the louvres of this box, where the entrance is, there is a recess about 10mm deep . We have recommended to South Cambridgeshire District Council that they fill this recess with Polyfilla in order to reduce the entrance to 30mm.
If Starlings are occupying your Swift boxes, then reduce the entrance sizes and put up Starling boxes some distance away from the Swift boxes.
You can read more about entrance sizes in this post