Squatters!

We put up two swift nest boxes last year.  The house sparrows moved in.  This year we have fitted "screechers" to try to attract the swifts to use the boxes but again the sparrows got into one first.  The other box has been colonised by tree bumblebees.  

It's all very fascinating to watch.

https://www.bumblebeeconservation.org/tree-bumblebee-bombus-hypnorum/