I have kept an occasional eye on the box recently and the first bird I saw in it was a blue tit, a male. He sat and looked around for several minutes, preening occasionally but otherwise just looking.
Then the female came in and they both spent some time looking around. The male is on the right (brighter colours and more white on the back of his head).
Later I got a short video. At first just the male is in the box. Then his mate comes in and you can see the difference in colouration.
Having decided this was an oversight and not a new nest I cleaned out this box as well, so now we are back to square one.
Since then Mrs B-T has continued to keep an eye on things.
Mr B-T pops in from time to time as well.
I also heard a commotion outside the box one time and what looked like the tip of a starling's beak poking in the entrance (which is on the left as we look at it). A few minutes later I saw a pair of starlings trying to get into this tree sparrow box below. The entrance hole is too small for starlings but that didn't stop them trying. The frightened sparrow was inside but eventually made its escape.
This is the male starling, with a blue base to his bill.
These two sparrows were keeping a close guard on their box while watching the starlings a few metres away.
Blue tits have also been challenging the ownership of this tree sparrow box.
In previous years the blue tits in the camera box have started bringing in moss in late March or early April so I guess at this stage they are just looking. I'll let you know what happens.