These birds look perfectly normal.
This must be the female. She is performing a nesting wriggle.
It is now shortly after midday, and the bird here is the male,
When the female arrives both birds are momentarily in the box . . .
. . . then the male departs, leaving the female by herself.
An hour or so later, the same thing happens again.
Some pairs do this a lot in the early part of the season as this pair seems to be doing.
Again, the male goes almost immediately, leaving the female by herself.
Off he goes!
A rather nice shot of the male leaving the nest box.
They go on like this for most of the day.
Clearly the female doesn't seem ready to start building a nest yet. Let's hope she comes round to the idea eventually.