As usual, at the beginning of the season, female bluetits take as much nesting material out of their nest boxes as they bring in.
As an example, this picture was taken at 9:15am. The nest is coming along nicely one might think.
By 7:22pm on the same day, most of the nesting material had been taken out again!
My theory is that this activity ensures that other bluetits in the area are aware that this nesting site has been taken.