18:59: I think the hatchling are almost ready to leave the nest.
I have the feeling that they are being invited by one (or both parents) to leave the warm and comfort of the nesting box. The parent bird flies to the entrance of the box and then flies away again. Sometimes it has a worm in its beak. The hatchlings are really active and are continously chirping. I wonder if the hatchlings will leave tonight or not. Last year the Great Tits left at 8 pm so it is not too late yet.