Nesting Box Activity

I'm not quite sure what's going on. There's a lot of coming and going but I don't hear any squeaking chicks. I've added an extra of the bird which is also the main. It looks young but I wonder why it's exiting the box with something in its beak. Maybe it's clearing the box out for a new nest. Any ideas? **

It's a warm and slightly sticky day.  Hair cut first thing which is a relief then I have to do a bit of baking for tomorrow. That's going to be hot. Otherwise I'm sitting outside in the shade. 

** Thanks to Blipper Shutterup I now know this to be a faecal sac. Tidy!