When we first had cameras in successful boxes in St. Luke's Church, Kinoulton, I remember being very surprised that, after the chicks had fledged, the adults continued to roost in the box for up to a week. My memory (which is far from reliable!) is that this happened when the chicks had fledged early (end of July, beginning of August) but not when they were late (end of August/beginning of September). I think we concluded that those adults which felt they had time to take a well-deserved rest between parenting and migrating did so, whereas those who felt they needed to get on with migration asap left straight away. Not very scientific but it seemed to explain what we saw!
Carol