Fluffy is clearly getting ideas about Box2.
"Do you think the nest cup should be here?"
Another rapid exchange - this time between Fluffy and Flash.
The nest in Box1 proceeds apace.
"Yes, a bit of hay there."
Fred comes to inspect progress.
"Come on then - time's up."
Fred's off!
When a bird is thinking about entering the nest box, and is hovering about the entrance hole outside, the light intensity inside the box can alter substantially. We can always tell whether it is the cock or the hen about to enter as the hen charges boldly in, after all, it is her nest box, whereas the cock hesitates for ages before plucking up the necessary courage. Presumable, he would get a telling off if he went into the box when the hen was already inside!