Immediately the bird first inside departs. This is typical if the two
birds are male and female and have formed a partnership. The male has now
left leaving the female in the nest box.
So what's new there?
The thing that is unusual is the hesitancy shown by both birds. They both
spent ages appearing to be plucking up courage to go into the box as though it was
new to them or in the case of the second bird, the other bird was inhibiting them.
When the original bird had left, the newcomer spent the time just looking around.
Is this a new pair trying to take over the box?