The next thing I see is one coming out of the left box. Now I know they have no adjoining door inside, but it did look like in one side, pop across, out of the other as a sort of game.
Fairly soon I noticed a second, I think the female, collecting nesting material around the garden. She quickly returned to the Box.
When she arrived it was straight into the left hand box, a process repeated dozens of times during the day.
While this was going on he either followed her in, or sometimes he went into the right hand box, and quickly came out, seemingly puzzled by the arrangement. Other times he sat on top of the vine and sang. Not the greatest song, but he was very enthusiastic.