Sunrise through the grapevines

Checked the birdhouses today. All six houses w/ tree swallow babies are now empty. Within a weeks time they all had taken flight. The parents carefully clean the nest right after the babies hatch but later stop. My guess it's part of their strategy to make the nest box so disgusting that the little ones are happy to leave. I had the unpleasant task of cleaning them out today. My neighbor told me that his friend doesn't clean out nest boxes until the end of the season since tree swallows only have one brood a year. I looked it up in one of my books and that's what it said too. Maybe the birds haven't read that book since the last nest box I checked (it had been unused so far this season) has new born tree swallows.