Not only do the adult Wrens work endlessly feeding the young…they are also required to do clean up as the young do tend to poop allot.
This is done by who knows how but everything is put into a fecal sac (doo doo bags) and removed from the nest.
I’m only going to assume that this would be done by any cavity nester.
Besides I would have charged them extra for clean up if they hadn’t done so themselves.