I spotted a wasp nest (I believe it is a species from Europe similar to our yellow jackets--but, luckily, minus the temper). Perhaps they could have lived in harmony with the tree swallow family, but I wasn't taking any chances, so, after taking a picture, I grabbed a stick and knocked the nest onto the ground.
Shortly after I walked away, one of the parents (I can't tell them apart) went over to check that everything was okay. :)
Perhaps this pair of tree swallows will learn to trust me, but I'm guessing after the young have hatched, they won't take as kindly to my monitoring the box.