I tried the grass trick with a dandelion leaf so I could differentiate it from my other grass if it fell. The leaf was gone when I got home, but no more eggs, and the leaf was not incorporated into the nest. It is wicked cold here the next two days, so maybe I will wait through that and then remove the nest and egg. And I will be careful with the guard. In the last four years, I have lost two clutches, both to wrens, and once the eggs weren’t fertile, so the female bluebird tossed them and laid again. This is the early laying, so wrens aren’t as active here yet as they are later in the season.
Thanks so much for your advice.