I am an amateur bird observer in Eugene, Oregon and decided to put up multiple nestboxes this year. One is directly outside my window from where I work at home and can observe all day. I was super excited when it was chosen by a pair of Bewick's wrens. For a couple of days they were very busy bringing twigs to the box, clearly building a nest inside. I only saw them in the mornings. Suddenly they stopped coming and I have not seen any sign of them in two days. Is this normal?