Here's an interesting video
posted to YouTube that shows what appears to be a falcon flying past earlier today while Durand watches from the ledge. (Note the fly-by takes place at 1:43; below is a saved photo from the video.)
Who would this mystery bird be? There's a good chance it is not the resident male as it just doesn't make sense that it wouldn't land on the ledge and/or assist with incubation as he had been doing. Durand's acceptance of this bird tells us it isn't a female as were that the case she would likely be airborne trying to chase the intruder out of town.
Breaking News! As I was typing this update the "new" bird made an appearance in the nestbox!!! Here he is:
This falcon is in subadult plumage (brownish back, some streaking on the breast and legs still evident) which would mean he hatched last year. I also noted a purple leg band on the right leg and what looks like a black over red band on the left leg. Hopefully we will get a closer look at the codes and be able to confirm where he came from!