Eastern Bluebird, Sialia sialis

Enjoying a picnic lunch today at Potter’s Lake, I noticed a Bluebird flying into a Willow. Whenever I picked up my binoculars for a closer look, the bird was gone. Finally, Lorna solved the problem by pointing out that that bird was going in-and-out of a hole in dead branch on the tree. I wonder why that picked that site to nest instead of one of my two bluebird boxes nearby.