A couple years ago I spotted a chickadee going and coming from a hole in a snag outside my window. A day or so later I saw a Downy Woodpecker vigorously excavating the same hole. Much to my surprise another day or so later the chickadee was back and I observed activity over a period of weeks that indicated the chickadees nested in the hole. I don't remember if the chickadees were black-capped or chestnut-backed, I have some of both year round.

So, in addition to Steve's question, is it possible Downy Woodpeckers contract out tree excavating services?