This is an old birch snag that, during the late spring was first excavated by chickadees. If you look closely, you can see at the top of the opening, where a nice circular hole goes right back into the snag. However, the chickadees ended up not using this cavity. Much later, woodpeckers took turns enlarging the hole, looking for insects, and even possibly contemplating this as a nest site. But it was much too low down and was not used by them either!