There are actualy two nest boxes at the Oak Creek Power Plant. Last year, a pair of Great Horned Owls took over the peregrine nest box, and raised young there. A second nest box was installed for the peregrines, who moved into it as soon as the owlets had fledged from the other box.
This year, the owls returned, and the peregrines again used the newer nest box. Yesterday, a pair of peregrines was on the ledge of the owl nest box. I'm pretty sure this is the pair from the other nest.