The Nature Conservancy has a fantastic photo feature on the Saguaro for Summer 2008.
When Gila Woodpeckers, gilded flickers or other excavating animals abandon nest cavities in saguaros, opportunists… move in.
Photo © John Cancalosi/naturepl.com
Did I say it was fantastic? Well, I only borrowed the one photo, while there are dozens more where that came from, including a shot of a western screech owl moving into that giant hole the woodpecker pecked out.