On my drive to Kingman today, I passed lots of saguaro cactus, but was not lucky enough to find "saguaro boot". What is that?
Several species of birds make holes for nests in the cactus. The plant tissue around the hole hardens, to protect the plant from water loss. While the cactus is alive, the holes are used by many creatures. When the cactus dies, the hardened part is more resistant than the rest of the plant, and because the holes go in and then down, it has a somewhat boot-like shape, which gives the saguaro boot its name.