When a woodpecker excavates a nest cavity in a Saguaro (Carnegiea gigantea), the cactus responds defensively by sealing off its living tissue and produces a protective callus material heavily impregnated with lignin. As it continues its excavation, the bird first drills an entry hole into the cactus and then turns downward to hollow out a space for its nest. This results in a boot-shaped callus-lined cavity. When the Saguaro dies, the softer fleshy tissues rots away, leaving behind the woody ribbed skeleton of the Saguaro as well as any “boots” created by nesting woodpeckers. This Saguaro boot was found amid the woody ribs of a long dead Saguaro in the Rincon foothills of Arizona.