Saguaro boot

A saguaro boot is the hard shell of callus tissue, heavily impregnated with lignin, that a saguaro cactus (Carnegiea gigantea) creates to protect the wound created by a bird's nesting house .[1] The bird pecks through the cactus skin, then excavates downward to hollow out a space for its nest.[2] When the saguaro dies, its soft flesh rots, but its woody infrastructure lasts much longer. So does the hollowed-out callus whose roughly boot-like shape gives it the name of "saguaro boot."[3]

Saguaro boot with US quarter to show scale

Several different kinds of birds create nest holes in saguaro cactus. The Gila woodpecker (Melanerpes uropygialis) creates small holes (about 5 cm across) at midlevel on the cactus, where the ribs are far apart,[4] feeding on larvae under the cactus skin.[5] The larger gilded flicker (Colaptes chrysoides) drills bigger holes higher up,[6] where ribs are close together, because its beak is strong enough to break through rib tissue.[4]

The saguaro responds to the bird's damaging its tissue by secreting a resinous sap that, over time, hardens into a bark-like shell that prevents the cactus from losing fluid and also protects the nest hole by making it waterproof.[4] The bird's nesting hole requires not only the bird's making a hole but also the cactus's lining the hole - it is not ready for use as a nest until a year after its creation.[4] Many saguaros are home to multiple nests; if birds excavate adjoining hollows, a saguaro boot may be formed with more than one opening.

Native Americans of the Seri group used saguaro boots to store or carry water.[7] It is now illegal to collect saguaro boots from the wild in Arizona.[8]

Some desert moth caterpillars also make tunnels inside saguaro cactus. The resulting dried callus that forms around their tunnels has a flattened disk structure where the caterpillar exits instead of the larger hole seen on a saguaro boot.[1]