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 hole. The bird pecks through the cactus skin, then excavates downward to hollow out a space for its nest. 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."
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, feeding on larvae under the cactus skin. The larger gilded flicker (Colaptes chrysoides) drills bigger holes higher up, where ribs are close together, because its beak is strong enough to break through rib tissue.
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. 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. Many saguaros are home to multiple nests; if birds excavate adjoining hollows, a saguaro boot may be formed with more than one opening.
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.
- "Sonoran Desert Field Guide". Sonoran Desert Naturalist. Retrieved 10 March 2011.
- Don, Glass. "A Moment of Science: Saguaro Boot". Indiana Public Media. Retrieved 11 March 2011.
- Flora of North America: North of Mexico. Magnoliophyta : Caryophyllidae. Caryophyllales. (Pink order), Volume 4, Part 1. Oxford University Press. 2004. p. 185. ISBN 0-19-517389-9.
Scar tissue that forms around nest cavities excavated in saguaro stems is later encountered among the remains of a dead saguaro as a hard, brown shell known as a "saguaro boot" because of its shape.
- Mark Elbroch; Eleanor Marie Marks; C. Diane Boretos (2001). "saguaro+boot" Bird tracks and sign. Stackpole Books. p. 311. ISBN 0-8117-2696-7.
Both gilded flickers and Gila woodpeckers make these cavities for nesting but they often choose different locations on the cactus.
- "Gila woodpecker". Nature Conservancy. Retrieved 2011-01-24.
The marks signal “Here be larvae” to the bird, and as it excavates the insects, it also cuts away the diseased tissue. As the sap hardens, the cactus is healed, and the excavation becomes a convenient nesting site.
- "Gilded Flicker". National Audubon Society. Archived from the original on 2007-12-03. Retrieved 2011-01-24.
A nest hole is excavated in a tall saguaro cactus, about 9 feet from the top and 11 to 25 feet from the ground.
- "The Long Arm of the Saguaro". Smithsonian. Archived from the original on 19 October 2011. Retrieved 11 March 2011.
- Blocker, Carol S. "The Saguaro Boot". Cactus and Succulent Society of America. Retrieved 10 March 2011.
Close-up of living saguaro with hole, wound-response lignin, quarter shown for scale