When a saguaro cactus dies, it leaves behind a skeleton that supported it. But also there are boot-like structures, called boots. It’s the combined work of the saguaro and various birds which nest there.
Saguaros can grow as high as 50 feet so birds can rest easy knowing they are safe from predators. The bird begins by burrowing its way through the leathery skin of the cactus. Once inside, it digs downward, hollowing out a space for its nest.
As a result, the saguaro loses water through evaporation. Normally such a wound cause the saguaro to continuously lose water and causing it to be susceptible to infection. But the saguaro has a built-in mechanism for protection—it secretes a resinous sap all around the hollow. The sap hardens into a kind of permanent scab or bark, which remains intact after the saguaro dies. Once the bird creates the hole it leaves giving the sap enough time harden before it returns.The cavities remain relatively cool during the day and warm at night, offering relief from the desert’s extreme temperatures.
After a saguaro dies, its boots may provide shelter to more creatures, including snakes, scorpions, and spiders.
Saguaro boots were also used by the local Tohona O’Odham Indians as water containers like a primitive canteen.