This is a ‘saguaro boot’. Let me explain what it is. Two species of woodpecker, Gilded Flicker (Colaptes chrysoides) and Gila Woodpecker (Melanerpes uropygialis) have adapted to live in arid desert environments in the SW USA and NW Mexico. Both species excavate nest holes in the fleshy trunks and limbs of saguaro cacti, but the plants treats these intrusive cavities as wounds and react by secreting a lignin sap that hardens around the hole chambers, effectively making protective sealing shell. Cleverly, the woodpeckers do not use the holes until the following year, waiting until the sap has dried out and the scar tissue of the chamber wall has become solid and watertight. When a cactus dies, its outer flesh rots away, but the tougher woody interior and the callus around any holes remain. These shells are angular in shape, keeping the form of the entrance and the chamber of the woodpecker cavity. These so-called ‘saguaro boots’ can be found on the ground amongst the debris of dead cacti. Interestingly, the boot in this photo is a double one! Thanks to Noel Snyder for sending me this remarkable image from Arizona, USA.