Medium:

Bark of a tree that was used by an acorn woodpecker (Melanerpes Formicivorus).

The acorn woodpecker as their name implies relies heavily on acorns as it’s food supply, so the woodpecker creates these “granary trees” by drilling thousands of holes into a tree and storing its supply of acorns. Each granary may contain close to 50,000 holes in it.
They are the ultimate hoarders of the bird kingdom...

Each year the woodpeckers reuse the same tree, filling up the old holes with acorns and drilling some new ones. The acorns are wedged in so tightly that it’s very difficult for other animals to remove them.
After they’ve been stored for a while,
The fit becomes looser as the acorn dries out-group members periodically check their stored acorns and move the loose ones to smaller holes, and store new ones in the old holes.