Picking up from the previous entry about chickadees:
The black-capped chickadee dwells in tree hollows, those spaces within the trunks of trees that have been hollowed out by birds (especially woodpeckers searching for insects) or by injury or from sim[ple decay. These hollows are often deep and include pulpy wood. The hollows are relatively safe and comfortable … for birds.
Not only birds but humans can find especially large tree hollows attractive dwelling-places. A relevant hagiographical example is the 12th century Dutch hermit Gerlac.
Gerlac Valkenberg had been a secular, worldly character most of his life, a soldier and mercenary. When his wife died, he began wandering, reaching Jerusalem on a penitential pilgrimage. Upon his return to his Netherlands home, Gerlac renounced his possessions and became a hermit, finally settling into a oak tree hollow as his dwelling.
Gerlac’s eremitic life upset the abbott of a nearby monastery, who tried to convince Gerlac to join the monastery. Gerlac refused, and the abbot began spreading rumors about Gerald’s supposed thievery and robberies, even persuading the townsfolk that Gerlac was hiding stolen money and goods in his tree hollow. A mob marched to the tree and chopped it down, only to discover nothing hidden in the tree, vindicating Gerlac. Towards the end of his life, it is said, Gerlac communicated with Hildegard of Bingen and was counted her friend.