It builds exclusively, so far as my experience goes, in earthen banks and cliffs, in holes which it excavates for itself, always, I think, in close proximity to water, and by preference in places overhanging or overlooking running water.
"The Nests and Eggs of Indian Birds, Volume 1"
Allan O. Hume
When the woodpecker in the fall excavates a lodge in a dry limb, we know he is prompted to it by the instinct of self-preservation, but the birds themselves obey the behests of nature without knowledge.
"Ways of Nature"
John Burroughs
When he excavates it in an upright tree-trunk, he usually chooses a spot beneath a limb; the limb forms a sort of rude hood, and prevents the rainwater from running down into it.
"The Wit of a Duck and Other Papers"
John Burroughs