What technique do some woodpeckers, who nest in tree cavities, use to protect the nestlings?

Answer 1

They excavate and maintain resin wells immediately below and around the nest cavities
The woodpecker work the resin wells regularly, giving a copious and persistent resin flow that coats the tree trunk, a resinous barrier against snakes. Some woodpeckers also scale loose bark from the cavity trees and closely adjacent trees, making a slick difficult-to-climb surface.