Fox hole? Hardwood trees really do seem to make the best cavities. I am always on the lookout for mammals or even bats, which must live in some of these goose pens.
From my naturalist point of view, I believe over time the exclusion of fire is having a profound impact on forest mammals, especially cavity nesters like the Pacific Fisher or porcupine. The Pacific fisher is currently a ‘candidate’ to be placed on the State Endangered Species Lists. Seen one lately? Porcupines?
All of the larger cavities I come across are fire scared. And I get the impression that numerous fires over decades if not a century or two formed them. Now without fire, there is very little recruitment for fire formed cavities, which really puts pressure on the ones that do exist. Its a shame that the vast majority of cavities I see are in the form of large hardwood trees, generally on the verge of collapse from being over topped by Douglas-fir.