Earlier quoted context omitted.
When birds make nests outside of their base anatomical functions, are they "unnatural"?
No, because birds are not humans. To be fair, "unnatural" is sometimes extended to refer to "life" instead of "humans", typically only when talking about the evolution of life or the search for extraterrestrial life. In that sense, then, yes - bird nests or termite mounds or coral reefs are unnatural. A more common wording for this same idea is something like "not created by geological processes".
What gives humans the special designation of their byproducts being unnatural?