Happenstance, if you ask me.

And if we're talking philosophy, I think the feather doesn't exist at all, so it's even harder for me to imagine that it's a product of conscious effort. I imagine that what we humans consider feathers have near-infinite, similar-looking predecessors, proto-feathers, which differ so slightly from actual feathers that it's hard to say where the non-feather featherlike skin protrusions end, and feathers begin. What we can do of course is agree on such a line, but that further proves to me that feather is just a human concept, and that non-humans don't actually consciously interact with whatever we happen to call feathers.

So, feathers just kinda happened over time.

"Discovery" fits since we're just uncovering what's already there, not creating it. "Invention" is for stuff we actually make. Unsure if just a mistake or creationist perspective in the article.

Evolution.

"Survival of the fittest" doesn't mean "That gene was gorgeous like Mr. Universe, so he won!" It means "Life threw something at a wall and this is what stuck. It died less in the face of actual real world conditions."

After you winnow away all the failed stuff, you have stuff that works. Do it for millions of years and amazingly complex and elegant stuff can emerge from the process.

Discovery implies that it already existed before and it wasn't found yet.

I'd say invention is closer but does seem to imply agency.

I think feathers are the evolution here. Something evolved into a feather.