Someone once told me, but I can't remember whether it was an aviator or an ornithologist, that a bird with a metal ring on its leg could cause total destruction of a jet engine. Of course they don't attach metal rings to birds any more and haven't done so for a long time, and I'm not sure how big the risk was even with the chunkiest of old-fashioned rings.

(What about a bird with an arrow stuck through it, I wonder? https://en.wikipedia.org/wiki/Pfeilstorch )