You CAN do the "purple note sacrifice", though.
Not all animals have the same "value", I guess - it's an old fact that only some animals are "robust enough" for the "bigger sacrifice".
No idea how it works, but it has been this way since forever.
And I think the "Folklore hint" is inaccurate in the sense that it mentions the FIRST possible prerequisite out of a few dozens (because it simply can't post ALL of them here).
But the logic is sound: You need a "single-animal" folklore first, only then you can build the "category folklore" on top of it, or something like this.
Now, the "feathers" thing looks weird, because I recall it working the other way around - may be a bug, or just bad "hint".
Too busy now to test the save, sorry.I have all sorts of various birds and I can't make the