A note on Feathers.
I'm not sure how Toady has it worked into the game, but you can obtain feathers as a body part by designating the feathers as a shearable tissue. However, like with all tissues, the game seems to keep track of how "big" the sheared tissue is once severed, for the purpose of things such as spinning into thread (if it doesn't hit some threshold, the game won't use it). This indicates some product dimension type mechanic is involved (just like with butchering).
Just like with organs and other tissues when butchering, if the BODY_SIZE*REL_SIZE(factor of body part rel_size and tissue thickness I believe) of the tissue in question doesn't hit some threshold, then it won't display as a product of the butchering. Shearing is a special case where the game will output the tissue after the shearing task, but it will have the pseudo-product_dimension below the game's desired critical value, possibly making it unusable (at least for hard-coded reactions, such as spinning thread or carving crafts).
Some quick science:
I took turkeys and cranked their body size up to 5000000 (three orders of magnitude). I also increased the tissue layer thickness for all feather to 10 (10x increase). After butchering, "Feather" was observable in the butchering results. But it was only that, 'Stray Turkey Gobbler Feather'. No number of units, just like skin. There were 110 Meat and 72 bones though.
I thought to myself "Hey, if that were hair, it would have said something like 'Stray Turkey Gobbler Hair(17)' or some such, because its so damn big that all the different body part hair layers should yield individual butchering returns". So I changed the Tissue_Shape of the Feather tissue to be STRANDS to match hair. The result: Still only one single feather body part after butchering.
My current conclusion:
I haven't done enough science to make a conclusive statement, but I have a suspicion. I think Feather, like skin, is treated specially by the game, with a restriction on multi-body-part butchering returns, and some critical body size vs. relative size equation that is currently elusive. Making a creature large enough, and drastically increasing the size of the Feather Tissue, does allow one to obtain Feather from butchering. Just what steps you are willing to pursue further on this course is up to you.