If I recall, it's Jeff Mattocks of Fertrell who says chickens can fight intestinal worms by ingesting certain kinds of plant fiber that make the gut inhospitable.
He's a big proponent of ranging and says he's never had to chemically worm chickens because they self medicate naturally. That caught my ear back when our first chickens kept coming up with intestinal infections due to roundworms (according to fecal analysis at the vet).
Mattocks' discussions on podcasts were motivators to start letting our chickens range more (this thread has been another). Their big, protected yard contains diverse soil conditions, moss, delicious invertebrates, and plants, but there's a whole world outside the fence they seem extremely interested in taste testing, most likely because it's essential to their health.
Mattocks isn't a source I can vouch for since I've never asked where his science comes from, and he's promoting products for Fertrell (good products, IMHO), but he presents a lot of thought-provoking concepts that shake up traditional feeding approaches. His audience tends to be breeders who make their own feed mixes.