A plant we always grew in our chicken run was called Artemisia Abrotanum also called 'Southernwood' or 'Boyslove'. we grew it for the express purpose of keeping the chickens in tip top condition and to prevent feather picking. Through the 20 odd years we kept chickens we never had a case of feather picking, so whether it was the plant which helped to keep the mites and other insects down I don't know, but I shall certainly be putting it in my new run when I build it next week. It's certainly worth a try and cost only pennies for a plant unless you know where there is one and strike a sprig off yourself. THis will not stop chickens from feather picking once they have started as they get the taste for it, but should help to prevent them from starting.