I had feather picking problems in the past and looked around quite a bit for answers on how to stop it. The solution that has worked best for me was from an article that suggested sodium deficiency in a chicken’s diet and to put another free choice, small waterer out with a teaspoon or less of sea salt diluted in a liter or more of water. This stopped the feather picking within a day. I am told there’s a bit of blood in the base of the feather that has sodium in it, which is sometimes how it starts if there’s a deficiency in their diet. I noticed the same behavior you described with a couple hens pecking and eating feathers out of our rooster, Crotty’s neck. Putting out the diluted salt water every other day for a couple times seemed to stop the behavior. I just noticed it again this morning after a month of peace. Time to make more salt water. I hope this helps alleviate your issue. Please let me know if it does.