Is the only feed they are getting scratch?  Scratch is really not a good feed to meet the complete nutrient needs of the chicken.
If so, I'm guessing it's lack of some nutrient that's causing them to peck each other's feathers.  While the sunflower seeds are higher in protein, they may not be getting the right combination of nutritents to keep them A-100% satisfied, so they are pecking feathers to fill in gaps in the diet.
Are they of laying age?  if so, do they have a calcium source?  If you aren't feeding layer pellets, you must provide oyster shell or similar if they are of laying age.  Lack of calcium will also cause them to peck feathers.
I recommend you get them the correct feed for their stage in life (either grower if not laying age yet, or laying pellets if they are), and free feed all they want, along with fresh water.
Save the other things you mentioned as treats, not the main diet.  AND limit scratch.  It's like junk food.