Brittle feathers are a sign of protein deficiency. Or it could be feather picking going on in the flock. Observe the birds for awhile during the day and see how they are behaving. If it is protein deficiency, animal proteins are superior to plant proteins, plus they contain B12, so we should supplement their regular feed ration every so often with adult chickens. Some folks use dog or cat food. With the numerous ingredients in both cat and dog foods you run the risk of giving a chicken too much of the wrong nutrient/ingredient. I tried giving mine Calf Manna once, then tried a fish-based cat kibble. They wouldn't touch either. Canned salmon, tuna that is salt free or very low sodium is good every so often. Non-fat dry milk mixed in a wet mash is a good supplement too. Fish meal itself is hard to come by anymore, at least in my area. Crimped oats are good too as a regular addition and the hulls improve feathering in older chickens. Some folks make it simple by mixing in, or giving gamebird crumbles/pellets every so often, especially when chickens begin to molt.