I've been getting extremely fragile shells of late. I mix the oyster shells into the feed so I think they are getting enough. One BIG scoop per day. (Free choice just didn't work.) My question is this....could they be getting TOO much protein? I've switched to a 20% protein brand. Any thoughts? I haven't id'd the culprits yet. I do have some older birds 2-3 years. Perhaps need to cull them out? Any other ideas?