Yes, there should be an overabundance of feed. One of the main reasons not to ration it out is that the dominant hens will take most of it, shutting out the less aggressive ones, who are left with nothing if there is not plenty of it. I'll bet your really thin girl is at the bottom of the pecking order and so is getting less feed than everyone else, especially if they are having to jostle to get it early, before it runs out. Are you using a feeder, or just putting it in a dish? Take a look online at "chicken feeders" if you don't already have one - it really is the best way to go, because it prevents so much waste and yet gives them unlimited, constant food - which is really very necessary. You really do not need to measure - they need unlimited access to food and water at all times. I agree about the scratch - definitely a treat, not a feed. I found that my girls were diving into the BOSS that I was giving them each morning, and not eating enough layer pellets - I let them out to free range first thing, scattered some BOSS around, and they rarely went back to the coop for a real meal. Now I keep them in their run & coop long enough in the mornings that I know they are eating a meal, and not just snacking. Look into a feeder so you don't have to worry about or think about how much they are eating. Kitchen scraps are great, but again, that unlimited feed is what will keep your less dominant hens healthy. Keep us in the loop, so I can stop worrying about hungry chickens; can't stand to think of any animal being remotely hungry! (We are also all plump, cats included!)
