If they're eating chick starter unsupplemented, then they are almost certainly not getting enough niacin. It is quick and easy to fix, but you must act quickly before it worsens.
Go to the local pharmacy and buy gel caps of niacin. It's best to go with the NON flush-free and NON time-release, but if that's all you can get, then fine. Mix 150mg per gallon of water and make it the only source of water in the brooder. You should see very quick improvement (24-48 hours) if this is the problem.
There are other ways to add niacin, but this method uses materials everyone has access to. Nutritional yeast works, and also mixing chick starter 50/50 with game bird starter, but you have to have access to those materials in order to do it.
I also recommend changing their food immediately to a fresh batch, just in case it was a bad batch. When they have recovered, you can try a small batch of the old feed and watch carefully for problems. If no problems, then it was niacin and you can use up all the feed. If they develop problems, then you'll know the feed caused it and can switch straight back again.
I'd put money, if I were the gambling sort, on this being a niacin issue though. Treat it quickly before they deteriorate further. Good luck!
