Last May, I started 15 black star pullets from Murray McMurray hatchery. They were vaccinated for Marek's and fed medicated chick feed for 20 weeks. At about 16 weeks, I divided my coop and put the pullets in one side with sole access to the run and left five hens from my old flock (that had not shown any health issues in the more than 2 years I had them)into the other side with access to free range. After several weeks of this arrangement, I finally blended the two flocks. A few months ago, I noticed a five-month old pullet appeared to be somewhat lethargic. She was a healthy-appearing bird - glossy feathers, clear eyes, no discharge, no difficulty breathing - absolutely nothing wrong other than she seemed "off" somehow. I found her dead the next day. Several weeks later, another pullet also became lethargic and showed signs of runny green poo, but nothing more. She had been remarkably healthy until this event. Laying, eating, everything. I isolated her and she managed to hang in there for about five days before succumbing. Three weeks ago, I found another dead pullet inside the run. I assumed that she had not made it inside the coop before the automatic door closed and that she had succumbed to the cold. It looked like she had been roosting on a ladder that I keep in the run and that she had simply died and fell Yesterday, I noticed a hen (one of the older birds) behaving a little "off" although she was very healthy in appearance and had been laying and everything up to this point. This morning I found her dead. Obviously, something is going on here. I've had chickens for five years now and have never experienced this. I haven't brought in any new birds. I feed 18% Nutrena flock raiser, free choice grit and oyster shell. Any ideas?