I received 18 day old chicks thru the mail. 1 died 2 days after I received them, it was a Bresse. The rest were fine for 6 weeks. One Bielfelder female died. I thought it was smothered in a chick pile. Even though there are 2 heat lamps they all prefer 1. I put them closer together and they seem to spread out. All was fine until yesterday, they are 9 weeks. I found another bielfelder female very weak, laying on her side. She felt light. I pulled her out and feed her vitamin water, yolk and chick start with a dropper all day long. At first she wouldn't eat but by evening she ate a little on her own and stood up. Breathing better. She was pooping. The poops looked normal. An hour later she died. All of the other chickens were fine but I still cleaned everything. I even checked on them this morning before feeding the outside chickens, all seemed well. When I came in, another Bielfelder female was on her side. I have other breeds and Bielfelder males. No problems with the others. They seemed to be eating and drinking. They have all had Mareks vaccination. They were eating chick start but for the last few days I have been giving them layer pellets. I had to work and couldn't get to the feed store before they closed. [ Please let me know if anyone has any clue. There was no blood in any of the poops from any of them.