Oh I am positvie they are not showing signs of marek disease. They are much too young for that. Is your brooder a galvanized stock tank? It looks rusty from the picture, but I might be mistaken. If so, I would take an old sheet to put around the inside and just tuck it into the bedding, in case they decide to start pecking at the sides. It is probably just a small thing, but I raise chicks in a Rubbermaid fiberglass stock tamk, and they love to peck at the mineral deposits on the sides, LOL. You are doing a great job, and sometimes one will die. It just happens.