I have 5 chicks that I purchased a few weeks ago from a local guy. They are about 8 or 9 weeks old now (almost fully feathered). All were doing fine until one died today. I can't figure out why and need some help. Nothing got to him and they have plenty of flock raiser and water. I don't see anything visibly wrong with him. What should I do to identify the cause? How do I protect the other 4? THANKS!