Unfortunately this does happen. I have a nursery full of 5 week plus chicks and I have found three dead in about a 3 week period. They get cold, the get trampled, they get dehydrated, they get a genetic problem, etc. . It really is just part of the mean old chickie raising thing. Usually it is your favorite one, which just blows. I mean why can't the black roosters that you have no need for not make it to the adult hood, no problem. I even threw a black roo that was only about 6 weeks old into the barn becuase he was beating the crud out of all the rest of his nursery mates. He I doing just fine and the temperatures have been in the low 20's here at night. He just does the baby chicken thing and finds a nice warm hen to get under, what a turd. He is thriving, eating, drinking and running about in the day.