I started out with 26 cornish x and they are about 6 weeks old. I had 5 die up to this point and the rest looked really good. This morning I have another one dead and it looks like it was trampled. There was actually still birds on top of it this morning. Should I be putting heat in my tractor? It was in the 30's here last night. The birds are fully feathered so I figured they could handle the cold. Maybe this is my problem?