I am new to this website and new to chickens in general. We started with 5 Buff Orpingtons and 1 Auracana in April. They have been happy, healthy birds up until last week. I went to collect the eggs last Thursday and I noticed one of my buffs was very dirty and missing quite a few feathers on her breast. Unfortunately, I didn't do anything about it. I thought she was just in need of a dust bath. The next morning, I found her in the run area and the other birds were eating her...alive!!!! I separated her from the group, but she died a few hours later. I've been checking the other birds daily, and came upon another of my Buffs with some feathers missing on her breast. She isn't dirty but there are some runny poops in the coop. I isolated her and she did lay an egg today. I am so worried that my flock has a disease and I am going to lose them all. The rest of the birds all look healthy, although one has some black spots on her comb. I live in New England and the weather has recently gotten very cold, is this frostbite or are my birds sick. Please help!! Thanks in advance.