Need help! my chickens seem to be picking on each other and have bare butts and blood

Don't know how to solve problem. I had 8 chickens now down to 5. My chickens are picking on each other. Have looked at them. Most of their butt feathers are picked off and have red butts with blood around vents. Have washed off the blood and sprayed with BLU_KOTE. .The tail feathers where tail connected to body look bad. I don't think they are molting because their neck feathers have never fallen out. They will be a year old march 23 of this year. With 5 chickens get 4 to 5 eggs a day. Would dusting help? Cleaning and disinfecting the coop help?. My chickens I have noticed don't like going out in high wind or temps around 30 degrees or below.
If they have mites would the heat help mites grow. I have an old milk shed with insulation behind the wood. Why has the chickens peck at each other to the point of killing it?