Our chickens are 20 months old. We started with 26 and now have 20. 1 or maybe 2 were lost due to egg bound. We've lost 3 in the last month. They were fine Friday when we had higher temps & I let them out of the coop/run to roam. They were doing dust baths and pecking. When my son went out tonight to shut the gate he found one of our roosters just sitting on the floor. We brought him in & bathed him and found mites on him. We put some dust on him & put him in our heated garage in a wire cage. The coop isn't as clean as we keep it in the summer because of the weather but it's not terrible either. I have an exhaust fan on about an hour a day to keep ammonia down. We do have mice this winter. We didn't last winter.