One of my Dark Cornish hens has taken sick again - huddled up, sluggish, eyes often closed, very weak. She got like this in the last cold spike a few days ago, but recovered to normal activity after a night under a heat lamp. She's 20 months old, and has been healthy until this cold snap. No visible injuries. No nasal/eye discharge or stuck droppings. She's not in molt - is fully feathered. No signs of frostbite. She feels thin to me - keel bone pokes out more than on the other hens. All of the others (mix of other breeds, including 2 other DC) are active and healthy. They spend nights in a mostly enclosed coop. I cover most of the ventilation areas; but leave the main door open during daylight to air out the coop. During the day they spend most of their time under the coop, in an adjacent shelter or on the ground surface where I've laid out hay. There is lots of room in those areas to shelter from wind, though it's still cold. Food consumption for the flock is normal for this tie of year. As I have 2 other DCs, I can't tell when this girl is eating. No signs of aggression in the flock - nobody being chased away from the feeder that I've observed. Other than bringing her in and keeping her warm, any suggestions?