This is my first winter with chickens. We have had a very cold December with nightly temps consistently below zero and highs either still below zero or in single digits. The last couple nights temps have reached -21 to -10. I don't heat my coop, but the night it reached -21 I did put a lamp out there. I have 6 standard chickens in a 8x4 coop. I've noticed in the morning the chickens all have frost on their feathers between their shoulder blades. Just one little patch 2x1 inches. What does this mean? I have the gable vents open, they are approximately 8x8 inches. But I close up the door to the run. Too much moisture? Not enough venting? I do keep food and water in the coop although the water is not heated. Any thoughts? Thanks!