Thank you! The entire area around the top is ventilated but clearly it needs more. I'll have to add some closer to the base and hopefully we won't have this problem again. Thank you again for responding!Apparently humidity is usually the culprit rather than just the cold. My sister has a tiny coop with good ventilation and even her Leghorns don't have any problems with their combs, but my smaller-combed girls have some black spots. I need to install some vents. I have been applying Vaseline to their combs on cold nights since then, and none of them have gotten any worse, even though the temps have dropped again.