Four out of 10 of the chickens comb tips are turning black. They went from red to white to black in about a day and a half. I looked up what it might be and saw some disease that had worms in their stool or frostbite. So I looked at their stools and never saw any worm and the temps haven't gotten below 30's at night for over a week. Before that the temps got down to the teens at night with the highs in the upper 20's but the chickens have been in the same environment before and never had any problems with frostbite. Either way, it definitely isn't cold enough for it right now. Any thoughts as to what this might be? I started them on corrid just in case it is worms.