Go to your feed store and look for Poultry Cell vitamins and give 3 ml per day orally. Other brands may be put in the water or food, just read the label, and make sure that they contain riboflavin (vitamin B2.) I have used Poultry Booster in feed, Vitamins With Lactobacillus, ans SaveAChick, all brands you may find. A cheaper alternative would be to give 1/2 of a vitamin B complex tablet for humans, crushed into some egg or yogurt daily. Just make sure there is riboflavin. Mareks could be a possibility, but injuries and vitamin or mineral deficiency should be ruled out.