I just noticed 3 of our 6 Golden Sex Link chickens (19 weeks old) have these black blotches on their tail feathers that look like mold. I can't rub or scrape off the black so I'm hoping it's just black discoloration in the feathers? I can't find mites or lice and they haven't shown any signs of that. We also have 1 Salmon Faverolle and two Easter Eggers that don't have any symptoms.Thanks. so much!