My ladies are 3 years old and we've had them since they were chicks. My barred rock now has a greasy looking head, see pics. No other chickens are exhibiting any symptoms and are laying as often as you would expect 3 year old chickens to lay. The hen in question has not laid regularly for at least 9 months, well before this ailment. Any ideas? They have a large run and get to free range in our fenced backyard every few days.