"Greasy" head on hen

[brubend]

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.

 

[Dreamleaf39]

Is her tail up? Have those black spots always been on her comb? Is she preening as normal?