green check back feathers changing colors

by CeeJay
(Rockwall Texas)

I have had my green cheek for 8 years and he seems to be very healthy and spirited as always, however his back feathers on both shoulders are turning black over time but they are not frayed in anyway. someone had told me that is was stress and someone else told me it was probably oil coming from my hands or hair products since he is on my shoulders about 80% of the time. he isn't showing any problems with anything and there is no difference in his attitude or spirit and his diet hasn't changed and he gets fresh fruit everyday along with pellet food does this sound like something to be concerned about?Ilove my baby he is my mate of sorts and want to make sure I am doing everything to make and keep him healthy and happy