Palm Cockatoo : Feather problem

Hi, everyone. I am new here.

I have a question regarding my friend Palm cockatoo's feather. (not efford to own one yet ) Its feather starting to change from black to partial yellowish/whitish and dropping. Wat causing this?? any medicine 4 tat??

My other friend palm is having the same problem 2 yrs ago, now only left some feather around the head. The feather was not plucking, simply dropped off. The bird health is absolute ok, eat well, eyes, beak, nostrils, legs and feces are alright. Is just the feather can not develop properly. So far it seem happened to palm only. I dun see this on other parrots.

HELP!!