Dirty looking substance around nostril holes

by Holly
(Elmira, New York, USA)

My 2 year old Double Yellow Headed Amazon has the slightest darkening substance around her nostril holes... I can not rub it off... and she is healthy otherwise... She is eating and drinking well, weight is good, her eyes are clear, We do have a vent free room heater... But it has never caused her any problems over the 6 years we have had it... We have the heater professionally cleaned... and the flame burns blue... as the manufacturer said it would if all was cleaned correctly and no residue or build-up is present... So, I would be surprised if it was the heater... Is it possible that this happens as they mature? Otherwise, I am confused, as to the cause... But fear to disregard it... Thanks in advance... for any help you can offer