 The male yellow @-@ tailed black cockatoo has a black bill , a dull yellow patch behind each eye , and pinkish or reddish eye @-@ rings . The female has grey eye @-@ rings , a horn @-@ coloured bill , and brighter and more clearly defined yellow cheek @-@ patches . Immature birds have duller plumage overall , a horn @-@ coloured bill , and grey eye @-@ rings ; The upper beak of the immature male darkens to black by two years of age , commencing at the base of the bill and spreading over ten weeks . The lower beak blackens later by four years of age . The elongated bill has a pointed maxilla ( upper beak ) , suited to digging out grubs from tree branches and trunks . Records of the timing of the eye ring changing from grey to pink in male birds are sparse , but have been recorded anywhere from one to four years of age . Australian farmer and amateur ornithologist John Courtney proposed that the similarity between juvenile and female eye rings prevented adult males becoming aggressive to younger birds . He also observed the eye rings to flush brighter in aggressive males . Moulting appears to take place in stages over the course of a year , and is poorly understood . 
