 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.
