 The three species of the subgenus Zanda have been variously considered as two , then as a single species for many years . In a 1979 paper , Australian ornithologist Denis Saunders highlighted the similarity between the short @-@ billed and the southern race xanthanotus of the yellow @-@ tailed and treated them as a single species with the long @-@ billed as a distinct species . He proposed that Western Australia had been colonised on two separate occasions , once by a common ancestor of all three forms ( which became the long @-@ billed black cockatoo ) , and later by what has become the short @-@ billed black cockatoo . However , an analysis of protein allozymes published in 1984 revealed the two Western Australian forms to be more closely related to each other than to the yellow @-@ tailed , and the consensus since then has been to treat them as three separate species . 
