The three species of the genus "Zanda" have been variously considered as two, then as a single species for many years. During the 1970s, Australian ornithologist Denis Saunders analysed the two white-tailed taxa and found that Baudin's black cockatoo also has a longer wing, and wider and higher skull than Carnaby's black cockatoo. Furthermore, there was no overlap in the range of (bill) lengths. In a 1979 paper, 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. 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.