Cockatoo - Taxonomy

Taxonomy

The cockatoos were first defined as a subfamily Cacatuinae within the parrot family Psittacidae by the English naturalist George Robert Gray in 1840, with Cacatua the first listed and type genus. This group has alternately been considered as either a full or subfamily by different authorities. The American ornithologist James Lee Peters in his 1937 Check-list of Birds of the World, Sibley and Monroe in 1990 maintained it as a subfamily, while parrot expert Joseph Forshaw classified it as a family in 1973. Subsequent molecular studies indicate that the earliest offshoot from the original parrot ancestors were the New Zealand parrots of the family Strigopidae, and following this the cockatoos, now a well-defined group or clade, split off from the remaining parrots, which then radiated across the southern hemisphere and diversified into the many species of parrots, parakeets, macaws, lories, lorikeets, lovebirds and other true parrots of the family Psittacidae.

The relationships between various cockatoo genera is largely resolved, although the placement of the Cockatiel (Nymphicus hollandicus) at the base of the cockatoos remains uncertain. The Cockatiel is alternatively placed basal to all other cockatoo species, as the sister taxon to the black cockatoo species of the genus Calyptorhynchus or as the sister taxon to clade consisting of the white and pink cockatoo genera as well as the Palm Cockatoo. The remaining species are within two main clades, the first consisting out of the black species of the genus Calyptorhynchus while the second clade contains the remaining species. The first species in this second clade is the majestic black Palm Cockatoo, followed first by a small clade consisting of the gray and reddish Galah and the Gang-gang Cockatoo and then the pinkish Major Mitchell's Cockatoo. The remaining species are mainly white or slightly pinkish and all belong to he genus Cacatua. The genus Cacatua is further subdivided into the subgenera Licmetis, commonly known as corellas, and Cacatua, referred to as white cockatoos. The genera Eolophus, Lophochroa and Cacatua are hypomelanistic and do not show sexual dimorphism. Confusingly, the term "white cockatoo" has also been applied to the whole genus, and even the whole clade. The five cockatoo species of the genus Calyptorhynchus are commonly known as black cockatoos, and are divided into two subgenera—Calyptorhynchus and Zanda. The former group are sexually dichromatic, with the females having prominently barred plumage. The two are also distinguished by differences in the food begging calls of juveniles.

The fossil record of cockatoos is even more limited than that of parrots in general, with only one truly ancient cockatoo fossil known: a species of Cacatua, most probably subgenus Licmetis, found in Early Miocene (16–23 million years ago) deposits of Riversleigh, Australia. Although fragmentary, the remains are similar to the Western Corella and the Galah. In Melanesia, subfossil bones of Cacatua species which apparently did not survive early human settlement have been found on New Caledonia and New Ireland. The bearing of these fossils on cockatoo evolution and phylogeny is fairly limited, although the Riversleigh fossil does allow tentative dating of the divergence of subfamilies.