Hornbill
Taxonomy

The family Bucerotidae was introduced (as Buceronia) by the French polymath Constantine Samuel Rafinesque in 1815; it comes from the genus name Buceros given by Carl Linnaeus in 1758 from the Greek word βούκερως bōukeros which means "cow horn".

There are two subfamilies: the Bucorvinae contain the two ground hornbills in a single genus, and the Bucerotinae contain all other taxa. Traditionally they are included in the order Coraciiformes (which includes also kingfishers, rollers, hoopoes and bee-eaters). In the Sibley-Ahlquist taxonomy, however, hornbills are separated from the Coraciiformes into an order of their own, Bucerotiformes, with the subfamilies elevated to family level. Given that they are almost as distant from the rollers, kingfishers and allies as are the trogons, the arrangement chosen is more a matter of personal taste than any well-established taxonomic practice. All that can be said with reasonable certainty is that placing the hornbills outside the Coraciiformes and the trogons inside would be incorrect.

Genetic data suggests that ground hornbills and Bycanistes form a clade outside the rest of the hornbill lineage. They are thought to represent an early African lineage, while the rest of Bucerotiformes evolved in Asia. However, another study claims that the ground hornbills diverged first, followed by Tockus. Within Tockus, two clades have been identified based on genetics and vocal types—'whistlers' and 'cluckers'. The 'cluckers' have been placed in a separate genus, Lophoceros.

Bycanistes belongs to a clade of mostly African species that also includes Ceratogymna and Tropicranus. Another member of this clade is the Black dwarf hornbill. The Black dwarf hornbill is typically classified in the genus Tockus but in this study, is a sister species to the White-crested hornbill. If these two species are classified in congeneric, Tropicranus becomes a junior synonym of Horizocerus, as that was one of the old names used for the Black dwarf hornbill. This clade also includes one Southeast Asian species, the white-crowned hornbill.

As for the other Asian hornbill species, Buceros and Rhinoplax are each other's closest relatives, Anorrhinus is part of a clade that has Ocyceros and Anthracoceros as sister taxa, and Aceros, Rhyticeros, and Penelopides form another clade. However, according to this study, Aceros is polyphyletic; the rufous-headed hornbill, writhed hornbill, and wrinkled hornbill form a clade with the Sulawesi hornbill, and are in turn more closely related to Penelopides. These four species have been classified in a separate genus, Rhabdotorrhinus. Similarly, the knobbed hornbill is more closely related to Rhyticeros, leaving the rufous-necked hornbill the only member of the genus Aceros.

The following cladogram showing the relationships between the genera is based on a molecular phylogenetic study by Juan-Carlos Gonzalez and collaborators that was published in 2013. The number of species in each genus is taken from the list of world birds maintained by Frank Gill, Pamela C. Rasmussen and David Donsker on behalf of the International Ornithologists' Union.
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