Nicobar Pigeon - Systematics

Systematics

Based on cladistic analysis of mtDNA cytochrome b and 12S rRNA sequences, the Nicobar Pigeon is sometimes called the closest living relative of the extinct didines (Raphinae), which include the famous Dodo (Raphus cucullatus). But the original study only yielded this result as just one possibility of several even from a rather limited sample of taxa, and not with a very high confidence either. In any case, nDNA β-fibrinogen intron 7 sequence data agrees with the idea of the Raphinae as a subfamily of pigeons (and not an independent family, as was previously believed due to their bizarre apomorphies) that was part of a diverse Indopacific radiation, to which the Nicobar Pigeon also belongs.

The following cladogram, from Shapiro and colleagues (2002), shows the Nicobar Pigeon's closest relationships within Columbidae, a clade consisting of generally ground-dwelling island endemics.

A similar cladogram was published in 2007, differing only in the inverted placement of Goura and Didunculus, as well as in the inclusion of the Pheasant Pigeon and the Thick-billed Ground Pigeon at the base of the clade.

C. nicobarica is a quite singular columbiform (though less autapomorphic than the flightless Raphinae), as are for example the Tooth-billed Pigeon (Didunculus strigirostris) and the crowned pigeons (Goura), which are typically considered distinct subfamilies. Hence, the Nicobar Pigeon may well constitute another now-monotypic subfamily. And while any of the semi-terrestrial pigeons of Southeast Asia and the Wallacea cannot be excluded as possible closest living relative of the Raphinae, the Nicobar Pigeon makes a more plausible candidate than for example the group of imperial-pigeons and fruit-doves, which seems to be part of the same radiation.

Whether it is possible to clarify such deep-time phylogenies without a comprehensive study of all major lineages of living Columbidae remains to be seen. The primitive molecular clock used to infer the date the ancestors of the Nicobar Pigeon and the didines diverged has since turned out to be both unreliable and miscalibrated. But what little evidence is available still suggests that the Nicobar Pigeon is distinct from all other living lifeforms since the Paleogene – most likely some time between 56-34 million years ago during the Eocene, which makes up the bulk of the Paleogene period.

From subfossil bones found on New Caledonia and Tonga, an extinct species of Caloenas, the Kanaka Pigeon (C. canacorum) was described. It was about one-quarter larger than the Nicobar Pigeon. Considering that it must have been a good source of food, it was most likely hunted to extinction by the first human settlers of its home islands. It probably was extinct by 500 BC. The Liverpool Pigeon ("C." maculata) is a more recently extinct species from an unknown Pacific locality; it probably disappeared in the 19th century and most likely succumbed to introduced European rats. It is placed in Caloenas as the least awkward possibility; its true affinities are presently undeterminable and it is perhaps more likely to represent a distinct genus of the Indopacific radiation of Columbidae.