Distinctiveness of Pacific Robin subspecies in Vanuatu revealed from disparate patterns of sexual dichromatism, plumage colouration, morphometrics and ancient DNA

Abstract

The Pacific Robin (Petroica multicolor) is a polytypic species, with 14 subspecies recognised in the tropical south-western Pacific Ocean. Like most of the avifauna of this region, the Pacific Robin has received little taxonomic attention since early works, such as those by Ernst Mayr in the 1930s–1950s. Here, we used mitochondrial DNA (mtDNA), spectrophotometry of plumage colouration and morphological data to examine patterns of sexual dichromatism, and phenotypic and genetic variation of Pacific Robins in the Vanuatu Archipelago. We have shown for the first time that subspecies in Vanuatu display three types of sexual dichromatism: typical marked sexual dichromatism, and reduced sexual dichromatism, in which both sexes have either elaborate masculinised plumage or drab feminised plumage. Different types of sexual dichromatism were not correlated with phylogenetic relationships. We also find that distinctive mtDNA, bill-length and colour of the throat plumage support the naming of a new subspecies for the population on Tanna Island, P. m. tannensis subsp. nov., which has long been treated as consubspecific with that on nearby Aneityum Island. Our study highlights the importance of revisiting the taxonomy and evolutionary history of other Pacific Ocean avifauna with molecular data and quantitative tests of phenotypic differentiation between island forms.