Population structure and inter-species admixture within a likely extinct yet formerly widespread Hawaiian honeycreeper

The Hawaiian honeycreepers simultaneously represent one of the most spectacular avian adaptive radiations and are one of the most endangered avian groups. This clade’s few geographically widespread species can serve as a model to understand population-level processes shaping differentiation and characterizing decline. One such species is the likely extinct ʻōʻū (Psittirostra psittacea), a parrot-like beaked honeycreeper with a frugivorous feeding ecology. We compiled morphological and hybridization-captured ancient DNA datasets for the ʻōʻū from museum specimens from across the Hawaiian archipelago. We find (i) genomic differentiation among ʻōʻū from Kauaʻi, Lānaʻi, and the remaining Hawaiian Islands and (ii) a larger phenotype on Kauaʻi and smaller Maui Nui morphological phenotypes. While the differentiated population on Kauaʻi is likely a result of Kauaʻi’s geographical isolation, the divergent population on Lānaʻi is harder to explain by biogeography alone. Thus, we investigated whether the unexpected divergence of Lānaʻi ʻōʻū could be attributed to inter-species admixture with the geographically overlapping, now extinct ‘parrot-billed’ Lānaʻi hookbill (Dysmorodrepanis munroi) or a critically endangered Maui endemic, the kiwikiu (Pseudonestor xanthophrys). We detect significant admixture between the Lānaʻi ʻōʻū population and the Lānaʻi hookbill, possibly explaining the observed population structure and associating interspecific breeding with populations on the precipice of extinction.