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 'ō'ū (<i>Psittirostra psittacea</i>), a parrot-like beaked honeycreeper with a frugivorous feeding ecology. We compiled morphological and hybridization-captured ancient DNA datasets for the 'ō'ū from museum specimens from across the Hawaiian archipelago. We find (i) genomic differentiation among 'ō'ū from Kaua'i, Lāna'i, and the remaining Hawaiian Islands and (ii) a larger phenotype on Kaua'i and smaller Maui Nui morphological phenotypes. While the differentiated population on Kaua'i is likely a result of Kaua'i's geographical isolation, the divergent population on Lāna'i is harder to explain by biogeography alone. Thus, we investigated whether the unexpected divergence of Lāna'i 'ō'ū could be attributed to inter-species admixture with the geographically overlapping, now extinct 'parrot-billed' Lāna'i hookbill (<i>Dysmorodrepanis munroi</i>) or a critically endangered Maui endemic, the kiwikiu (<i>Pseudonestor xanthophrys</i>). We detect significant admixture between the Lāna'i 'ō'ū population and the Lāna'i hookbill, possibly explaining the observed population structure and associating interspecific breeding with populations on the precipice of extinction.