Avian malaria implicated in the functional extinction of Oreomystis bairdi (‘Akikiki), a Hawaiian forest bird

Abstract

The native forest bird community on Kaua‘i, one of the main Hawaiian Islands, is facing an extinction crisis, with the endangered endemic Oreomystis bairdi (‘Akikiki) at the forefront. The standardized survey in 2018 estimated the global population size of O. bairdi at 120–886 individuals, while the 2023 survey detected none. Extensive range-wide searches in 2024 detected only 4 individuals, and by August 2025 only one female was known to persist. She has not been seen since, so the species is now likely extinct in the wild with only a small captive flock preventing complete extinction. The leading hypothesis that increased prevalence of avian malaria is driving Hawaiian honeycreeper declines is based on correlations between observations of encroaching disease and declining bird populations. However, no study of Hawaiian honeycreepers has directly measured the vital rates of a rapidly declining species to assess competing causes. Here, we combine data on O. bairdi survival, numbers of pairs, reproduction, and disease prevalence from 2015 to 2022 in the former core of their range, Halepa‘akai, that strongly implicate disease as a driver of its extinction. At Halepa‘akai, the number of breeding pairs fell from 35 to 2 pairs between 2015 and 2021, with no breeding in 2022. Annual adult survival declined from 80% (95% CI: 58–92%) in 2016 to 27% (95CI: 12–50%) in 2022. Avian malaria prevalence in O. bairdi across their range increased from 0% in 2011–2015 to 33% in 2021–2022. Productivity at Halepa‘akai varied within this period from 0.62 to 1.0, but did not change significantly, indicating that mortality drove the demise of O. bairdi, with all evidence pointing to disease-induced mortality. The extinction of O. bairdi in the wild is the leading edge of a looming crisis in honeycreepers across Hawaii as avian malaria continues to decimate this unique avian family.