Population trends of native and non-native forest birds on the island of Molokai, Hawaii

Hawaiʻi’s avifauna has undergone profound changes over the past 1,000 years, with many endemic bird species driven to extinction and non-native introductions reshaping forest ecosystems. On the island of Molokaʻi, habitat degradation and the introduction of mosquito-borne disease, mainly avian malaria (Plasmodium relictum), have caused extinctions and severe declines in native forest bird populations. To assess these changes, point-transect distance sampling surveys were initiated in 1979 and 1980 and repeated six times, most recently in 2021, covering a 3,527 ha area. This study analyzes species composition, population densities, abundances, and long-term trends for species with sufficient detections using log-linear regression. Since 1979–1980, a total of 17 non-native and 3 native species were detected across all six surveys. The non-native Warbling White-eye (Zosterops japonicus) had the highest density and abundance in every survey, while abundance of Red-billed Leiothrix (Leiothrix lutea) significantly trended upward. The Japanese Bush Warbler (Horornis diphone) irrupted in the survey area, increasing in abundance by more than 99% since 1979–1980. Among native species, ʻApapane (Himatione sanguinea) populations increased by 33%, while Hawaiʻi ʻAmakihi (Chlorodrepanis virens) declined by 78% from their peak in 1995. Federally threatened ʻIʻiwi (Drepanis coccinea), highly susceptible to avian malaria, have not been recorded on Molokaʻi since 2010, and the endemic Olomaʻo (Myadestes lanaiensis) has not been detected since 1980 and is possibly extinct. Our results underscore the ongoing conservation crisis for Molokaʻi’s forest birds and the continued vulnerability of native birds to disease and habitat loss.