Disease-driven collapse of the native Kauaʻi avifauna and the rise of introduced bird species

Hawaii hosts one of Earth’s most unique and threatened avifaunas. Upslope migration of mosquito-vectored avian malaria on Kauaʻi (maximum elevation 1,598 m) has likely caused its rapid loss of avifaunal diversity; only 8 of 13 historic forest bird species remain. We update the status and trends of Kauaʻi forest bird populations since the original (1981) surveys using the latest (2023) survey data and distance sampling. We fit detection functions to species-specific count data and stratified estimates across the Interior (since 1981) and Exterior (since 2000) survey areas, and between low (900–1,100 m), medium (1,100–1,300 m) and high (> 1,300 m) elevation bands (since 2000). Log-linear trends of ʻakekeʻe (Loxops caeruleirostris), ʻanianiau (Magumma parva), ʻiʻiwi (Drepanis coccinea), and Kauaʻi ʻamakihi (Chlorodrepanis stejnegeri) steeply declined across the timeseries, with extinction of ʻakekeʻe and ʻiʻiwi expected before 2050. Undetected in 2023, ʻakikiki (Oreomystis bairdi) were excluded from analysis. ʻApapane (Himatione sanguinea), Kauaʻi ʻelepaio (Chasiempis sclateri), Chinese hwamei (Garrulax canorus), and white-rumped shama (Copsychus malabaricus) were stable overall. Northern cardinal (Cardinalis cardinalis) steadily declined, whereas Japanese bush warbler (Horornis diphone) and warbling white-eye (Zosterops japonicus) exponentially increased. Taxonomic and functional diversity did not vary greatly across our timeseries, while the proportion of introduced species in the Exterior increased from 34 to 59%. However, introduced species do not replace the losses of ecological functions from native species, whose populations are likely declining from avian malaria. Future monitoring can be used to evaluate forest bird population responses to mosquito suppression using the Incompatible Insect Technique.