Nihoa and Laysan Island passerines population abundances, trends, and habitat utilization

Nihoa and Laysan Island, part of the Northwestern Hawaiian Islands, are host to three endangered passerine species—Nihoa finch (Telespiza ultima), Nihoa millerbird (Acrocephalus familiaris kingi), and Laysan finch (Telespiza cantans). Using point-transect distance sampling survey records from 2010 to 2022 for Nihoa and 2013 to 2019 for Laysan Island, we estimated the density and abundance of all three species. We also compared densities between habitats for both islands, classifying Nihoa habitat as Eragrostis (grass) or mixed shrub and Laysan Island as open or dense habitat. The population of Nihoa finch remained stable with slight fluctuations among years, with a population size of 6,592 (4,954–8,655) birds for the most recent sampling year, 2022. Laysan finch also remained stable with a total population in both dense and open habitat of 17,657 (11,994–23,320) for the most recent sampling year, 2019. The Nihoa millerbird significantly increased on both Nihoa and Laysan Island with a global population of 1,907 (1,291–2,766) in 2019, the most recent year both islands were sampled. Overall, the populations of these three endangered species were stable or increasing. The increase of Nihoa millerbird on Laysan Island, after being translocated to the island in 2011 and 2012, represents the successful establishment of a second viable population, leading to a decrease in the species’ IUCN extinction risk status (from critically endangered to endangered). Continued population monitoring of these endangered species can help inform future management actions and ensure their preservation into the future.