Fifty years of conservation under the Endangered Species Act: Himantopus mexicanus knudseni (Ae‘o or Hawaiian Stilt), a conservation success story in progress

ABSTRACT The U.S. Endangered Species Act (ESA) has provided the legal framework to prevent many extinctions, yet recovery remains elusive for hundreds of species. Himantopus mexicanus knudseni (Ae‘o or Hawaiian Stilt), listed as federally Endangered in 1973, provides a valuable case study for assessing long-term demographic responses to conservation. We synthesized reproductive data from 1963 to 2020 across nine wetlands on the island of O‘ahu, Hawai‘i, USA, where most studies have taken place for this species, and analyzed trends in vital rates using linear mixed models. Nest and fledging success increased modestly, indicating that conservation actions likely reduced nest failure and improved chick survival. In contrast, clutch size, hatching success, and fledgling density did not significantly increase. Importantly, site-level variation was substantial, with the highest reproductive outputs occurring in locations with predator exclusion and minimal flooding. Expanding wetland habitat that is managed for invasive predators through partnerships with private, non-profit, and community-led efforts will be critical for species recovery. While the species remains listed, conservation actions over five decades under the ESA have prevented extinction and produced measurable improvements in key life stages, marking H. m. knudseni as a conservation success story in progress.