Occupancy and habitat use of the endangered Akikiki and Akekee on Kauai Island, Hawaii

Limited resources for biodiversity conservation demand strategic science-based recovery efforts, particularly on islands, which are global hotspots of both endemism and extinction. The Akikiki (Oreomystis bairdi) and the Akekee (Loxops caeruleirostris) are critically endangered honeycreepers endemic to the Hawaiian island of Kauai. Recent declines and range contraction spurred investigation of the habitat characteristics influencing range-wide occupancy of these species. We surveyed Akikiki and Akekee and habitat covariates within 5 study areas on the Alakai Plateau of Kauai along a gradient of forest conditions. Occupancy rates for both species increased from west to east along the plateau (Akikiki: Psi = 0.02 +/- 0.07 to 0.55 +/- 0.21; Akekee: Psi = 0.03 +/- 0.10 to 0.53 +/- 0.33), but were low throughout the ranges of both species. Canopy height was positively correlated with occupancy for both species, which suggests the damage done by hurricanes in 1982 and 1992 may be one factor restricting these birds to the most intact forest remaining. Vegetation surveys revealed several key differences in forest composition and structure between areas, indicative of broader changes occurring across the plateau. Invasive plants such as Himalayan ginger (Hedychium gardnerianum) were dominant in the western portion of the plateau, where there was a corresponding decline in native plant cover. Conversely, ground disturbance by feral ungulates was higher in more eastern native-dominated areas. These results highlight the need to protect habitat in the regions where Akikiki and Akekee occupancy is highest, and restore habitat in other parts of their range. These actions should occur in concert with the mitigation of other known threats to Hawaiian honeycreepers such as avian disease. Without significant investment to address these threats and protect suitable habitat for these species, it is unclear how long these birds will persist.