Update on the status of the avifauna of Lehua Islet, Hawai'i, including initial response of seabirds to rat eradication

Abstract

Lehua Islet is a small volcanic islet located 1 km north of Ni‘ihau, protected by the State of Hawai‘i as a Seabird Sanctuary because of its large seabird colonies. Between December 2011 and May 2021, data was collected using a variety of methods to provide an update of the islet’s avifauna, with a particular focus on seabirds. In 2017 a rat eradication project was undertaken to remove the Polynesian Rat Rattus exulans and the islet was officially declared rat free on 20 April 2021. This report therefore provides both an update on the status of the avifauna of Lehua and an assessment of the initial impact of the rat eradication project on breeding seabirds. A total of 39 bird species were recorded on Lehua over the study period, including 18 seabird species (9 confirmed breeding). The most numerous breeding seabird was the Wedge-tailed Shearwater Ardenna pacifica, estimated at 22,226 ± 2,981bp. Lehua is therefore an important refuge for this species, which suffers significant predation by cats and dogs across the Main Hawaiian Islands. Several species, particularly burrow-nesting seabirds, responded positively to rat eradication. The most dramatic changes occurred in the Bulwer’s Petrel Bulweria bulwerii breeding population. This species was rarely encountered before eradication, but numbers of burrows located, the percentage of burrows where adults were confirmed breeding and two measures of nest success increased after eradication. Wedge-tailed Shearwaters also responded positively, with Mayfield Nest Success Estimates significantly higher in the three years after rats were eradicated than in the two years before eradication (t=2.37, p=0.02). Conversely, one species - the endangered Band-rumped Storm-petrel Oceanodroma castro - appeared to decline after rat eradication (as measured via call rates on acoustic sensors), although the change was not significant. Rat eradication has been a critical milestone in the conservation of Lehua Islet, and further natural recovery of seabirds can be expected. Future management actions will be key for capitalizing on the early successes of the rat eradication project. These include social attraction of seabirds that have been extirpated from the islet and habitat restoration through invasive plant removal and outplanting of native species. Regular control of the two remaining non-native predators (Barn Owl Tyto alba and Cattle Egret Bubulcus ibis) on the islet by well-trained predator control specialists will also be critical, as Barn Owls could reverse conservation gains and prevent new seabird species from colonizing. Lastly, ensuring a rigorous biosecurity program and monitoring strategy is vital to prevent reinvasion.