Habitat Use and Status of the Bokikokiko or Christmas Island Warbler (Acrocephalus aequinoctialis)1

The Bokikokiko or Christmas Island Warbler (Acrocephalus aequinoctialis) is found only on Kiritimati (Christmas) and Teraina (Washington) Islands in the Republic of Kiribati. The population on Kiritimati Island is threatened by habitat degradation, sea level rise, and predation from feral cats (Felis catus), Pacific rats (Rattus exulans), and recently arrived black rats (Rattus rattus). There is scant information about distribution and abundance of the Bokikokiko. From 2007 to 2012, we conducted surveys with song playbacks at 83 points on 12 transects in the northern half of Kiritimati Island to measure abundance of the Bokikokiko and begin monitoring for possible declines associated with the spread of rats, and we collected habitat data to investigate factors that influenced Bokikokiko abundance. Song playbacks resulted in a 263% increase in detection rate over passive listening. We detected an average (±1 SE) of 0.63 ± 0.11 birds per point using playbacks. Average population density was 0.36 ± 0.06 birds per hectare, but abundance varied among regions, and no birds were detected in some areas with apparently suitable habitat. Range of the Bokikokiko encompassed an area of 14,180 ha but was fragmented by many lagoons and bare ground, and only about half that area was actually occupied. Estimated population size was 2,550 ± 425 birds. Bokikokiko were more abundant in areas with taller Heliotropium trees and taller Scaevola shrubs, and less abundant in areas with more Suriana shrubs, bare ground, and grass. Conservation actions needed for the Bokikokiko include ongoing removal of rats from islets within the lagoons of Kiritimati Island, protection of preferred habitat from development and fires, and translocations to create additional populations on rat-free atolls.