The other subpopulation of Christmas Island White-eye Zosterops natalis (Aves: Zosteropidae): a historic introduction has led to an enduring subpopulation on Horsburgh Island, Cocos (Keeling) Islands group

The Christmas Island White-eye Zosterops natalis occurs naturally only on the 135 km2 Christmas Island. Between 1888 and 1900 (remarkably soon after it was first discovered and described), it was introduced to the 1 km2 Horsburgh Island in the Cocos (Keeling) Islands group. There has been limited subsequent documentation of the fate of this translocated population. Based on transect sampling in 2013, we estimate the population size to be 1084 individuals (with 95% confidence limits of 731 to 1716). This represents a substantially larger estimate than the previous estimate (of 400 individuals in 1941), and indicates a greater abundance than the most recent (1982) non-quantitative record of its status. In contrast to previous documentation, this species is widespread on Horsburgh Island, and abundant in a mosaic of natural and modified vegetation (dominated by coconut Cocos nucifera plantation with Scaevola taccada shrub layer). Contextualising the conservation significance of this introduced population is difficult because there is no reliable estimate of the population size on Christmas Island, but it is plausible that the Horsburgh Island subpopulation now comprises c. 5% of the species’ total population size. However, more importantly, its significance lies in the provision of a second location that may reduce extinction risk.