Effects of alien rodent control on demography of the O'ahu 'Elepaio, an endangered Hawaiian forest bird

Abstract

The O'ahu 'Elepaio Chasiempis sandwichensis ibidis is an endangered monarch flycatcher endemic to the Hawaiian island of O'ahu. Nest predation by alien Black Rats Rattus rattus is one of the main causes of the decline of this forest bird. From 1996 to 2000 we monitored demography of the largest remaining O'ahu 'Elepaio population and we controlled rats with snap traps and poison bait stations in an attempt to begin recovery of the species. Nest predation by Black Rats influenced many aspects of 'Elepaio demography. Rat control resulted in a 112% increase in 'Elepaio reproduction, a 66% increase in survival of female 'Elepaio, and restored mate fidelity, site fidelity, female age structure, and female recruitment age to more natural conditions. The 'Elepaio population growth rate (lambda) was 0.76 without rat control and 1.00 with rat control, indicating rat control stabilized 'Elepaio populations, but was not sufficient to cause population increase due to the added threat of alien mosquito-borne diseases. Rat control may facilitate evolution of disease resistance by providing birds that have greater natural immunity an increased chance of reproducing, thereby increasing the proportion of resistant birds in each subsequent generation more quickly.