Pre- and Post-El Nino Southern Oscillation Comparison of Nest Sites for Red-Tailed Tropicbirds Breeding in the Central Pacific Ocean

Abstract

Red-tailed Tropicbirds (Phaethon rubricauda) breeding on Christmas Island, Central Pacific Ocean, exhibited stable preferences for nest sites over time, such that vegetative characteristics associated with active nests were similar in 1980 and 1984. Before the El Niño Southern Oscillation (ENSO, 1980) the vegetative structure of randomly placed points differed from that of actual nests. After the ENSO (1984) the vegetative structure at randomly placed points resembled that of actual nests, suggesting that the overall availability of preferred sites may have increased. Records of the thermal environment surrounding actual nests and other points within the same shrub suggest that shifts in vegetative structure of nesting habitat created a favorable thermal environment for tropicbird chicks. This post-ENSO shift in habitat structure was accompanied by an increase in the number of active nests at the Y-site colony relative to pre-ENSO breeding seasons. These data support the hypothesis that suitable nest sites may limit short-term reproductive opportunities of tropicbirds, hence influence the rate of population growth and time course of recovery from catastrophic events such as ENSO..