Survival rates of a tropical island endemic following conservation introduction on a rehabilitated island: the case of the endangered Seychelles White-eye

Abstract

Survival rates of the endemic Seychelles White-eye Zosterops modestus were investigated in a small transferred population on Frégate Island, Seychelles. The population has been studied since the first conservation introduction of 37 Seychelles White-eyes in 2001-2003, in an attempt to secure the long term future of this critically endangered species. Re-sighting data of 206 birds from direct systematic surveys were analysed with Cormack-Jolly-Seber (CJS) models using the software MAR K 5.1. Here we present the first robust estimates of adult annual survival published for this species. We also attempted to provide a preliminary survival rate of first-year birds. Adult survival probability (φ) was found to be 76 % in our best-supported model, and survival was constant over time. Annual adult survival rate did not differ significantly between the transferred white-eyes (φ = 72, n = 31) and the Frégatois (those born on Frégate) (φ = 78, n = 175), nor did it between males (φ = 77, n = 117) and females (φ = 74, n = 86). Juvenile survival rate could not be modelled due to insufficient data on chicks marked at the nest or shortly after fledgling. However, 24 out of 27 (89 %) white-eyes marked as chicks/ fledglings on Frégate between 2002 and 2008 survived to one year old, indicating potentially high juvenile survival rates, comparable to the adults. More data are needed on marked chicks/ fledglings to determine reliable and robust juvenile survival rates through modelling. Our study revealed the importance of having good monitoring design, regular, systematic and long-term data collection for such transferred populations, which will aid in the estimation of reliable survival rates along with other demographic and ecological parameters. We recommend the continued collection of both protocol-driven and empirical data, as well as an increased effort to estimate juvenile survival rates which will lead to more accurate assessments of population status. Close monitoring of transferred threatened birds on small islands offers further opportunities to better understand fundamental principles of population dynamics and ecology in these environments.