Recent Dynamics and Trends of Landbird Populations on Saipan, Northern Mariana Islands

Abstract

The avifauna of Saipan, Northern Marianas Islands, includes a variety of endemic and range-restricted species of conservation concern. Monitoring population dynamics, trends, and vital rates of these species can help inform their conservation. Here we apply reverse symmetry models to 11 years (2008–2018) of capture–recapture data from 6 mist-netting stations on Saipan to estimate adult apparent survival probabilities, recruitment rates, population growth rate, and demographic contributions to population growth rate for five bird species: Mariana Kingfisher (Todiramphus albicilla albicilla), Rufous Fantail (Rhipidura rufifrons saipanensis), Bridled White-eye (Zosterops conspicillatus saypani), Golden White-eye (Cleptornis marchei), and Micronesian Myzomela (Myzomela rubratra saffordi). Population growth rate estimates suggested declines for all species except Golden White-eye. Survival was the largest contributor to annual population growth rate for all species except Micronesian Myzomela, for which population change was largely determined by recruitment (including 1-yo recruits and immigrating adults). Hierarchical models applied to the two species with the most captures and recaptures, Rufous Fantail and Golden White-eye, showed greater temporal than spatial variation in vital rates. Annual variation in vital rates confirmed the importance of survival in explaining population change, but also showed recruitment to be especially important in driving population growth in years of population increase. Our results provide new insights into drivers of population change in these species and highlight information gaps, such as a need for understanding seasonal habitat use and the scale and timing of movements.