Status of forest birds on Rota, Mariana Islands

Abstract

The western Pacific island of Rota is the third largest human inhabited island in the Mariana archipelago, and is designated an Endemic Bird Area. Between 1982 and 2012, 12 point-transect distance sampling surveys were conducted to assess population status. Surveys did not consistently sample the entire island; thus, we used a ratio estimator to estimate bird abundances in strata not sampled during every survey. Occupancy models of the 2012 survey revealed general patterns of habitat use and detectability among 11 species that could be reliably modeled. The endangered Mariana crow ( Corvus kubaryi ) was dispersed around the periphery of the island in steep forested habitats. In contrast, the endangered Rota white-eye ( Zosterops rotensis ) was restricted to the high-elevation mesa. Precision of detection probabilities and occupancy estimates and effects of habitat types, sampling conditions, and specific observers varied considerably among species, indicating that more narrowly defined classifications and additional observer training may improve the accuracy of predictive modeling. Population estimates of five out of ten native bird species, including collared kingfisher ( Todiramphus chloris orii ), Mariana crow, Mariana fruit-dove ( Ptilinopus roseicapilla ), Micronesian myzomela ( Myzomela rubrata ), and white-throated ground-dove ( Gallicolumba xanthonura ) declined over the 30-year time series. The crow declined sharply to fewer than 200 individuals (upper 95% confidence interval). Trends increased for Micronesian starling ( Aplonis opaca ), rufous fantail ( Rhipidura rufifrons mariae ), and white tern ( Gygis alba ). Rota white-eye numbers declined from 1982 to the late 1990s, but returned to 1980s levels by 2012. The trend for the yellow bittern ( Ixobrychus sinensis ) was inconclusive. The alien Eurasian tree sparrow ( Passer montanus ) apparently increased in number despite an unreliable trend assessment. Declines were noted in the other two alien birds, black drongo ( Dicrurus macrocercus ) and island collared-dove ( Streptopelia bitorquata ). Total bird densities on Rota were similar to those on Saipan and Tinian, which were lower than densities on Aguiguan. Overall, bird trends on Rota declined, whereas trends observed for the same period on Saipan and Tinian were mixed, and trends on Aguiguan were stable to increasing. We identified several sampling design and protocol procedures that may improve the precision of occupancy, status, and trend assessments. Continued monitoring and demographic sampling are needed to understand why most bird species on Rota are declining, to identify the causative agents, and to assess effectiveness of conservation actions for rare species, especially the Mariana crow.