Current Land Bird Distribution and Trends in Population Abundance Between 1982 and 2012 on Rota, Mariana Islands

Abstract

The western Pacific island of Rota is the fourth largest human-inhabited island in the Mariana archipelago and designated an Endemic Bird Area. Between 1982 and 2012, 12 point-transect distance-sampling surveys were conducted to assess bird 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. Trends in population size were reliably estimated for 11 of 13 bird species, and 7 species declined over the 30-y time series, including the island collared-dove Streptopelia bitorquata, white-throated ground-dove Gallicolumba xanthonura, Mariana fruit-dove Ptilinopus roseicapilla, collared kingfisher Todiramphus chloris orii, Micronesian myzomela Myzomela rubratra, black drongo Dicrurus macrocercus, and Mariana crow Corvus kubaryi. The endangered Mariana crow (x̄ = 81 birds, 95% CI 30–202) declined sharply to fewer than 200 individuals in 2012, down from 1,491 birds in 1982 (95% CI = 815–3,115). Trends increased for white tern Gygis alba, rufous fantail Rhipidura rufifrons mariae, and Micronesian starling Aplonis opaca. Numbers of the endangered Rota white-eye Zosterops rotensis declined from 1982 to the late 1990s but returned to 1980s levels by 2012, resulting in an overall stable trend. Trends for the yellow bittern Ixobrychus sinensis were inconclusive. Eurasian tree sparrow Passer montanus trends were not assessed; however, their numbers in 1982 and 2012 were similar. Occupancy models of the 2012 survey data revealed general patterns of land cover use and detectability among 12 species that could be reliably modeled. Occupancy was not assessed for the Eurasian tree sparrow because of insufficient detections. Based on the 2012 survey, bird distribution and abundance across Rota revealed three general patterns: 1) range restriction, including Mariana crow, Rota white-eye, and Eurasian tree sparrow; 2) widespread distribution, low abundance, including collared kingfisher, island collared-dove, white-throated ground-dove, Mariana fruit-dove, white tern, yellow bittern, black drongo, and Micronesian myzomela; and 3) widespread distribution, high abundance, including rufous fantail and Micronesian starling. The Mariana crow was dispersed around the periphery of the island in steep forested land-cover types. In contrast, the Rota white-eye was restricted to the high-elevation mesa. Only for the white-throated ground-dove was there a significant difference among cover types, with lower occupancy in open field than in forested areas. Vegetation was included in the best-fit occupancy models for yellow bittern, black drongo, Micronesian myzomela, and Micronesian starling, but vegetation type was not a significant variable nor included in the top models for the remaining five species: white tern, island collared-dove, Mariana fruit-dove, collared kingfisher, and rufous fantail. Given declining population trends, the Rota bird-monitoring program could benefit from establishing threshold and alert limits and identifying alternative research and management actions. Continued monitoring and demographic sampling, in conjunction with ecological studies, are needed to understand why most bird species on Rota are declining, identify the causative agents, and assess effectiveness of conservation actions, especially for the Mariana crow.

Landscape-level bird loss increases the prevalence of honeydew-producing insects and non-native ants

“…Thus, we suggest that the lack of a difference in HPIs between Guam and Rota is mostly an issue of low statistical power. Rota’s intermediate level of HPIs could also reflect the lower abundance of insectivorous birds on Rota compared to Saipan (Camp et al 2009 , 2015 ), which may contribute to reduced direct predation of HPIs by birds. This possibility is corroborated by the findings of Davis et al ( 2008 ), who showed in a similar island system with invasive ants and HPIs that Christmas Island white-eyes ( Zosterops natalis ) forage extensively on ant-tended scale insects.…”

“…Recently estimated densities of Rufous fantails are slightly higher on Rota (651 ± 16 individuals/km 2 ) compared to Saipan (469 ± 63 individuals/km 2 ), although Saipan has much higher densities of bridled white-eyes ( Z. conspicillatus ) (4713 ± 387 individuals/km 2 ) and golden white-eyes ( C. marchei ) (711 ± 112 individuals/km 2 ) compared to densities of Rota white-eyes ( Z. rotensis ) (450 ± 14 individuals/km 2 ) (Camp et al 2009, 2015); Rota white-eyes have not been recorded at our study sites. Finally, Saipan also has insectivorous Mariana swiftlets that are locally extirpated from Rota.…”

“…The nearby islands of Rota and Saipan have intact bird assemblages when compared to Guam, including forest-dwelling insectivores such as Rufous fantails ( Rhipidura rufifrons ) and white-eyes ( Cleptornis marchei , Zosterops conspicillatus , Z. rotensis ) that likely encounter arboreal ants and HPIs. Recently estimated densities of Rufous fantails are slightly higher on Rota (651 ± 16 individuals/km 2 ) compared to Saipan (469 ± 63 individuals/km 2 ), although Saipan has much higher densities of bridled white-eyes ( Z. conspicillatus ) (4713 ± 387 individuals/km 2 ) and golden white-eyes ( C. marchei ) (711 ± 112 individuals/km 2 ) compared to densities of Rota white-eyes ( Z. rotensis ) (450 ± 14 individuals/km 2 ) (Camp et al 2009 , 2015 ); Rota white-eyes have not been recorded at our study sites. Finally, Saipan also has insectivorous Mariana swiftlets that are locally extirpated from Rota.…”

Vertebrate seed dispersers maintain the composition of tropical forest seedbanks

“…2013 ). The potential impact of recent declines in some frugivorous bird species on Rota ( Camp et al . 2014 ) was not evident in our study, as a similar per cent of seeds in the seedbanks on Rota as on Saipan likely arrived through vertebrate dispersal (i.e.…”

“…In contrast, the nearby islands of Saipan ( Camp et al . 2009 ) and Rota ( Camp et al . 2014 ), which both support similar limestone forest to Guam, have a more intact disperser assemblage, though some populations in Rota are declining ( Camp et al .…”