Terrestrial bird population trends on Aguiguan (Goat Island), Mariana Islands

Abstract

SummaryThe island of Aguiguan is part of the Mariana archipelago and currently supports populations of four endemic species, including one endemic genus,Cleptornis. Bird population trends since 1982 were recently assessed on the neighbouring islands of Saipan, Tinian, and Rota indicating declines in some native species. Point-transect surveys were conducted in 2008 by the U.S. Fish and Wildlife Service to assess population densities and trends on Aguiguan. Densities for six of the nine native birds—White-throated Ground-doveGallicolumba xanthonura, Collared KingfisherTodiramphus chloris, Rufous FantailRhipidura rufifrons, Golden White-eyeCleptornis marchei, Bridled White-eyeZosterops conspicillatusand Micronesian StarlingAplonis opaca—and the non-native bird—Island Collared-doveStreptopelia bitorquata—were significantly greater in 2008 than in 1982. No differences in densities were detected among the surveys for Mariana Fruit-dovePtilinopus roseicapilla, and Micronesian MyzomelaMyzomela rubratra. Three federally and locally listed endangered birds—Nightingale Reed-warblerAcrocephalus luscinius, Mariana SwiftletCollocalia bartschi, and Micronesian MegapodeMegapodius laperous)—were either not detected during the point-transect counts, the surveys were not appropriate for the species, or the numbers of birds detected were too small to estimate densities. The factors behind the increasing trends for some species are unknown but may be related to increased forest cover on the island since 1982. With declining trends for some native species on neighbouring islands, the increasing and stable trends on Aguiguan is good news for forest bird populations in the region, as Aguiguan populations can help support conservation efforts on other islands in the archipelago.