Status and trends of the land bird avifauna on Tinian and Aguiguan, Mariana Islands

Avian surveys were conducted on the islands of Tinian and Aguiguan, Marianas Islands, in 2008 by the U.S. Fish and Wildlife Service to provide current baseline densities and abundances and assess population trends using data collected from previous surveys. On Tinian, during the three surveys (1982, 1996, and 2008), 18 species were detected, and abundances and trends were assessed for 12 species. Half of the 10 native species—Yellow Bittern (Ixobrychus sinensis), White-throated Ground-Dove (Gallicolumba xanthonura), Collared Kingfisher (Todiramphus chloris), Rufous Fantail (Rhipidura rufifrons), and Micronesian Starling (Aplonis opaca)—and one alien bird—Island Collared-Dove (Streptopelia bitorquata)—have increased since 1982. Three native birds—Mariana Fruit-Dove (Ptilinopus roseicapilla), Micronesian Honeyeater (Myzomela rubratra), and Tinian Monarch (Monarcha takatsukasae)—have decreased since 1982. Trends for the remaining two native birds—White Tern (Gygis alba) and Bridled White-eye (Zosterops saypani)—and one alien bird—Eurasian Tree Sparrow (Passer montanus)—were considered relatively stable. Only five birds—White-throated Ground-Dove, Mariana Fruit-Dove, Tinian Monarch, Rufous Fantail, and Bridled White-eye—showed significant differences among regions of Tinian by year. Tinian Monarch was found in all habitat types, with the greatest monarch densities observed in limestone forest, secondary forest, and tangantangan (Leucaena leucocephala) thicket and the smallest densities found in open fields and urban/residential habitats. On Aguiguan, 19 species were detected on one or both of the surveys (1982 and 2008), and abundance estimates were produced for nine native and one alien species. Densities for seven of the nine native birds—White-throated Ground-Dove, Mariana Fruit-Dove, Collared Kingfisher, Rufous Fantail, Bridled White-eye, Golden White-eye (Cleptornis marchei), and Micronesian Starling—and the alien bird— Island Collared-Dove—were significantly greater in 2008 than 1982. No differences in densities were detected between the two surveys for White Tern and Micronesian Honeyeater. Three native land birds— Micronesian Megapode (Megapodius laperouse), Guam Swiftlet (Collocalia bartschi), and Nightingale Reed-Warbler (Acrocephalus luscinia)—were either not detected during the point-transect counts or the numbers of birds detected were too small to estimate densities for either island. Increased military operations on Tinian may result in increases in habitat clearings and the human population, which would expand human-dominated habitats, and declines in some bird populations would be likely to continue or be exacerbated with these actions. Expanded military activities on Tinian would also mean increased movement between Guam and Tinian, elevating the probability of transporting the brown tree snake (Boiga irregularis) to Tinian.