The National Park of American Samoa (NPSA) was surveyed for landbirds and habitat characteristics from June through August, 2011. This information provides the first data in the time-series of landbird monitoring for long-term trends in forest bird distribution, density, and abundance within the NPSA. The NPSA survey area was comprised of the terrestrial portions of the Ta‘u and Tutuila Units. Each Unit was surveyed using point-transect distance sampling to estimate bird abundance. Sampling was conducted using a split-panel design where legacy transects are visited during each sampling occasion and newly, randomly located transects are visited only during one sampling occasion. This design optimizes trend detection while allowing for measuring and correcting for estimator bias.
A total of 2,516 birds was detected from 13 species in both Units. All species were either endemic or indigenous to the islands of American Samoa. Numbers of detections ranged from 7 to 1,111. Nearly every species detected was broadly distributed in the predominantly native forests of NPSA. Sufficient detections were made of seven species, allowing for density estimation. Densities of species were higher in the Tutuila Unit; with the exception of the Wattled Honeyeater (Foulehaio carunculata), which was the most abundant species in both Units. The species occurred at nearly every station sampled and had densities much higher than the Samoan Starling (Aplonis atrifusca), Polynesian Starling (Aplonis tabuensis), and Collared Kingfisher (Halcyon chloris) which occurred in modest densities. The remaining species detected occurred at less than 20% of stations sampled and we were only able to determine the number of birds per station and percent occurrence. The White-rumped Swiftlet (Aerodramus spodiopygius) and Cardinal Honeyeater (Myzomela cardinalis) were detected in small numbers, but both species can be difficult to detect in closed canopy forests. The Purple Swamphen (Porphyrio porphyrio) and Banded Rail (Gallirallus philippensis) were most often detected in areas close to villages and agroforestry plantations. The Blue-crowned Lorikeet (Vini australis) and Fiji Shrikebill (Clytorhynchus vitiensis) only occur in the Manu‘a Island Group. The former was detected in most survey areas and the latter was patchily distributed in the Ta‘u Unit. The Many-colored Fruit-dove (Ptilinopus perousii), a species of concern, was detected in very small numbers in both Units. The Spotless Crake (Porzana tabuensi), which is extirpated on Tutuila Island, has been incidentally detected in small numbers on Ta‘u Island. However, the species was neither seen nor heard during this survey and remains a species of concern.
NPSA canopy and understory composition was predominantly native, and trees formed a dense closed canopy at nearly 90% of the stations sampled. More than half of the tree heights in both units were taller than 5 m and the majority of slopes were steeper than 20 degrees. There were no clear dominant tree species in the mixed native forests. The most common tree species documented included Syzygium spp., Dysoxylum spp., Ficus spp., Hibiscus tiliaceus and Rhus taitensis (among others). There were significant differences in the distribution of bird densities between legacy and random transects. Determining differences in detection probabilities cannot be definitively assessed from a single survey. We recommend both panels be sampled in the future until bias in density and abundance can be evaluated, or if sampling may be reduced.
|Title||Pacific island landbird monitoring annual report, National Park of American Samoa, Ta‘u and Tutuila units, 2011|
|Authors||Seth W. Judge, Richard J. Camp, Visa Vaivai, Patrick J. Hart|
|Publication Subtype||Other Government Series|
|Series Title||National Park Service Natural Resource Technical Report|
|Record Source||USGS Publications Warehouse|