Population density, distribution, and trends of landbirds in the National Park of American Samoa, Ta‘ū and Tutuila units (2011–2018)

Judge, Seth; Camp, Richard; Vaivai, Visa; Hart, Patrick

doi:10.36967/nrr-2284409

The National Park of American Samoa (NPSA) was surveyed for landbirds from June through July, 2018. Surveys were conducted using point-transect distance sampling methods to estimate bird densities. This information provides the second datum in the time-series of landbird monitoring for long-term trends in landbird distribution, density, and abundance within NPSA. The Ta‘ū Unit and Tutuila Unit, each on separate islands, were first surveyed in 2011 and we tested for changes in densities between each survey year. A total of 1,766 birds was detected of 14 species in both units. All species were either endemic or indigenous to the islands of American Samoa. Sufficient detections were made of seven species, allowing for density estimation. Birds per station (BPS) and percent occurrence were calculated for the remaining species. The Polynesian Wattled Honeyeater (Foulehaio carunculatus) was the most widely distributed and abundant species, with an estimated 50,794 ± 11,149 SE birds in the 2,229 ha Ta‘ū Unit and 18,147 ± 3,230 birds in the 1,021 ha Tutuila Unit. The Samoan Starling (Aplonis atrifusca) occurred at an estimated abundance of 12,801 ± 3,596 birds in the Ta‘ū Unit. We observed a significant decline in Samoan Starling densities in the Tutuila Unit–our abundance estimate of 4,994 ± 597 birds was approximately 54% of the 2011 estimate. The Polynesian Starling (Aplonis tabuensis) also occurred broadly in both units–we estimated an abundance of 9,153 ± 1,943 birds in the Ta‘ū Unit and 3,707 ± 805 birds in the Tutuila Unit. The Pacific Imperial Pigeon (Ducula pacifica) occurred at an abundance of 654 ± 303 birds in the Ta‘ū Unit and 1,050 ± 362 birds in the Tutuila Unit. The Crimson-crowned Fruit Dove (Ptilinopus porphyraceus) occurred in a relatively low abundance in both units, with an estimated 815 ± 386 birds in the Ta‘ū Unit and 484 ± 187 birds in the Tutuila Unit. Estimated abundance of the Pacific Kingfisher (Todirhamphus sacer) was 280 ± 87 birds in the Ta‘ū Unit and 202 ± 62 birds in the Tutuila Unit. The Blue-crowned Lorikeet (Vini australis) in the Ta‘ū Unit declined significantly from 2011 estimates to an abundance of 2,686 ± 1,007 birds. The Cardinal Myzomela (Myzomela cardinalis) occurred at 31.5% of the stations sampled in the Tutuila Unit, but the species has been historically absent from Ta‘ū Island. The White-rumped Swiftlet (Aerodramus spodiopygius) occurred at 35.4% and 45.2% of stations sampled in the Ta‘ū Unit and Tutuila Unit, respectively. The remaining species detected occurred at less than 20% of stations sampled. The Purple Swamphen (Porphyrio porphyrio) and Banded Rail (Gallirallus philippensis) were most often detected in areas close to villages and agroforestry plantations. The Samoan Shrikebill (Clytorhynchus powelli powelli) was limited to one area on the southern portion of the Ta‘ū 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), also a species of concern, was incidentally detected at one location in the Ta‘ū Unit.

Population density, distribution, and trends of landbirds in the National Park of American Samoa, Ta‘ū and Tutuila units (2011–2018)

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