Relative abundance, habitat use, and long-term population changes of wintering and resident landbirds on St. John, U.S. Virgin Islands
St. John, U.S. Virgin Islands, is one of the most forested islands in the West Indies and provides an opportunity to conserve both resident birds and wintering neotropical migrants.We conducted double-observer point counts of landbirds in December 2005 and 2006 in Forest Inventory and Analysis (FIA) plots and National Park Service (NPS) trails in Virgin Islands National Park (VINP) to assess population trends of birds in subtropical dry and moist forests. We recorded 2,270 individual birds representing 35 species at 150 point count stations in 2005, and 3,092 individuals of 32 species at 143 of these stations in 2006. The increase in birds per point from 2005 (15.1) to 2006 (21.6) was due to resident species, 17 of which were recorded more frequently in 2006. The 17 species of neotropical migrants composed 11.8% of all registrations in 2005 and 2006. Subtropical moist and dry forest habitats differed strongly in vegetation characteristics and plant species, but no species of birds exhibited a strong affiliation with either habitat type on FIA plots. Data from NPS trails showed that most migrant species were detected more often in moist, mature forest. The resident Bridled Quail-Dove (Geotrygon mystacea) also was correlated with mature forest. Plant and bird species co-occurrence with positive correlations that may carry a signal of preferred frugivory included Guettarda odorata (Rubiaceae) with Bridled Quail-Dove, and Myrciaria floribunda (Myrtaceae) with Pearly-eyed Thrasher (Margarops fuscatus). Migrant species did not exhibit strong long-term changes in relative abundance since founding of VINP in 1957, but four open-country resident species declined significantly between 1957 and 2006 as the forest matured. Forest maturation should continue on St. John, yielding a bright future for most of its landbirds barring catastrophic hurricanes, pathogens, or invasive plants.