Temporal Variation in Forest Bird Survey Data from Tutuila Island, American Samoa

Abstract

Avian census data from tropical Pacific islands often are limited to brief, one-time surveys. These efforts yield information about species' presence and distribution but reveal little about variation in abundance through time. This variation may be important for refining and optimizing survey methods and, in turn, assessing habitat preferences, population status, activity patterns, or the impact of disturbance on the abundance and distribution of island birds. The objective of this study was to determine if intra- or interannual patterns exist in the recorded abundance of resident land birds. Forest birds on Tutuila Island, American Samoa, were surveyed each month from 1992 to 1996 at 35 stations on six transects distributed around the island. We used multiple regression techniques to determine that seasonal patterns in detected abundance exist in several species, most notably the Purple-capped Fruit-dove, Ptilinopus porphyraceus, and the Wattled Honeyeater, Foulehaio carunculata. Intraannual patterns may be associated with seasonally variable vocalizations or with concentrations of birds at particular resources. Interannual trends in abundance were not islandwide for any native species during the study period; they were localized and as such may be attributable to small-scale changes in habitat rather than to overall changes in population size. The results of this study, especially that the abundance of nonmigratory island birds is seasonally variable, reinforce the importance of year-round monitoring in the study and conservation of Pacific birds.