絶滅危惧種アフリカオウムの個体数推定法3つの比較

Obtaining reliable estimates of population sizes and their temporal trends is essential for assessing the conservation status and guiding the management of threatened species. Parrots (order Psittaciformes) are among the most diverse yet most threatened groups of birds worldwide, but information on their population sizes remains scarce. Their ecological traits and low densities complicate the application of widely used distance sampling methods for estimating population densities, which correct for declining detectability with increasing distance. Researchers have proposed simple encounter rates-the number of birds or groups detected per hour of observation during casual walks-as an alternative approach to address these limitations when estimating the abundance of rare species. Previous studies of the globally endangered African grey parrot (Psittacus erithacus) demonstrated that encounter rates derived from walk transects serve as reliable surrogates for densities estimated using distance sampling models. In this study, we evaluated whether car surveys and point counts, two other commonly used methods for estimating bird abundance, produce encounter rates comparable to those obtained from walk transects, thereby allowing greater methodological flexibility. To this end, we conducted a nationwide survey of African grey parrots in Equatorial Guinea using walk transects, car transects, and point counts (199 surveys, 1973 km, 192 h), which yielded 1166 encounters and 2972 recorded individuals. Three aspects of parrot detectability (the frequency of aural and visual detections, whether birds were perched or in flight, and detection distances) did not differ among the three survey methods. Encounter rates and the number of individuals observed per hour varied among sampled regions but did not differ among survey methods. These findings support the use of the three methods, either individually or in combination, to estimate the abundance of this globally endangered species and provide a basis for testing and applying this approach to other parrot species and geographic regions.