Population assessment and habitat associations of the Visayan Hornbill Penelopides panini in Northwest Panay, Philippines

Abstract

BackgroundSeven out of 10 hornbill species in the Philippines are threatened with extinction. Among these is the Endangered Visayan hornbill Penelopides panini, found on Panay and Negros islands. Threatened by habitat loss and hunting, its population size is thought to have declined from 1,800 individuals 20 years ago to less than 1,000. However, a recent study on Negros estimated 3,564 individuals across three core forest blocks. This study aims to quantify the Visayan hornbill population size in and around the Northwest Panay Peninsula Natural Park (NWPPNP) on Panay, the largest contiguous low-elevation forest landscape remaining across its range, and its broad habitat associations across a gradient of environmental degradation. MethodsHornbills were surveyed using 10-minute Distance sampling point counts (n = 362) along transects (average length 1.1 km). Habitat variables were recorded, while habitat was classified into: primary forest, secondary forest, plantation or open habitat. Using Distance software, population densities were estimated for, and post-stratified by habitat, with the overall population estimate taken as a mean of habitat density estimates weighted by habitat area. Using logistic binary regression, hornbill occurrence was modelled against reduced habitat factors extracted from factor analysis of the habitat data. ResultsSurveys covered 204.4 km2 of the 374.8 km2 Northwest Panay Peninsula. Hornbills were not recorded in plantation or open habitats. The estimated density of hornbills was significantly higher in primary forest (17.7 individuals km-2 {+/-} 29.7% CV) than in secondary forest (5.0 individuals km-2 {+/-} 36.7 %CV; z = 9.538, P < 0.001). The overall population estimate is 2,231 individuals {+/-} 24.4 %CV for the NWPPNP and environs, and 2,949 individuals {+/-} 23.1 %CV for the entire Northwest Panay Peninsula. One habitat factor, described by increasing numbers of large trees, elevation and distance from the Parks boundary, had a significant positive effect in explaining hornbill occurrence, with hornbills significantly more likely to occur in primary forest than the other habitat types. ConclusionsOur study demonstrates the habitat preference of the Visayan hornbill, highlights the importance of the NWPPNP for the species conservation, and provides strong evidence for re-assessing the global population size.