Changes in endangered hornbill populations over space and time and potential ecological impacts in peninsular Thailand

Hornbills are particularly important seed dispersers in tropical Asian forests, but most of the species in the region are declining. In addition, long-term data on the extent of the declines and how forest degradation impacts hornbills remain mostly unquantified. Here, we used multicovariate distance sampling to estimate the densities of hornbills in 2021–2022 in Hala-Bala Wildlife Sanctuary and Budo Su-Ngai Padi National Park protected areas in southern Thailand. These two sites are particularly useful for comparison due to differences in protection levels, with Bala being relatively less disturbed than Budo Mountain and because Bala was surveyed 20 years prior (2001–2002). Fig tree watches were also conducted in 2021–2022 to assess the potential impact of forest disturbance on hornbill visitation rates. Results indicated significant differences in hornbill densities between the two sites. The mean density of the Rhinoceros (Buceros rhi­

noceros) and Wreathed Hornbill (Rhyticeros undulatus) was 3.2 and 5.1 times higher in Bala than

in Budo, while the mean density of the Great Hornbill (Buceros bicornis) was 2.2 times higher in

Budo than in Bala. The density of the Helmeted Hornbill (Rhinoplax vigil) was notably lower than

other hornbill species. There were also significant changes over the past 20 years in large hornbill

densities, with Great Hornbill and Wreathed Hornbill significantly increasing, while the Helmeted

Hornbill declined more than 90 %. Also, the probability of hornbills visiting fruiting Ficus trees

was higher in the less-disturbed Bala than the Budo Mountain, making hornbills more reliable

visitors in the less-disturbed site. We demonstrate the applicability of the multi-covariate distance

sampling approach to estimate densities of rare hornbills, the negative impacts of habitat

degradation on hornbills and the ecological role they play as seed dispersers. Additional surveys

are needed throughout tropical Asia to establish further sites for long-term monitoring, especially

in the Sunda region, where the Helmeted Hornbill is under extreme threat