Estimating the availability of potential hornbill nest trees in a secondary forest patch in Kinabatangan, Sabah

• Main Authors: Kaur, Ravinder, Ramli, Rosli, Ancrenaz, Marc, Hassan, Nelson, Ahmad, Eddie, Ratag, Mahathir, Elahan, Berjaya, Sinyor, Hasbollah, Rajak, Abdul
• Format: Article
• Published: Oriental Bird Club 2020
• Subjects: QH301 Biology; SD Forestry
• Online Access: http://eprints.um.edu.my/37056/
• Institution: Universiti Malaya

Eight species of hornbill occur in the Lower Kinabatangan Wildlife Sanctuary, Sabah. Hornbills are secondary cavity-nesting birds and one of the limiting factors to sustain their numbers is the availability of naturally-formed tree cavities. Past timber extraction has left behind highly degraded forest patches without large emergent trees that usually provide suitable cavities for nesting hornbill pairs. Therefore, we conducted a study to assess how widespread this key resource is and to estimate the proportion of a forest patch currently occupied by potential nest trees, i.e. trees with cavities. In a 10 km(2) study site, eight trained observers systematically visited 30 250 m x 250 m plots and recorded tree cavities that appeared suitable for hornbills based on a pre-established list of criteria. Nineteen trees with cavities were located, measured and identified. We anticipated that cavities could go undetected by the observers; we therefore used a zero-inflated process occupancy model to address this measurement error and to analyse data obtained along transects. The observers detected trees with cavities in 10 out of 30 plots, translating into an observed proportion of roughly 33%. However, our model indicated that trees with cavities might actually occupy 25 out of 30 plots, i.e. 82% of the forest patch area. Our modelling approach incorporates imperfect detection through hierarchical modelling and constitutes a quick and cost-effective assessment tool that can be used to investigate the spatial presence of potential nest trees, an important resource for hornbills.