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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 km2 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.
Little is known about the nesting behaviour of the Helmeted Hornbill Rhinoplax vigil because it occurs in low numbers and nests are difficult to locate. The nest cavity is usually high and hidden amidst thick foliage and the cavity’s opening is inclined upwards, making it hard to see from the ground. A nesting pair of Helmeted Hornbills was observed in the Kinabatangan Wildlife Sanctuary between 2013 and 2017. We sought to determine the nesting period and associated behaviour, and to identify the type and amount of food provided to the female and chick over the nesting cycle. The nest was located inside the nub of a broken branch of a Shorea pauciflora tree, 37 m up on the trunk. The pair began nesting in May, in the drier months, and the single chick fledged in November the same year. The pair and the fledged young stayed together for at least six months. The male made a maximum of 11 visits per day to bring food to the nest midway through the breeding period. Food brought to the nest consisted of mainly figs, including Ficus stupenda, F. benjamina, F. stricta and F. crassiramea. The adult Helmeted Hornbills delivered stick insects, beetles and praying mantis, while the chick itself caught and consumed a giant millipede at the nest entrance. The specific fig diet and nest cavity preferences make the species extremely vulnerable to environmental changes caused by logging and agricultural expansion. The added pressure from hunting it for casques may be driving it to extinction. Therefore we recommend that their nests be located and offered protection by local authorities and communities through nest adoption schemes.