Data Zone
Justification of Red List category
Rhyticeros cassidix is listed as Vulnerable as a deforestation models predict a 31% reduction in forest cover over the next three-generations. The species has specific nesting and feeding requirements, which make it particularly susceptible to habitat loss and degradation which is widespread across its range. Additionally, illegal hunting of the species and potential future fire impacts contribute additional threats which result in a suspected 40% reduction in the population over the next three-generations.
Population justification
Described as fairly common (Eaton et al. 2021). There have been no surveys to attempt to estimate the global population size of Rhyticeros cassidix. Line transects in April 1993-March 1994 in Tangkoko-DuaSudara Nature Reserve in North Sulawesi yielded a density estimate of 50-85 birds/km2 (O'Brien and Kinnaird 1996) but this extremely high density may have been skewed by gatherings at fruiting trees. Non-breeding densities have also been estimated at 51 birds/km2, with nesting densities of 10 pairs/km2 (Kemp and Boesman 2020). The densities however are tightly linked to food availability, and can range between 9-84 birds/km2 (Kinnaird and O'Brien 2007), lending high uncertainty to the average density of the species. No population size is provided due to the high uncertainties in species density over its range, although the large area of suitable habitat and the densities report suggest that the global population size is not small.
Trend justification
R. cassidix is a lowland species, that depends on the prevalence of Ficus spp. (figs) and natural tree cavities for feeding and nesting, respectively (Suryadi et al. 1994, Cahill 2003, Kemp and Boesman 2020). Over half the species’ diet is comprised of figs (Walker 2007), and hornbill populations have been shown to fluctuate over time and space in relation to Fig densities and fruit production, emphasising the importance of conserving preferred feed trees (Kinnaird et al. 1996, Kinnaird and O'Briend 2007). Therefore, forest loss, suspected to be c. 18% over the past three-generations (Global Forest Watch 2024, based on data from Hansen et al. [2013] and methods therein), is likely to underestimate declines in the population where degradation such as selective logging disproportionally removes figs and suitable nesting trees. Over the past three generations therefore, rates of decline are suspected to have been equal to 20-29%.
Forest loss may be higher in the future. A study by Voigt et al. (2021) projected c. 31% loss of forest cover from 2018 to 2053 (a time-range similar to the 2025-2051 three-generation period), using a dynamic deforestation model that incorporates drivers and patterns of forest loss in Sulawesi. Their study split the region into several sub-regions to calculate sub-regional annual rates of deforestation and is considered more reliable than speculatively projecting the average forest loss rate of the past five-years over the next three-generations (27 years). Additionally, although the Voigt et al. (2021) study included 880 km2 of unsuitable mangrove forest in their deforestation rates, this represents only a small fraction of their total (49,570km2) predicted forest loss, and any overestimations of the impact on the species by including mangrove forests is expected to be negligible.
Further, the species is subject to hunting for commercial markets and personal consumptions in Sulawesi (O’Brien and Kinnaird 1996, Kemp and Boesman 2020). Illegal hunting has led to the population declines in R. vigil (Prawiradilaga 2020), and a similar impact would be expected for R. cassidix. Overall future rates of declines are therefore suspected to fall in the range of 20-40%.
R. cassidix is endemic to Sulawesi and offshore islands of Lembeh, Togian Islands, Muna and Butung, Indonesia (Kemp and Boesman 2020). The species is locally very common, particularly in areas with a high density of fig trees, though the distribution is increasingly patchy (Kemp and Boesman 2020).
R. cassidix occupies evergreen forest up to 1,800 m elevation, but primarily occupies lowland forests below 1,100 m (Kemp and Boesman 2020). The species also will travel locally during the non-breeding season and extend into patches of secondary forest, woodlands, and plantations, moving in flocks of up to 50 birds and covering up to 30-50 km2 (Kemp and Boesman 2020). Intact mature forest is of particular importance to the species, as the majority of its diet constitutes figs (Ficus spp.), and they will congregate in larger numbers and spend more time feeding in stands with a higher density of feed trees (Suryadi et al. 1994, Kinnaird and O'Brien 2007). The species nests in naturally forming tree cavities that occur in larger mature/emergent trees (Cahill 2003).
R. cassidix is primarily threatened from habitat loss caused by a combination of agricultural and unsustainable logging activities, as well as gold-mining and fire impacts (Kemp and Boesman 2020), as well as hunting for meat and the illegal trade of ivory (O’Brien and Kinnaird 1996, Kemp and Boesman 2020, Prawiradilaga 2020). The species is particularly prevalent in lowland evergreen forest in areas with high densities of fig trees (Kemp and Boesman 2020), habitat that has been extensively cleated in the late 19th century (FWI/GFW 2002, Whitten et al. 2002). An estimated 89% loss of lowland forest was lost in the region between 1985 and 1997 (FWI/GFW 2002, Whitten et al. 2002), and forest loss is predicted to continue at high rates into the future (e.g., Voigt et al. 2021).
R. cassidix also has specific ecological requirements which increase its risk of declines. The species is highly dependent on Ficus spp. for its diet (Suryadi et al. 1994), and also nests in natural tree cavities, has a high degree of nest site fidelity (Kinnaird and O’Brien 1999), and prefers nesting in tall, mature trees, with large girth, particularly large emergent trees (Cahill 2003). When impacts (i.e., selective logging or fire) remove these specific habitat features, the species is likely to be disproportionally impacted.
R. cassidix is also susceptible to fire impacts across much of its range (Aldiansyah and Saputra 2024), either from the loss of nesting and feeding sites, or direct mortality as a consequence of the female sealing themselves in the nest cavity during nesting (Cahill and Walker 2000). Fire impacts can directly reduce the nesting and recruitment success of the species (Cahill and Walker 2000) and represents an additional cumulative impact.
Conservation measures underway
CITES Appendix II. The species is common in several small reserves, as well as Dumoga-Bone National Park and Lore Lindu National Park (Kemp and Boesman 2020).
Conservation measures proposed
Further surveys are needed to ascertain the population size and the distribution of the population across its range. Additionally, an understanding of the species movements and population responses to habitat disturbance are needed. Further research is needed to understand the numbers at which the species is hunted. Key feeding and nesting sites needs to be protected. Improved fire management across the Island is necessary, particularly outside of reserves where the species is most susceptible to fire (Aldiansyah et al. 2024). Research into the causes of mortality after fledging will also aid in identifying conservation priorities (Kinnaird and O.Brien 1999).
70–80 cm; male 2360–2500 g. Large hornbill with white tail and high casque. Male crown and nape deep rufous, head sides and neck paler rufous to creamy, body and wings black; bill yellow, ridged base orange and brown, wrinkled casque red ; bare skin around eye pale blue; extensive bare throat skin light and dark blue and black. Female smaller, with head and neck black, smaller casque yellow, throat skin with smaller black band. Juvenile like adult male, but small casqueless bill plain yellow. Sounds: A repeated, short gruff bark “grrok”, sometimes interspersed with more subdued croaking notes.
Text account compilers
Richardson, L.
Contributors
Lambert, F., Mulyawati, D., O'Brien, T., Holmes, D., Kinnaird, M., Butchart, S., Derhé, M., Westrip, J.R.S., Ekstrom, J. & Gilroy, J.
Recommended citation
BirdLife International (2024) Species factsheet: Knobbed Hornbill Rhyticeros cassidix. Downloaded from
https://datazone.birdlife.org/species/factsheet/knobbed-hornbill-rhyticeros-cassidix on 23/12/2024.
Recommended citation for factsheets for more than one species: BirdLife International (2024) IUCN Red List for birds. Downloaded from
https://datazone.birdlife.org/species/search on 23/12/2024.