Data Zone
Justification of Red List category
This species qualifies as Vulnerable because it is restricted to low-lying forest in a region where this habitat-type is being cleared and degraded at such a rate that rapid and continuing population declines are suspected.
Population justification
The population size of this species has not been quantified, however this species is considered a specialist of stunted forest types (Sheldon 1987), the area of which is difficult to map. Within suitable habitat, it may be regularly observed and can be quite common (Posa and Marques 2012, RER 2017, Eaton et al. 2021, eBird 2022) but the area of available habitat has greatly diminished over the past 20 years, such that the population size, although unquantified here, is likely to be much smaller than three generations ago. On Sumatra, the area of suitable habitat is small and greatly fragmented, with recent records from only a handful of sites on the island's east; here, however, it is always thought to have been rare (see review by van Marle and Voous 1988). On Borneo, the area of habitat is much larger and records (Mann 2008, eBird 2022) suggest it remains widespread.
Trend justification
Setornis criniger is thought to be declining rapidly in response to extensive forest cover loss across its range. In the absence of population data, forest cover loss is used as a proxy to estimate the rate of reduction over the past three generations (10.5 years; Bird et al. 2020), however doing this accurately relies on robust knowledge of the species' distribution when forest cover was more extensive. There is, however, much uncertainty on how widespread it was on Sumatra historically, and its elevational limits on Borneo (where it has on occasion been recorded in kerangas forest up to 1,000 m [Mann 2008]). Under a range of mapped scenarios, forest cover loss in the species' mapped range is thought to have reduced by 19-42% over the last three generations (Global Forest Watch 2022, based on data from Hansen et al. [2013] and methods disclosed therein). Precautionarily, towards the upper limit of these losses (30-40%) is accepted here given the species confinement to specialised habitat. This rate of loss is also suspected to occur into the future, with comparatively little of the species' range circumscribed by protected areas (UNEP-WCMC 2022), and most of it accessible for exploitation.
Setornis criniger is confined to Borneo (including Sabah and Sarawak, Malaysia, Brunei, and Kalimantan, Indonesia) and the Indonesian islands of Sumatra and, historically, Bangka (BirdLife International 2001), although there is probably no suitable habitat left on the latter.
It is strongly associated with nutrient-poor vegetation on acid soils (Sheldon 1987). These include peatswamp (lowland evergreen forest characterised by low tree species diversity and strong adaptation to a fluctuating water-table and heath forest (kerangas, dense, low forest of thin-boled, small-leaved and often sclerophyllous trees). It has also been recorded in abandoned rubber plantations, ridge-top heath forest (to 1,000 m), sometimes tolerating secondary forest, but generally avoiding dryland primary forest.
Forest destruction in the Sundaic lowlands of Indonesia and Malaysia has been extensive, principally for timber and conversion to agriculture and, more locally, for urban expansion. In particular the rapid expansion of oil palm and rubber has driven the conversion of the majority of remaining lowland forest in the region. Between 2010 and 2021, tree cover was reduced in its range by 19–42% (Global Forest Watch 2022, based on data from Hansen et al. [2013] and methods disclosed therein). Moreover, the impact of forest degradation will have added to rates of decline. Associated with the forest loss has been an increase in fire frequency, extent and severity, particularly during strong El Nino events (as in 1998).
Conservation Actions Underway
This species occurs in a handful of protected areas on Borneo, in all range states, however the majority (>80%) of suitable habitat remains unprotected (UNEP-WCMC and IUCN 2023). Occurs in the Kampar Peninsula, which is subject to protection and monitoring (see RER 2017).
19-20 cm. Drab, forest-dwelling bulbul. Warm brown crown, rest of upperparts mid-brown. Distinctive face pattern with whitish supercilium, blackish eye-stripe and moustachial divided by greyish cheeks. Pale whitish-buff underparts, greyer on flanks. Brown outer tail feathers broadly tipped white. Strong and hook-tipped bill. Similar spp. Puff-backed Bulbul Pycnonotus eutilotus also has white-tipped tail feathers, but is warmer brown, usually has visible crest and red eyes, and lacks supercilium and eye-stripe. Voice Relatively quiet, but gives loud, harsh crruk.
Text account compilers
Berryman, A.
Contributors
Davidson, G., Lambert, F., van Balen, B.S., Benstead, P., Gilroy, J. & Taylor, J.
Recommended citation
BirdLife International (2024) Species factsheet: Hook-billed Bulbul Setornis criniger. Downloaded from
https://datazone.birdlife.org/species/factsheet/hook-billed-bulbul-setornis-criniger on 23/11/2024.
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 23/11/2024.