6 thoughts on “Sulawesi Hornbill (Rhabdotorrhinus exarhatus): Revise global status?”
This proposed change may well be correct, based on the assessed criteria, but it is an important decision and as such probably bears further discussion, as there are some aspects of the proposal that give cause for concern.
Generally, there has been a recent trend of several Sulawesian species being reassessed and assigned a lower threat level (with some major exceptions like Maleo going the other way) due to research showing they are either more widespread or more tolerant of forest disturbance than previously thought. I agree with those assessments, but this species does not really fall into either of those ‘lower-risk-than-thought’ categories.
It is a difficult decision given there is not much data specifically published on this species. We do know, however, that it is dependent on mature, closed canopy forest (O’Brien & Kinnaird 1996) which is associated with its dependence on large trees for nesting (Winarni & Jones 1996), as acknowledged in the proposed account. It is also known to be primarily a lowland forest species, with a core altitudinal range of <650m.
A key point of this reassessment is focussed on an improved knowledge of the species’ generation time, which is fair enough – this is solid, quantitative data that relates to assessment criteria.
My concerns focus more on the species’ habitat associations, as described above, and potential mismatches with the area of occurrences given in this account.
So the salient concern I would have is that high quality lowland forest <650m (i.e. the core habitat of the species) is an increasingly scarce habitat on Sulawesi. The map (and indeed some of the text) in the proposed account gives the impression that the species probably occurs across much of Sulawesi, barring some of the most developed areas of South Sulawesi Province and high altitude areas. However, given knowledge of the species ecology, this is almost certainly not the case. A cursory examination of a google satellite image of Sulawesi quickly indicates that many of the areas mapped do not support high quality forest (especially in South and South-eastern Sulawesi, and coastal parts of Western Sulawesi). This is more helpfully quantified in Cannon et al. (2007), who even 15 years ago showed that most good quality forest in Sulawesi remained in higher altitude areas, with comparative low cover of high quality forest cover in lowlands (especially in South Sulawesi, where there is almost none). Voigt et al. (2021) also provide a map of closed canopy forest. I think an adapted GIS map showing all forested areas indicated by Voigt et al. (2021) or, more conservatively, Cannon et al. (2007) filtered to also give only those areas <650m, would show how potentially limited this species habitat really is. Voigt et al. (2021) also raise serious concerns about the fate of these remaining forested habitats in the coming decades, especially within lowland Key Biodiversity Areas (of which this species is present in several). They also raise concerns about growing threats of forest fragmentation, which are likely to impact this species disproportionately.
So, the species in likelihood has a much more limited habitat than current range maps suggest, and almost certainly a smaller EOO than indicated. The EOO for this species (and indeed all Sulawesi endemics I have looked at), appears strange, incidentally. For the species, the EOO is given as 415,000 km2. This is considerably more than twice the size of the entire island of Sulawesi (180,000 km2) and so cannot possibly be correct even taking into account this species habitat associations (unless there is some metric to these EOO estimates that I am unaware of, like accounting for terrain rugosity, which I guess is possible?).
Population estimates may also be a somewhat misleading in this account. Its complete true that no formal population estimates of the species yet exist, so there is an element of guesswork involved here and best available sources must be used, at any rate these estimates/descriptive sources are not binding in any way. However, the statement “the species is generally described as not uncommon (Eaton 2021) is very coarse and potentially misleading, and there is no data on what such an assessment is based on. Published literature indicates the species may occur at different abundances in different parts of its range. Martin et al. (2012) indicate it to be a fairly common species in Lambusango Forest Reserve, Buton Island (a vast tract of lowland forest highly suitable for the species). O’Brien & Kinnaird (1996), meanwhile, considered it a rare species in Northern Sulawesi. There is no data at all on its abundance from most parts of its range, including smaller forest patches with strong edge effects and accessibility for logging, and where bird watchers and researchers seldom visit. It could actually be rare throught most of its range – we just don’t know.
It is also perhaps worth providing a quick comparison with the better-studied Knobbed Hornbill (Rhyticeros cassidix), which is also currently classed as Vulnerable, with no proposal to change this status. It is true that Knobbed Hornbills may face higher hunting pressure in parts of their range than Sulawesi Hornbills (O’Brien & Kinnaird 1996) (although hunting pressures on both species remain poorly explored), and Knobbed Hornbills also seem to be somewhat less tolerant of anthropogenic disturbance (Winarni & Jones 1996). However, Knobbed Hornbill has a core habitat zone extending at least to 1100m (nearly twice that of Sulawesi Hornbill) and that difference encompasses a lot more “good forest” habitat as shown in maps by Cannon et al. (2007) and Voigt et al. (2021). So it might be less pressured by habitat loss overall despite its higher intrinstic vulnerabilites.
As at the start, I do not say the proposed assessement to reclassify this species is wrong, but I hope the concerns raise above are useful for further discussion on this proposal. The species’ ecology and strong associations with increasingly rare lowland forest ecosystems make it sensitive, so some care is needed here. It would be good to hear if any other researchers had similar concerns to myself.
O’Brien, T. & Kinnaird, M.F. (1996). Changing populations of birds and mammals in North Sulawesi. Oryx 30, 150-156.
Cannon, C.H., Summers, M., Harting, J.R. & Kessler, P.J.A. (2007). Developing Conservation Priorities Based on Forest Type, Condition, and Threats in a Poorly Known Ecoregion: Sulawesi, Indonesia. Biotropica 39, 747–759.
Martin, T.E., Blackburn, G.A., Kelly, D.J., Heriyadi, D. & Singer, H. (2012). The avifauna of the Lambusango Forest Reserve, Buton Island, South-East Sulawesi (with additional sightings from Southern Buton). Forktail 28: 107-112.
Winarni, N.L. & Jones, M. (2012). Effect of anthropogenic disturbance on the abundance and habitat occupancy of two endemic hornbill species in Buton island, Sulawesi. Bird Conservation International 22: 222-233.
Voigt, M., Supriatna, J., Deere, N.J., Kastanya, A., Mitchell, S.L., D Rosa, I.M., Santika, T., Siregar, R., Tasirin, J.S., Widyanto, A., Winarni, N.L., Zakaria, Z., Mumbunan, S., Davies, Z.G. & Struebig, M.J. (2021). Emerging threats from deforestation and forest fragmentation
in the Wallacea centre of endemism. Environmental Research Letters 16, 094048.
Also, as a factual correction, after checking with a friend, and caveats about potential region differences given above not withstanding, the second edition of Eaton et al. actually says this species is “uncommon”, not “not uncommon”. An important distinction.
Many thanks to everyone who has contributed to this discussion. We greatly appreciate the time and effort invested by so many people in commenting. The window for consultation is now closed and we are unable to accept any more comments until 21 February 2022. We will now analyse and interpret the new information, and we will post a preliminary decision on this species’s Red List status on this page on 21 February 2022, when discussions will re-open.
Upon investigating modelled input parameters for calculating the species’s generation length, the mean age at first breeding was strongly suspected to be deficient at two years. The view of the IUCN SSC Hornbill Specialist Group in previous discussions is that this value is very unlikely to be below three years for Asian forest hornbills. Recalculating the generation length using a mean age at first breeding of three years results in a revised generation length for Rhabdotorrhinus exarhatus of 6.86 years, rounded to 7 years.
Using this revised generation length and restricting this species to a lower elevation of below 600 m (where the highest densities are assumed to occur) reveals suspected losses of c.30% over three generations, based on forest loss alone (likely to be greater due to fragmentation, selective logging and some localised hunting). These rates are predicted to continue or accelerate in the future.
The extent of occurrence has indicated in the original forum topic is correct. Following IUCN Guidelines, EOO values are calculated drawing a minimum convex polygon around the species’s range. In many instances, especially islands with complex shapes containing peninsulas etc. such as Sulawesi, the EOO value generated can therefore be larger (occasionally much) than the total area of the island itself.
Based on the available information therefore, our preliminary proposal for the 2022.1 Red List would be to list Sulawesi Hornbill as Vulnerable under Criteria A2cd+3cd+4cd.
There is now a period for further comments until the final deadline on 27 February 2022, after which the recommended categorisations will be put forward to IUCN.
The final 2022.1 Red List categories will be published on the BirdLife and IUCN websites in July 2022, following further checking of information relevant to the assessments by both BirdLife and IUCN.
Many thanks to everyone who has contributed to this discussion. We greatly appreciate the time and effort invested by so many people in commenting. The window for consultation is now closed and we are unable to accept any more comments. We will analyse and interpret the new information, and we will post a final decision on this species’ Red List status on this page on 7 March 2022.
Recommended categorisation to be put forward to IUCN
The final categorisation for this species has not changed. Sulawesi Hornbill is recommended to be listed as Vulnerable under Criteria A2cd+3cd+4cd.
Many thanks for everyone who contributed to the 2022.1 GTB Forum process. The final 2022.1 Red List categories will be published on the BirdLife and IUCN websites in July 2022, following further checking of information relevant to the assessments by both BirdLife and IUCN.
This proposed change may well be correct, based on the assessed criteria, but it is an important decision and as such probably bears further discussion, as there are some aspects of the proposal that give cause for concern.
Generally, there has been a recent trend of several Sulawesian species being reassessed and assigned a lower threat level (with some major exceptions like Maleo going the other way) due to research showing they are either more widespread or more tolerant of forest disturbance than previously thought. I agree with those assessments, but this species does not really fall into either of those ‘lower-risk-than-thought’ categories.
It is a difficult decision given there is not much data specifically published on this species. We do know, however, that it is dependent on mature, closed canopy forest (O’Brien & Kinnaird 1996) which is associated with its dependence on large trees for nesting (Winarni & Jones 1996), as acknowledged in the proposed account. It is also known to be primarily a lowland forest species, with a core altitudinal range of <650m.
A key point of this reassessment is focussed on an improved knowledge of the species’ generation time, which is fair enough – this is solid, quantitative data that relates to assessment criteria.
My concerns focus more on the species’ habitat associations, as described above, and potential mismatches with the area of occurrences given in this account.
So the salient concern I would have is that high quality lowland forest <650m (i.e. the core habitat of the species) is an increasingly scarce habitat on Sulawesi. The map (and indeed some of the text) in the proposed account gives the impression that the species probably occurs across much of Sulawesi, barring some of the most developed areas of South Sulawesi Province and high altitude areas. However, given knowledge of the species ecology, this is almost certainly not the case. A cursory examination of a google satellite image of Sulawesi quickly indicates that many of the areas mapped do not support high quality forest (especially in South and South-eastern Sulawesi, and coastal parts of Western Sulawesi). This is more helpfully quantified in Cannon et al. (2007), who even 15 years ago showed that most good quality forest in Sulawesi remained in higher altitude areas, with comparative low cover of high quality forest cover in lowlands (especially in South Sulawesi, where there is almost none). Voigt et al. (2021) also provide a map of closed canopy forest. I think an adapted GIS map showing all forested areas indicated by Voigt et al. (2021) or, more conservatively, Cannon et al. (2007) filtered to also give only those areas <650m, would show how potentially limited this species habitat really is. Voigt et al. (2021) also raise serious concerns about the fate of these remaining forested habitats in the coming decades, especially within lowland Key Biodiversity Areas (of which this species is present in several). They also raise concerns about growing threats of forest fragmentation, which are likely to impact this species disproportionately.
So, the species in likelihood has a much more limited habitat than current range maps suggest, and almost certainly a smaller EOO than indicated. The EOO for this species (and indeed all Sulawesi endemics I have looked at), appears strange, incidentally. For the species, the EOO is given as 415,000 km2. This is considerably more than twice the size of the entire island of Sulawesi (180,000 km2) and so cannot possibly be correct even taking into account this species habitat associations (unless there is some metric to these EOO estimates that I am unaware of, like accounting for terrain rugosity, which I guess is possible?).
Population estimates may also be a somewhat misleading in this account. Its complete true that no formal population estimates of the species yet exist, so there is an element of guesswork involved here and best available sources must be used, at any rate these estimates/descriptive sources are not binding in any way. However, the statement “the species is generally described as not uncommon (Eaton 2021) is very coarse and potentially misleading, and there is no data on what such an assessment is based on. Published literature indicates the species may occur at different abundances in different parts of its range. Martin et al. (2012) indicate it to be a fairly common species in Lambusango Forest Reserve, Buton Island (a vast tract of lowland forest highly suitable for the species). O’Brien & Kinnaird (1996), meanwhile, considered it a rare species in Northern Sulawesi. There is no data at all on its abundance from most parts of its range, including smaller forest patches with strong edge effects and accessibility for logging, and where bird watchers and researchers seldom visit. It could actually be rare throught most of its range – we just don’t know.
It is also perhaps worth providing a quick comparison with the better-studied Knobbed Hornbill (Rhyticeros cassidix), which is also currently classed as Vulnerable, with no proposal to change this status. It is true that Knobbed Hornbills may face higher hunting pressure in parts of their range than Sulawesi Hornbills (O’Brien & Kinnaird 1996) (although hunting pressures on both species remain poorly explored), and Knobbed Hornbills also seem to be somewhat less tolerant of anthropogenic disturbance (Winarni & Jones 1996). However, Knobbed Hornbill has a core habitat zone extending at least to 1100m (nearly twice that of Sulawesi Hornbill) and that difference encompasses a lot more “good forest” habitat as shown in maps by Cannon et al. (2007) and Voigt et al. (2021). So it might be less pressured by habitat loss overall despite its higher intrinstic vulnerabilites.
As at the start, I do not say the proposed assessement to reclassify this species is wrong, but I hope the concerns raise above are useful for further discussion on this proposal. The species’ ecology and strong associations with increasingly rare lowland forest ecosystems make it sensitive, so some care is needed here. It would be good to hear if any other researchers had similar concerns to myself.
O’Brien, T. & Kinnaird, M.F. (1996). Changing populations of birds and mammals in North Sulawesi. Oryx 30, 150-156.
Cannon, C.H., Summers, M., Harting, J.R. & Kessler, P.J.A. (2007). Developing Conservation Priorities Based on Forest Type, Condition, and Threats in a Poorly Known Ecoregion: Sulawesi, Indonesia. Biotropica 39, 747–759.
Martin, T.E., Blackburn, G.A., Kelly, D.J., Heriyadi, D. & Singer, H. (2012). The avifauna of the Lambusango Forest Reserve, Buton Island, South-East Sulawesi (with additional sightings from Southern Buton). Forktail 28: 107-112.
Winarni, N.L. & Jones, M. (2012). Effect of anthropogenic disturbance on the abundance and habitat occupancy of two endemic hornbill species in Buton island, Sulawesi. Bird Conservation International 22: 222-233.
Voigt, M., Supriatna, J., Deere, N.J., Kastanya, A., Mitchell, S.L., D Rosa, I.M., Santika, T., Siregar, R., Tasirin, J.S., Widyanto, A., Winarni, N.L., Zakaria, Z., Mumbunan, S., Davies, Z.G. & Struebig, M.J. (2021). Emerging threats from deforestation and forest fragmentation
in the Wallacea centre of endemism. Environmental Research Letters 16, 094048.
Also, as a factual correction, after checking with a friend, and caveats about potential region differences given above not withstanding, the second edition of Eaton et al. actually says this species is “uncommon”, not “not uncommon”. An important distinction.
Many thanks to everyone who has contributed to this discussion. We greatly appreciate the time and effort invested by so many people in commenting. The window for consultation is now closed and we are unable to accept any more comments until 21 February 2022. We will now analyse and interpret the new information, and we will post a preliminary decision on this species’s Red List status on this page on 21 February 2022, when discussions will re-open.
Preliminary proposal
Upon investigating modelled input parameters for calculating the species’s generation length, the mean age at first breeding was strongly suspected to be deficient at two years. The view of the IUCN SSC Hornbill Specialist Group in previous discussions is that this value is very unlikely to be below three years for Asian forest hornbills. Recalculating the generation length using a mean age at first breeding of three years results in a revised generation length for Rhabdotorrhinus exarhatus of 6.86 years, rounded to 7 years.
Using this revised generation length and restricting this species to a lower elevation of below 600 m (where the highest densities are assumed to occur) reveals suspected losses of c.30% over three generations, based on forest loss alone (likely to be greater due to fragmentation, selective logging and some localised hunting). These rates are predicted to continue or accelerate in the future.
The extent of occurrence has indicated in the original forum topic is correct. Following IUCN Guidelines, EOO values are calculated drawing a minimum convex polygon around the species’s range. In many instances, especially islands with complex shapes containing peninsulas etc. such as Sulawesi, the EOO value generated can therefore be larger (occasionally much) than the total area of the island itself.
Based on the available information therefore, our preliminary proposal for the 2022.1 Red List would be to list Sulawesi Hornbill as Vulnerable under Criteria A2cd+3cd+4cd.
There is now a period for further comments until the final deadline on 27 February 2022, after which the recommended categorisations will be put forward to IUCN.
The final 2022.1 Red List categories will be published on the BirdLife and IUCN websites in July 2022, following further checking of information relevant to the assessments by both BirdLife and IUCN.
Many thanks to everyone who has contributed to this discussion. We greatly appreciate the time and effort invested by so many people in commenting. The window for consultation is now closed and we are unable to accept any more comments. We will analyse and interpret the new information, and we will post a final decision on this species’ Red List status on this page on 7 March 2022.
Recommended categorisation to be put forward to IUCN
The final categorisation for this species has not changed. Sulawesi Hornbill is recommended to be listed as Vulnerable under Criteria A2cd+3cd+4cd.
Many thanks for everyone who contributed to the 2022.1 GTB Forum process. The final 2022.1 Red List categories will be published on the BirdLife and IUCN websites in July 2022, following further checking of information relevant to the assessments by both BirdLife and IUCN.