Sumba Hornbill (Rhyticeros everetti): revise global status?

BirdLife species factsheet for Sumba Hornbill

Sumba Hornbill (Rhyticeros everetti) is endemic to the island of Sumba in Indonesia. It inhabits low-altitude semi-evergreen forest containing large trees with a dense canopy, but is occasionally also found at forest edges, in isolated trees or in parkland away from closed forest. Sumba Hornbill is however rare or absent in small forest fragments, though it may use them as stepping stones for movement (Sitompul et al. 2004). The species depends on large deciduous trees, especially Tetrameles nudiflora, for the creation of nest cavities in trunks.

Due to its restriction to large patches of closed-canopy forests, the main threat to Sumba Hornbill is the loss and degradation of its habitat through small-scale logging, firewood collection, clearance for agricultural cultivations and pastures and uncontrolled burning. The species is further threatened by trapping for the wildlife trade, thought this appears to have declined in recent years (D. Mulayawati in litt. 2010, M. Kinnaird in litt. 2016, Kemp and Boesman 2020).

The global population size is likely small. It had been hypothesised that the species is locally common, numbering well over 4,000 individuals (D. Muyawati in litt. 2010, Burung Indonesia in litt. 2011), but this number is now considered to be an overestimate. Based on density estimates in forest fragments of different size, the population was estimated to number 1,650 mature individuals in 2010, having declined from c. 3,900 mature individuals in 1995 (O’Brien et al. 1998, T. O’Brien in litt. 2010).

Sumba Hornbill is currently listed as Vulnerable under Criteria A2cd+3cd+4cd; B1ab(ii,iii,iv,v); C1+2a(ii) (BirdLife International 2020). However, concerns raised by the IUCN SSC Hornbill Specialist Group over the population size and rate of decline suggest that the species’s status is worse than previously believed. Therefore, Sumba Hornbill is here re-assessed against all Red List Criteria: 

Criterion A – Estimates of the population trend are complicated due to a lack of recent data on the population size and uncertainty surrounding previous estimates. Sumba Hornbill has a generation length of 8.4 years (Bird et al. 2020)*, hence 25.2 years (three generations) is the appropriate period over which to consider population reduction under this criterion.

Available data suggests that the species has declined from c. 3,900 mature individuals in 1995 (O’Brien et al. 1998) to c. 1,650 mature individuals in 2010 (T. O’Brien in litt. 2010). Assuming that declines are exponential and continuing at the same rate to the present day and into the future, this equates to a decline of 76.7% over three generations. Sumba Hornbill would consequently qualify for listing as Endangered under Criterion A2acd+3cd+4acd.

Criterion B – The Extent of Occurrence (EOO) for this species is 13,100 km2. The Area of Occupancy (AOO) has not been quantified following IUCN Guidelines (see IUCN Standards and Petitions Committee 2019). The EOO thus meets the threshold for listing as Vulnerable under Criterion B1, but the species cannot be assessed against Criterion B2. In order to be listed as threatened under Criterion B1, further conditions have to be met.

Sumba Hornbill is distributed across the island of Sumba, and all individuals are thought to belong to the same subpopulation. The species is therefore not severely fragmented sensu IUCN (IUCN Standards and Petitions Committee 2019). With the most severe threat being habitat loss through forest logging, the number of locations** of occurrence is ≤10, and condition a is met. Further, due to moderately rapid rates of habitat loss, the species is undergoing continuing declines in area of occupancy, area and qualify of habitat and population size; hence condition b(ii,iii,v) is met. There is no evidence that the habitat availability or population size are undergoing extreme fluctuation, and condition c is not met. Overall, Sumba Hornbill meets the threshold for listing as Vulnerable under Criterion B1ab(ii,iii,v).

Criterion C – Assuming an exponential decline from 3,900 mature individuals in 1995 to 1,650 mature individuals in 2010, the population of Sumba Hornbill may currently number around 900-1,000 mature individuals. This meets the threshold for listing as Endangered under Criterion C. However to do so further conditions have to be met.

The species is estimated to be declining at a rate of 76.7% over three generations. This meets subcriterion 1 at the level of at least Endangered. Furthermore, the species is thought to form one single subpopulation, and subcriterion 2a(ii) is met. There is no evidence of extreme fluctuations in the population size, and subcriterion 2b is not met. Hence, Sumba Hornbill qualifies for listing as Endangered under Criterion C1+2a(ii).

Criterion D – Assuming an exponential decline, the population of Sumba Hornbill would currently number roughly 900-1,000 mature individuals. This meets the threshold for listing as Vulnerable under Criterion D1. Further, the species is thought to be restricted to 6-10 locations**, and it does additionally warrant a listing as Near Threatened, approaching the threshold for listing as threatened under Criterion D2.

Criterion E – To the best of our knowledge no quantitative analysis of extinction risk has been conducted for this species. Therefore, it cannot be assessed against this criterion.

Therefore, it is proposed that Sumba Hornbill (Rhyticeros everetti)be listed as Endangered under Criteria A2acd+3cd+4acd; C1+2a(ii). We welcome any comments on this proposed listing and specifically request up-to-date information on the population size and trend. Newly available population size estimates from 1995 and 2010 allow us to investigate the timeframe when the species crossed the threshold for Endangered. Assuming an exponential decline from 3,900 mature individuals in 1995 to 1,650 mature individuals in 2010, the population size of Sumba Hornbill fell below 2,500 mature individuals in the year 2004, qualifying it for listing as Endangered during the period 2000-2004.

*Bird generation lengths are estimated using the methodology of Bird et al. (2020), as applied to parameter values updated for use in each IUCN Red List for birds reassessment cycle. Values used for the current assessment are available on request. We encourage people to contact us with additional or improved values for the following parameters; adult survival (true survival accounting for dispersal derived from an apparently stable population); mean age at first breeding; and maximum longevity (i.e. the biological maximum, hence values from captive individuals are acceptable).

*The term ‘location’ refers to a distinct area in which a single threatening event can rapidly affect all individuals of the taxon present, with the size of the location depending on the area covered by the threatening event. Where a taxon is affected by more than one threatening event, location should be defined by considering the most serious plausible threat (IUCN 2001, 2012).

Please note that this topic is not designed to be a general discussion about the ecology of the species, rather a discussion of its Red List status. Therefore, please make sure your comments are relevant to the discussion outlined in the topic. By submitting a comment, you confirm that you agree to the Comment Policy.

An information booklet on the Red List Categories and Criteria can be downloaded here and the Red List Criteria Summary Sheet can be downloaded here. Detailed guidance on IUCN Red List terms and definitions and the application of the Red List Categories and Criteria can be downloaded here.


Bird, J. P.; Martin, R.; Akçakaya, H. R.; Gilroy, J.; Burfield, I. J.; Garnett, S.; Symes, A.; Taylor, J.; Şekercioğlu, Ç. H.; Butchart, S. H. 2020. Generation lengths of the world’s birds and their implications for extinction risk. Conservation Biology online first view.

BirdLife International. 2020. Species factsheet: Rhyticeros everetti. (Accessed 20 May 2020).

IUCN Standards and Petitions Committee. 2019. Guidelines for using the IUCN Red List Categories and Criteria. Version 14.

Kemp, A. C.; Boesman, P. F. D. 2020. Sumba Hornbill (Rhyticeros everetti), version 1.0. In: del Hoyo, J.; Elliott, A.; Sargatal, J.; Christie, D. A.; de Juana, E. (eds.). Birds of the World. Cornell Lab of Ornithology, Ithaca, NY, USA. (Accessed 20 May 2020).

O’Brien, T. G.; Kinnaird, M. F.; Jepson, P.; Setiawan, I. 1998. Effect of forest size and structure on the distribution of Sumba Wreathed hornbills Aceros everetii. In: Poonswad, P. (ed.) The Asian hornbill: ecology and conservation. Thai studies in biodiversity No. 2. Bangkok, Thailand.

Sitompul, A. F.; Kinnaird, M. F.; O’Brien, T. G. 2004. Size matters: the effects of forest fragmentation and resource availability on the endemic Sumba Hornbill Aceros everetti. Bird Conservation International 14: S23-S37.

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5 Responses to Sumba Hornbill (Rhyticeros everetti): revise global status?

  1. Tim O'Brien says:

    Area of occupancy (AOO) for Sumba hornbill cannot exceed area of forest cover. I have used Global Forest Watch to calculate potential habitat. Using a canopy cover threshold of 30%, we have 1332 km2 of forest cover as of 2017. This is an estimated loss of 23% compared to 1998 and 10% of the Extent of occurrence (EOO). Annual tree cover loss averages 820.5 hectares per year.

  2. Tim O'Brien says:

    Following my previous comment, here are the annual deforestation rates. 2001 – 313 ha; 2002 – 820 ha; 2003 – 1023 ha; 2004 – 388 ha; 2005 – 637 ha, 2006 – 850 ha; 2007 – 464 ha; 2008 – 658 ha; 2009 – 543 ha, 2010 – 491 ha; 2011 – 977 ha; 2012 – 826 ha; 2013 – 336 ha; 2014 – 1766 ha; 2015 – 1826 ha; 2017 – 713 ha; 2017 – 1318 ha.

  3. Hi,
    Apart from suffering from trapping as a food source, to supply the illegal trade and forest loss this species is also suffering from lack of suitable nests. The competition about the remaining nesting trees is intense between this species, the Eclectus Parrots (Eclectus roratus cornelia), Citron-crested Cockatoos (Cacatua sulphurea citrinocristata) and Great-billed Parrots (Tanygnathus megalorynchos sumbensis). This is a fairly big problem for the wild population taken into consideration the decrease of suitable habitat in the whole of the island. What remains on the island is small fragments of the patches of forests that use to support the birds of Sumba. many species of birds adapt well to the changes but the species mentioned above depends of forests with large trees maybe not much for foodstuff but necessary for breeding.

  4. Red List Team (BirdLife International) says:

    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. We will analyse and interpret the new information and post a preliminary decision on this species’s Red List status on this page in early July.

    Thank you once again,
    BirdLife Red List Team

  5. Red List Team (BirdLife International) says:

    Preliminary proposal

    Based on available information, our preliminary proposal for the 2020 Red List would be to adopt the proposed classifications outlined in the initial forum discussion.

    There is now a period for further comments until the final deadline in mid-July, after which the recommended categorisations will be put forward to IUCN.

    Please note that we will then only post final recommended categorisations on forum discussions where these differ from the initial proposal.

    The final 2020 Red List categories will be published on the BirdLife and IUCN websites in December 2020/January 2021 (information on the IUCN Red List update process can be found here), following further checking of information relevant to the assessments by both BirdLife and IUCN.

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