Tickell’s Brown Hornbill (Anorrhinus tickelli): request for information.

Tickell’s Brown Hornbill (Anorrhinus tickelli) is predominantly found in dense forest of southern Myanmar and adjacent Thailand up to 1,500m (Kemp and Boesman 2017). As with many hornbill species in this region deforestation and hunting may be leading to population declines (see Kinnaird and O’Brien 2007; see also this additional topic), and the species is currently assessed as Near Threatened as a result of these threats (BirdLife International 2017).

The species’ population size has been suggested to be potentially small, yet there remains no current estimate. It had been considered to be more common on the Thai side of its range, but a recent paper by Trisurat et al. (2013) has suggested that the species may now be highly range-restricted in Thailand, and so the population size there may in fact be quite small. Therefore, given this information and using the new information regarding forest loss from Tracewski et al. (2016) (see here for further info), this species will be re-assessed against all criteria.


Criterion A – Tracewski et al. (2016) measured the forest loss within this species’ range between 2000 and 2012 as c.2,790km2. This roughly equates to a rate of forest loss of 8.4% over 3 generations (33.3 years) for this species, with the assumption that population changes may be proportional to forest change. Therefore, unless hunting may be considered to account for >15% declines over 3 generations, this species is unlikely to approach the threshold for Vulnerable under criterion A.


Criterion B – The Extent of Occurrence (EOO) of this species, calculated using a Minimum Convex Polygon (see IUCN 2001, 2012, Joppa et al. 2016) is 212,000km2. This far exceeds the threshold for Vulnerable under criterion B1.

Trisurat et al. (2013) estimated the extent of suitable habitat for this species in Thailand to be 1,864km2. While Trisurat et al. referred to this as an extent of occurrence, this actually would at best represent a maximum Area of Occupancy (AOO) value instead, because it does not meet the IUCN guidelines for classification as an EOO (see IUCN Standards and Petitions Subcommittee 2017). Tracewski et al. (2016) measured the amount of available forest (as a proxy for the Area of Occupancy) across the species’ whole range as c.86,450km2. It should be stressed that this is a maximum potential AOO, especially given the value of Trisurat et al. (2013), and so the species’ AOO is likely less than the value of Tracewski et al. (2016). However, given that the range value of Trisurat et al. for Thailand alone is so close the global threshold for Vulnerable under criterion B2 (2,000km2) it is very unlikely that the global AOO value would remain beneath this threshold. Therefore, the species likely does not warrant listing under this criterion.


Criterion C – Taking population density estimates for A. tickelli and closely related species, and assuming only a proportion of its range (using the max AOO value of Tracewski et al. [2016]) is occupied would give a population size of c. 23,000 mature individuals. However, this could still be far too large a population size estimate given the information from Trisurat et al. (2013). Using their max AOO value instead would give a population size of c.3,700 mature individuals in Thailand, and while the species’ AOO in Myanmar has not been measured, based on this figure the appropriate population size range is likely 2,500-9,999 mature individuals.

To qualify as Vulnerable, the species would still need to meet certain conditions. The species would not qualify as Vulnerable under criterion C1 because the rate of decline of this species is uncertain (a high level of confidence is required for this criterion). Also the population is likely fragmented, occurring in multiple sub-populations and is not known to undergo extreme fluctuations. Therefore, it would not qualify for Vulnerable under criteria C2a(ii) and C2b.

This leaves criterion C2a(i). To be listed as Vulnerable under this criterion would require the largest sub-population to contain ≤1,000 mature individuals, and given the already high level of uncertainty regarding population estimates it is uncertain whether any sub-population is this small. Therefore, information is urgently sought regarding both the overall population size, and sub-population sizes to see whether this species warrants listing as Vulnerable under this criterion.


Criterion D – Even though the population size is difficult to estimate clearly, it is still suspected to be >1,000 mature individuals, and the species is not limited to a very small number of ‘locations’ (see IUCN Standards and Petitions Subcommittee 2017 for IUCN definition of a ‘location’) or very small AOO. Therefore, it does not warrant listing under this criterion.


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


Therefore, it appears that the only criterion where the species may now approach or meet the threshold for Vulnerable is C2a(i). To better assess this species against this criterion, we therefore urgently request any information regarding the global population size, and sub-population sizes.


Please note that this topic is not designed to be a general discussion about the ecology of the species, rather a discussion of the species’ Red List status. Therefore, please make sure your comments are relevant to the information that is sought, or about the species’ Red List status.



BirdLife International. 2017. Species factsheet: Anorrhinus tickelli. Downloaded from http://www.birdlife.org on 14/11/2017.

IUCN. 2001. IUCN Red List Categories and Criteria: Version 3.1. IUCN Species Survival Commission. IUCN, Gland, Switzerland and Cambridge, U.K.

IUCN. 2012. IUCN Red List Categories and Criteria: Version 3.1. Second edition. IUCN Species Survival Commission. IUCN, Gland, Switzerland and Cambridge, U.K. Available at: www.iucnredlist.org/technical-documents/categories-and-criteria.

IUCN Standards and Petitions Subcommittee. 2017. Guidelines for Using the IUCN Red List Categories and Criteria. Version 13. Prepared by the Standards and Petitions Subcommittee. Downloadable from http://www.iucnredlist.org/documents/RedListGuidelines.pdf.

Joppa, L. N.; Butchart, S. H. M.; Hoffmann, M.; Bachman, S. P.; Akçakaya, H. R.; Moat, J. F.; Böhm, M.; Holland, R. A.; Newton, A.; Polidoro, B.; Hughes, A. 2016. Impact of alternative metrics on estimates of extent of occurrence for extinction risk assessment. Conservation Biology 30: 362-370.

Kemp, A. C.; Boesman, P. 2017. Tickell’s Brown Hornbill (Anorrhinus tickelli). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/55883 on 14 November 2017).

Kinnaird, M. F.; O’Brien, T. G. 2007. The Ecology & Conservation of Asian Hornbills. Farmers of the Forest. The University of Chicago Press, Chicago.

Tracewski, Ł.; Butchart, S. H. M.; Di Marco, M.; Ficetola, G. F.; Rondinini, C.; Symes, A.; Wheatley, H.; Beresford, A. E.; Buchanan, G. M. 2016. Toward quantification of the impact of 21st-century deforestation on the extinction risk of terrestrial vertebrates. Conservation Biology 30: 1070-1079.

Trisurat, Y.; Chimchome, V.; Pattanavibool, A.; Jinamoy, S.; Thongaree, S.; Kanchanasakha, B.; Simcharoen, S.; Sribuarod, K.; Mahannop, N.; Poonswad, P. 2013. An assessment of the distribution and conservation status of hornbill species in Thailand. Oryx 47(3): 441-450.

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