BirdLife species factsheet for Aceh Bulbul
The Aceh Bulbul is a treeline songbird that has become increasingly rare in the Aceh region of north-western Sumatra, Indonesia (del Hoyo et al., 2020; F. Rheindt in litt., 2020). Whilst previously the species was considered to occur across much of northern Aceh, it is now considered to only occur in the alpine area surrounding Mount Leuser, located in the Gunung Leuser National Park (total range c. 9,500 km2) (Global Forest Watch, 2020; F. Rheindt in litt., 2020). However, the species has not seen recent records here, although is thought to be common. The species may also occur north of Perlak, Aceh, and extend to the greater Leuser Ecosystem and south to Mount Sinabung (eBird, 2020). However, these observations remain uncertain and could suggest only sporadic occurrence of the species, not representing its true range (F. Rheindt in litt., 2020). Thus, it is assumed that the species range has been overestimated and is instead highly restricted, likely to only occur in one site. The species prefers scrubby clearings within submontane forests (del Hoyo et al. ,2020).
The species is heavily threatened by trapping for the songbird trade. In past studies for example, only low numbers of the species were recorded in Sumatran bird markets (Eaton and Collar, 2015). In recent years, this scarcity has remained, with only four Aceh Bulbuls observed in 2018 in a market in Takengon (Leupen and Gomez, 2019). The Aceh Bulbul may also be sold locally and therefore remain unrecorded in most high-selling bird markets (Eaton and Collar, 2015). However, the absence of trade records and its scarcity in markets is not thought to be the consequence of not being traded, rather the genuine rarity of the species leading to difficulty in locating and further trapping (Eaton and Collar 2015; Leupen and Gomez, 2019). It is therefore estimated that there are likely 250 (or more) mature individuals left (F. Rheindt in litt., 2020). To account for any uncertainty, it is tentatively placed here in the band of 250-999 mature individuals.
The Aceh Bulbul is currently listed as Vulnerable under Criteria C2a(ii), based on a declining and restricted population size, as well as the entire population existing in one subpopulation only. However, information regarding new population estimates and range may warrant a change in Red List status. Therefore, the species will be re-assessed against all criteria:
Criterion A – The population size reduction of this species has not been formerly quantified over a 3-generation period (11 years; Bird et al., 2020)*. Although the species has certainly undergone population declines therefore, in the absence of reliable evidence, we cannot fully assess the species against Criterion A.
Criterion B – The true range of this species remains highly uncertain. Whilst the minimum range may only include the immediate surroundings of Mount Leuser (F. Rheindt in litt., 2020), the maximum range could cover much of the greater Gunung Leuser National Park and even further beyond the Leuser Ecosystem (eBird, 2020; Global Forest Watch, 2020). Taking a precautionary middle-approach however, assuming the range of the species covers the extent of the Gunung Leuser National Park only (9,500 km2), the Extent of Occurrence (EOO) is estimated at 16,273 km2, rounded here to 16,300 km2. However, this is likely still overestimating the true range of the Aceh Bulbul. We can therefore suspect that the species could at least meet Vulnerable under Criterion B1.
The species must however meet at least two further conditions to warrant a listing as threatened under Criterion B1. For this assessment, we can consider sub-criteria (a) and (b). Firstly, under IUCN definitions, the term ‘location’ is a geographically or ecologically distinct area in which a single threatening event could eradicate all individuals of the taxon present (usually over one generation or 3-year period; whichever is longer) (IUCN Standards and Petitions Committee, 2019). In this instance, where the species may occur over a suspected larger range, it may alternatively only occur in an area as small as one site (surrounding Mount Leuser; F. Rheindt in litt., 2020). A known ‘site’ however does not necessarily translate to a known ‘location’ under IUCN definitions. In the absence of an exact rate of decline therefore, we can suspect that the number of locations would regardless be small, based on the known effect of trapping on similar species or congeners of the Aceh Bulbul. Given the species’s short generation length however (3.5 years), it is unlikely that this threat could eradicate the entire population in one generation (equating to one location). However, based on the restricted range and rarity of the species, it is possible that trade could influence all mature individuals, and thus drive them to possible extinction within two or three generations (7-11 years) (equating to two or three locations).
Taking a similar approach undertaken for Criterion B1 therefore, we can assume that at maximum, the species could tentatively occur in 2-5 locations, meeting at least a Vulnerable status under sub-criteria (a). With the influence of trapping also causing severe population declines, it is suspected that there could be continued decline in the number of mature individuals, thus meeting conditions of sub-criteria (b). The Area of Occupancy (AOO) was not quantified for this species due to uncertainty in range. Therefore, we can precautionarily assess the species as Vulnerable under Criterion B1ab(v). It is important to consider however that this may be an overestimated assumption, with some possibility that the EOO of the species could even fall below 5,000 km2, thus meeting an Endangered status under Criterion B1. In order to comprehensively assess the species under Criterion B therefore; we welcome any recent information on the distribution range of the species.
Criterion C – The number of mature individuals is estimated at 250-999. This initially meets the required thresholds for both Endangered and Vulnerable statuses. In the absence of quantified population declines however, the species cannot be assessed under Criterion C1. However, due to the restricted range of the species, and its small locality of approximately 2-5 locations, we can assume that all mature individuals belong to the same subpopulation. The species therefore meets an Endangered status under Criterion C2a(ii).
Criterion D – Assuming the number of mature individuals occurs between the band of 250-999, this initially meets a Vulnerable status under Criterion D1. Precautionarily assuming that the species only occurs in 2-5 locations, and with trapping for trade being an imminent threat that could drive the species to Critically Endangered, the species also meets Vulnerable under Criterion D2. Overall, the species may therefore meet Vulnerable D1+2.
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 suggested that the Aceh Bulbul (Pycnonotus snouckaerti) be reclassified as Endangered C2a(ii). The species was recognised at species-level in 2016 (lumped previously with the Orange-spotted Bulbul; Pycnonotus bimaculatus). This suggests that the Aceh Bulbul may have been unrecorded in trade markets in the greater Sumatra region prior to 2015-2016. The species may therefore have been very rare since the late 1990’s, with the population potentially falling below 10,000 mature individuals as early as 1996-2000 or 2000-2004. The species has also likely further declined below 2,500 mature individuals between 2016 and 2020. We therefore welcome any comments on the above proposed listing and specifically ask for information regarding the timeframe of the genuine deterioration of the Aceh Bulbul population, as well as on the distribution range.
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.
*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).
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.; Šekercioğlu, Ç.; Butchart, S. H. M. (2020). Generation lengths of the world’s birds and their implications for extinction risk. Conservation Biology online first view.
del Hoyo, J., Collar, N., and Kirwan, G. M. (2020). Aceh Bulbul (Pycnonotus snouckaerti), versión 1.0. In Birds of the World (J. del Hoyo, A. Elliott, J. Sargatal, D. A. Christie, and E. de Juana, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.orsbul2.01
Eaton, J. A. and Collar, N. J. 2015. The taxonomic status of Pycnonotus bimaculatus snouckaerti. Forktail 31: 107-110.
eBird. 2020. eBird: An online database of bird distribution and abundance. Ithaca, NY, USA Available at: http://www.ebird.org. Accessed 22nd May.
F, Rheindt. (2020). Asian Songbird Trade Specialist Group. In litt.
Global Forest Watch. 2020. Interactive Forest Change Mapping Tool. http://www.globalforestwatch.org (Accessed 22nd May 2020).
IUCN Standards and Petitions Committee. 2019. Guidelines for Using the IUCN Red List Categories and Criteria. Version 14. Prepared by the Standards and Petitions Committee. Downloadable from http://www.iucnredlist.org/documents/RedListGuidelines.pdf
Leupen, B. T. C., and Gomez, L. (2019). The trading of the Orange-spotted Bulbul Pycnonotus bimaculatus and Aceh Bulbul P. snouckaerti in Indonesia. BirdingASIA; 32, 102-107.