Chestnut-capped Thrush (Geokichla interpres): revise global status?

BirdLife species factsheet for Chestnut-capped Thrush

The Chestnut-capped Thrush is a songbird species resident throughout the Sundaland region of Southeast Asia; including both Peninsular and Borneo’s East Malaysia, Brunei, and across Indonesia (including parts of Sumatra, Java, Kalimantan and the Lesser Sunda islands) (Collar, 2020). The species also occurs on the southernmost range of peninsular Thailand and may be a rare resident in the Philippines (including the Sulu and Basilan Islands). The species prefers lowland habitats, including primary, tall secondary and logged forests, woodlands, and woodlots (Collar, 2020). The species is rare and local throughout most of its range, considered ‘fairly common’ only in Borneo and the Lesser Sundas. The species is a recent coloniser in Krakatau (Collar, 2020).

The species was previously considered to be undergoing a moderately rapid decline due to forest loss and trapping for the cage bird trade. A recent forest analysis of the Sundaland region suggests that the Chestnut-capped Thrush may be continuing to face high risk from surrounding habitat loss (Symes et al. 2018). Owed to a number of factors such as logging, land conversion for industrial plantations, and forest fires, the Sundaic lowlands had experienced a loss of over 70% of its original forest cover by 2010 alone (Symes et al., 2018). The species is also thought to fare poorly in logged forests, occurring in low densities in these habitats (D. Edwards in litt., 2020). The intensive trade for birds further compounds forest destruction and habitat loss, with the Chestnut-capped Thrush facing significant pressures in trapping for domestic and international markets (Symes et al., 2018).

The population size of the species was previously unknown. Applying appropriate conversions to population densities based on territories and singing birds however suggest that the species may number 2.75-100 pairs/km2 (Collar, 2020); converted here to 5.5-200 mature individuals/km2. The maximum of this estimate is however based on locally very high densities of the species and may therefore overestimate its presence (Collar, 2020). Thus, to account for the apparent rarity of the species in parts of its range, the lower estimate (c. 5.5 mature individuals/km2) is used therein. Precautionarily assuming that 10% of the total mapped range (c. 674,550 km2) is occupied, the population may therefore number 371,002 mature individuals. To account for uncertainty, the population is placed tentatively in the band of 100,000-499,999 mature individuals.

The Chestnut-capped Thrush is currently listed as Near Threatened, based on a moderately rapid decline rate that approaches a threatened status, resulting from forest loss and trapping for the cage bird trade. However, information regarding hunting and forest loss data may warrant a change in Red List status. Therefore, the species will be re-assessed against all criteria:

Criterion A – The Chestnut-capped Thrush is thought to be undergoing population declines due to habitat loss and trapping and hunting for trade. A recent study investigating both factors across the Sundaic region found that forest loss and hunting pressures had caused an estimated 67.5% decline over 10 years (Symes et al., 2018; see also Tracewski et al., 2016). Following recent findings, the generation length of the Chestnut-capped Thrush is thought to be 4.5 years (Bird et al., 2020)*. The relevant timeframe for an assessment against Criterion A is three generation lengths, which is equivalent to 13.5 years for this species. A population decline of 67.5% over 10 years equates to 78.1% decline over 13.5 years. The species therefore initially qualifies for an Endangered status under Criterion A.

However, it is important to consider that these population decline estimates covered only the Greater Sundaic range, and did not include the Lesser Sunda Islands where the species is regarded to be trapped intensely (F. Rheindt in litt. 2020). Its presence in the Lesser Sunda Islands is therefore becoming increasingly rare, albeit has not been formerly quantified. Given the overall large spread of the species throughout much of the Sundaic region however, and to account for any uncertainty in population declines therefore, it is suspected that declines are in any case ≥30% (F. Rheindt in litt. 2020); placed tentatively in the band of 30-49% decline. We can also assume that population declines are likely to continue into the future. Thus, the species may qualify for a Vulnerable listing under Criterion A2cd+A3cd+A4cd. We however welcome any recent information regarding population trends that cover the entirety of the species range.

Criterion B – The Extent of Occurrence (EOO) for this species is estimated at 4,270,000 km2. This is outside the required threshold for a threatened status under Criterion B1, and the species qualifies for Least Concern under Criterion B1. The Area of Occupancy (AOO) was not quantified for this species as required (see IUCN Standards and Petitions Committee, 2019); thus, the species cannot be assessed against Criterion B2.

Criterion C – The estimated population size of this species (100,000-499,999 mature individuals) is outside the required thresholds for a threatened status under this criterion. Therefore, the species qualifies for Least Concern under Criterion C.

Criterion D – The estimate population size and range of this species does not approach any relevant threshold for a threatened status under this criterion. Therefore, the species qualifies for Least Concern under Criterion D.

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 Chestnut-capped Thrush (Geokichla interpres) be reclassified as Vulnerable A2cd+3cd+4cd. We therefore welcome any comments on the above proposed listing and specifically ask for any recent population trends. We additionally ask for information on the timeframe of declines: Data from Symes et al. (2018) suggests that declines accelerated between 2000 and 2015, meaning that the species may have crossed the threshold for listing as Vulnerable during 2008-2012 or during 2012-2016. We welcome any comments to the proposed listing and suggested timing of change.

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.

References

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.

BirdLife International (2016). Geokichla interpres. The IUCN Red List of Threatened Species 2016.

Collar, N. (2020). Chestnut-capped Thrush (Geokichla interpres), version 1.0. In Birds of the World (J. del Hoyo, A. Elliott, J. Sargatal, D. A. Christie, and E. de Juana; Editors). Cornell Lab or Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.chcthr1.01

D. Edwards (2020). Asian Songbird Trade Specialist Group. In litt.

F. Rheindt (2020). Asian Songbird Trade Specialist Group. In litt.

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

Symes, W. S., Edwards, D. P., Miettinen, J., Rheindt, F. E., and Carrasco, L. R. (2018). Combined impacts of deforestation and wildlife trade on tropical biodiversity are severely underestimated. Nature Communications 9: 4052.

Tracewski, Ł., Butchart, S. H. M., Di Marco, M., Ficetola, G. F., Rondinini, C., Symes, A., Wheatley, H., Beresford, A. E., and 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.

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6 Responses to Chestnut-capped Thrush (Geokichla interpres): revise global status?

  1. Ingkayut Sa-ar says:

    I agree if the Chestnut-capped Thrush will be reclassified its conservation status as “Vulnerable”, because the species has been hunted for songbird and occurs in low density especially in Southern Thailand.

    In Satun, peninsular Thailand. I found three birds were caught by local poachers in difference of time — the first in 1999, the second in 2013 and the third in 2019 https://www.facebook.com/100001579074304/posts/2643431065719503/?d=n

    These are evidence indicate that the species has been caught for songbird in Southern Thailand.

    In addition, there were a few records of this species in Thailand after passed 1999 ; two birds were found at Krung Ching Khao Luang National Park in 2005, a pair was found at Taleban National Park in June 2018 and recent record was found by me on April 2020 at a rubber plantation in Satun province. https://www.facebook.com/100001579074304/posts/3082407905155148/?d=n

  2. Simon Mahood says:

    I don’t follow the logic here. You clearly present evidence that Chestnut-capped Thrush falls within the upper part of the EN decline band based on trends in the Greater Sundas (78.1% decline in 3 gens – almost a CR rate). Then you state that in the “Lesser Sunda Islands … the species is …. trapped intensely … therefore becoming increasingly rare … it is suspected that declines are in any case ≥30%”. Finally, you propose that the overall rate of decline (range-wide) falls in the band 30-49% and CCT therefore qualifies as VU under A.

    If the population in the Greater Sundas is declining at 78.1% over 3 gens, then for the range-wide rate of decline over 3 gens to be <50% would require a fairly large proportion of the total population to be found within the Lesser Sundas (even if it is assumed that there it is declining at only 30%). If the rate of decline in the Lesser Sundas is nearer 50% then you need most of the global population to be there for it to qualify as VU. Given the relative land-area covered by primary or tall secondary lower elevation forest in the Greater vs Lesser Sundas I would be surprised if this were the case.

    I would like to see someone more cleverer than me do the maths and show what proportion of the population must be in the Lesser Sundas for this species to qualify as VU so that we can evaluate if this is reasonable. Otherwise the evidence presented in this forum topics indicates that the species should be uplisted to EN.

  3. Simon Mahood says:

    Luckily I didn’t need to do the maths because I found this: https://www.iucnredlist.org/resources/criterion-a Using the multiple populations tab Following the data contained in the topic above I assumed a rate of decline of 78% over the past 3 gens in the GS and experimented with different proportions of the global population in the GS and LS. I found that if I made the population in the LS decline at 30% over the past 3 gens, then to get a range-wide rate of decline of 50% over 3 gens (ie VU) then I needed to assume that in 2020 82% of the global population is found in the LS. If the rate of decline in the LS is closer to 50% over 3 gens (it is given as 30-49% above) then you must assume that in 2020 >95% of the global population is in the LS. 82-95% of the global population in the LS does not seem plausible (based on a crude mental assessment of the area of suitable habitat), so the species is likely EN. Apologies if I’ve misused the spreadsheet and got it all completely wrong – if so please do not post this comment because it will only mislead others!

  4. Yong Ding Li says:

    The population densities and estimates seems unrealistically high (5.5-200 mature individuals/km2!!). They are certainly not backed by field data – including both field surveys, and camera trap data in the central Malay Peninsula. While camera traps set for mammals are not necessarily ideal for sampling birds (camera traps have found most phasianids, and hardly any thrushes!!). On the peninsula, the species seem dependent on lowland forests, especially riparian types that are now either heavily disturbed, or logged-over for palm oil. Combined with increasing trapping pressures for songbirds, the threats faced by the species in the Sundaic part of its range (which is really the larger part of its global distribution) is definitely in trouble.

  5. 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

  6. Red List Team (BirdLife International) says:

    Preliminary proposal

    Based on available information, and despite the potential overestimation of decline rates from the impacts of hunting alone, in the absence of other mitigating factors, it is precautionarily assumed that the species may be undergoing a decline exceeding the required threshold for a higher threatened category.

    Thus, our preliminary proposal for the 2020 Red List would be to list the Chestnut-capped Thrush as Endangered under Criterion A2cd+3cd+4cd.

    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, following further checking of information relevant to the assessments by both BirdLife and IUCN.

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