Chestnut-backed Thornbird (Phacellodomus dorsalis): revise global status?

BirdLife species factsheet for Chestnut-backed Thornbird

Chestnut-backed Thornbird (Phacellodomus dorsalis) is endemic to north-western Peru, where it occurs in the upper Marañón valley in the departments Cajamarca, Amazonas and La Libertad. The population size has been placed in the band 6,000-15,000 mature individuals, and the species is generally considered uncommon (Fjeldså and Krabbe 1990, Ridgely and Tudor 1994, Clements and Shany 2001).

Chestnut-backed Thornbird is found in dense, tangled thorn-scrub, pastures and hedgerows on bushy slopes with scattered Acacia trees (F. Angulo in litt. 2012). The only threat known is the loss of its habitat through fires and the conversion to agricultural land; large parts of the Marañón drainage have already deteriorated. However, the species’s tolerance of some degree of habitat alteration make it less vulnerable to ongoing habitat degradation.

Chestnut-backed Thornbird is currently listed as Vulnerable under Criterion B1ab(iii,v) (BirdLife International 2020). However, new information regarding the distribution range and population trend suggests that the species may warrant a change in Red List status. Therefore, it will be re-assessed against all Red List criteria:

Criterion A – The population trend has not been estimated directly. The only threat known to the species is the loss and degradation of its habitat. Tree cover loss within the range has been low over ten years (< 5%; Global Forest Watch 2020; one generation length being 2.8 years; Bird et al. 2020*). Given that the species does not depend on forest, but readily tolerates degraded and converted habitats, the rate of population decline is likely lower than the rate of forest loss. Therefore, while we can precautionarily suspect the species to be undergoing a small decline, the rate of decline does not approach the threshold for listing as threatened, and Chestnut-backed Thornbird may be considered Least Concern under Criterion A.

Criterion B – The Extent of Occurrence (EOO) for this species is 16,300 km2. This value meets the threshold for Vulnerable under Criterion B1 (EOO < 20,000 km2). The maximum Area of Occupancy (AOO), calculated as a 2 x 2 km2 grid over the area of mapped range, is 4,160 km2, which does not meet the threshold for listing as threatened under Criterion B2 (AOO < 2,000 km2).

In order to be listed as threatened under Criterion B1, at least two further conditions have to be met. Chestnut-backed Thornbird is found in at least eleven sites throughout its range (Angulo et al. 2008, F. Angulo in litt. 2020, see also eBird 2020). As such, there is no evidence to suggest that the species occurs at less than ten locations or is severely fragmented sensu IUCN (IUCN Standards and Petitions Committee 2019), and the species does not meet condition a. Within the range, habitat is lost at a low rate, but impacts on the species are likely not severe. However, we can precautionarily infer a decline in habitat quality, and so the species meets condition b(iii). There is no evidence of extreme fluctuations in the distribution range or population size, and condition c is not met.

Overall, the species does not fulfil sufficient conditions for listing as threatened under Criterion B. Therefore, it is considered Near Threatened, approaching the threshold for listing as threatened under Criterion B1ab(iii).

Criterion C – The population size of Chestnut-backed Thornbird has preliminarily been estimated at 6,000-15,000 mature individuals. Assuming that the true population size is closer to the lower end of the estimate, this meets the threshold for listing as threatened under Criterion C. However, to do so a species must meet further conditions.

It is suspected that the species is undergoing a slow population decline due to habitat loss. A suspected decline, however, precludes a listing as threatened under Criterion C. The species is thought to form several subpopulations, but it is unlikely that the largest subpopulation contains less than 1,000 mature individuals. Therefore, Chestnut-backed Thornbird qualifies for listing as Least Concern under Criterion C.

Criterion D – The population size and range are too large to warrant listing as threatened under Criterion D, and so the species may be considered Least Concern under this criterion.

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 Chestnut-backed Thornbird (Phacellodomus dorsalis) be listed as Near Threatened, approaching the threshold for listing as treatened under Criterion B1ab(iii). We welcome any comments on the proposed listing.

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

Angulo, F.; Palomino, W.; Arnal, H.; Aucca, C.; Uchofen, O. 2008. Corredor de Conservación de Aves Marañón – Alto Mayo: Análisis de Distribución de Aves de Alta Prioridad de Conservación e Identificación de Propuestas de Áreas para su Conservación. Asociación Ecosistemas Andinos – American Bird Conservancy, Lima, Peru.

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. 2020. Species factsheet: Phacellodomus dorsalis. http://www.birdlife.org (Accessed 09 March 2020).

Clements, J.F.; Shany, N. 2001. A Field Guide to the Birds of Peru. Lynx Edicions, Barcelona, Spain.

eBird. 2020. eBird: An online database of bird distribution and abundance [web application]. eBird, Ithaca, New York. http://www.ebird.org (Accessed 09 March 2020).

Fjeldså, J.; Krabbe, N. 1990. Birds of the High Andes. Apollo Books, Copenhagen, Denmark.

Global Forest Watch. 2020. World Resources Institute. http://www.globalforestwatch.org (Accessed 09 March 2020).

IUCN Standards and Petitions Committee. 2019. Guidelines for using the IUCN Red List Categoreis and Criteria. Version 14. http://www.iucnredlist.org/documents/RedListGuidelines.pdf.

Ridgely, R.S. and Tudor, G. 1994. The birds of South America. University of Texas Press, Austin, Texas, USA.

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4 Responses to Chestnut-backed Thornbird (Phacellodomus dorsalis): revise global status?

  1. Red List Team (BirdLife International) says:

    Global Forest Change data on tree cover loss up to 2019 have now been released and made available via Global Forest Watch. Based on these data, over ten years approximately 1.2% of tree cover with 30% canopy cover was lost from within the species’s range (Global Forest Watch 2020). This does not affect the above assessment under Criterion A.

  2. Red List Team (BirdLife International) says:

    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,
    BirdLife Red List Team

  3. 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 classification 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 those in 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.

  4. Fernando Angulo says:

    Angulo et al. (2008) p. 63 shows a much bigger distribution area than the map provided on the BLI website. This is another case of a bird believed to be found only on the w slope of the Marañon river. Angulo et al. (2008) shows (p. 122) at least 5 localities on the e side of the marañon valley. This, can affect the size of the EOO, turning it bigger than previosouly though.

    Angulo, F.; Palomino, W.; Arnal, H.; Aucca, C.; Uchofen, O. 2008. Corredor de Conservación de Aves Marañón – Alto Mayo: Análisis de Distribución de Aves de Alta Prioridad de Conservación e Identificación de Propuestas de Áreas para su Conservación. Asociación Ecosistemas Andinos – American Bird Conservancy, Lima, Peru.
    https://3pktan2l5dp043gw5f49lvhc-wpengine.netdna-ssl.com/wp-content/uploads/2015/05/Alto-Mayo-Conserverion-Corridor-Report-2008.pdf

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