Dusky Tetraka (Crossleyia tenebrosa): revise global status?

BirdLife species factsheet for Dusky Tetraka

Dusky Tetraka (Crossleyia tenebrosa) is endemic to eastern Madagascar. It inhabits lowland and mid-altitude rainforest, where it prefers the most humid and undisturbed areas. The species is not well known and appears to be extremely scarce (ZICOMA 1999). The population size has not been quantified, but is preliminarily placed in the band 1,500-7,000 mature individuals. The most severe threat known to Dusky Tetraka is habitat loss, mainly due to slash-and-burn cultivation by subsistence farmers, which converts the rainforest to degraded regrowth and eventually to bracken-covered areas or grassland (Du Puy and Moat 1996). Much of the eastern coastal plain in Madagascar has either already been cleared or is covered by highly degraded forest (Jenkins 1987), remaining habitat is under pressure from the increasing human population (Jenkins 1987), and commercial logging is an additional threat in some areas (A. F. A. Hawkins in litt. 1995).

The species has been considered Vulnerable under Criterion B1ab(i,ii,iii,v) (BirdLife International 2020). However, this is no longer tenable because this was based on an Extent of Occurrence (EOO) value calculated as the ‘area of mapped range’. This is no longer appropriate and the EOO should be calculated using a Minimum Convex Polygon (see IUCN 2001, 2012, Joppa et al. 2016), as EOO is a measure of the spatial spread of areas occupied by a species, not the actual area it occupies. The resulting EOO value now exceeds the thresholds required to maintain the species’s current listing, and as such it potentially cannot retain its current Red List status. Therefore, we have fully reviewed the species here against all criteria.

Criterion A – The population trend for this species has not been directly estimated. The only threat know to Dusky Tetraka is habitat loss, and Global Forest Watch (2020) report a loss of tree cover at a rate of 27% between 2001 and 2018 within its range. Assuming that the decline in tree cover is exponential and continuing at the same rate, this equates to a rate of habitat loss of 16% over ten years (one generation length being 2.7 years; Bird et al. 2020)*. Assuming further that the population declines at roughly the same rate as tree cover, we can tentatively place the population decline in the band 10-19% over ten years. This is too low to qualify for listing as threatened, and therefore Dusky Tetraka may be considered Least Concern under Criterion A.

Criterion B – The newly calculated Extent of Occurrence (EOO) for this species is 50,600 km2. This is too large to meet the threshold for Vulnerable under Criterion B1, and Dusky Tetraka may be listed as Least Concern under this criterion. The Area of Occupancy (AOO) has not been quantified following IUCN Guidelines (IUCN Standards and Petitions Committee 2019), and therefore the species cannot be assessed against Criterion B2.

Criterion C – The global population is estimated to number 1,500-7,000 mature individuals, but we do not have any information regarding the subpopulation structure and the size of the largest subpopulation. In view of the fragmentation of its habitat and the assuming that the true population size is closer to the lower end of the estimate, it is conceivable that the species forms several subpopulations, with the largest containing less than 1,000 mature individuals. The species is suspected to decline at 10-19% over ten years due to habitat loss. A suspected decline, however, precludes a listing as threatened under Criterion C, and therefore Dusky Tetraka may be listed as Near Threatened, approaching the threshold for listing as threatened under Criterion C1+2a(i).

Criterion D – The global population has been placed in the band 1,500-7,000 mature individuals. Assuming that the true population size is closer to the lower end of the estimate, Dusky Tetraka may be listed as Near Threatened, approaching the threshold for listing as threatened under Criterion D1.

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

Therefore, it is suggested that Dusky Tetraka (Crossleyia tenebrosa) be listed as Near Threatened, approaching the threshold for listing as threatened under Criteria C1+2a(i); D1.We welcome any comments to 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 the species’ Red List status. Therefore, please make sure your comments are relevant to the species’ Red List status and the information requested. 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.

BirdLife International. 2020. Species factsheet: Crossleyia tenebrosa. http://www.birdlife.org (Accessed 28 February 2020).

Du Puy, D. J.; Moat, J. 1996. A refined classification of the primary vegetation of Madagascar based on the underlying geology: using GIS to map its distribution and to assess its conservation status. In: Lourenço, W.R. (ed.), Proceedings of the International Symposium on the biogeography of Madagascar, pp. 205-218. ORSTOM, Paris.

Global Forest Watch. 2020. World Resources Institute. http://www.globalforestwatch.org (Accessed 28 February 2020).

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. www.iucnredlist.org/technical-documents/categories-and-criteria.

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.

Jenkins, M. D. 1987. Madagascar: an environmental profile. International Union for Conservation of Nature and Natural Resources, Cambridge, U.K.

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.

ZICOMA. 1999. Les zones d’importance pour la conservation des oiseaux à Madagascar. Ministère de l’Environnement, Madagascar.

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5 Responses to Dusky Tetraka (Crossleyia tenebrosa): revise global status?

  1. Michael Mills says:

    I’m surprised that there is enough information on the species to make any assessment. Personally I think Data Deficient is the most appropriate category. There are virtually no records for the past 20 years supported by any evidence.

  2. Callan Cohen says:

    When was the last reliable, documented sighting? Seems not for a loooong time

  3. 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 15.9% of tree cover with 75% canopy cover was lost from within the species’s range (Global Forest Watch 2020). This does not affect the above assessment under Criterion A.

  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 list Dusky Tetraka as Data Deficient.

    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. The final publication date will be publicised by IUCN here: https://www.iucnredlist.org/assessment/updates

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