Archived 2020 topic: Yellow-bearded Greenbul (Criniger olivaceus): Revise global status?

Birdlife International factsheet for Yellow-bearded Greenbul

Yellow-bearded Greenbul is found in Africa, in a range that encompasses Guinea, Liberia, Côte d’Ivoire, and Ghana (BirdLife International, 2020). It inhabits the midstorey of lowland primary forest, and can also be found in mature secondary forest (Gatter, 1997). The total population for this species is estimated to be between 100,000-499,999 individuals (BirdLife International, 2020). The only known threat to this species appears to be habitat loss and degradation resulting from deforestation for agricultural and mining activities (Freeman et al., 2018).

Yellow-bearded Greenbul has previously been considered Vulnerable under criteria A2c + 3c + 4c. However, recent data regarding the rate of forest loss may mean that this is no longer tenable and the species may warrant a change in Red List status. Yellow-bearded Greenbul has therefore been reassessed here against all the criteria.

Criterion A: IUCN guidelines stipulate that rates of decline should be measured over the longer of 10 years or 3 generations (IUCN Standards and Petitions Committee, 2019). The generation length for Yellow-bearded Greenbul has been recalculated to 3.1 years (Bird et al., 2020*). Therefore, the rates of reduction for this species are calculated over 10 years.

The population trend for this species has not been directly estimated. However, the only known threat for this species is habitat loss and degradation. Between 2000-2018, there was an 18% reduction in tree cover across this species’ range (Global Forest Watch, 2020). Interpreting the effects of this on population reduction is challenging. This species seems to prefer denser, more mature canopy, and it is not certain that it can exist in degraded habitat. Assuming that the population size declines at the same rate as habitat loss, this equates to a 10% reduction over the last 10 years. Additionally, Tracewski et al. (2016) found negligible levels of deforestation within the range in their analysis. A 10-year reduction rate of 10% is too low to trigger the threatened category (≥30% population size reduction) under this criterion. Yellow-bearded Greenbul may therefore be considered Least Concern under Criterion A.

Criterion B: The Extent of Occurrence (EOO) for this species is too high to trigger the threatened threshold (EOO <20,000 km²) (Birdlife International, 2020). Yellow-bearded Greenbul may be considered Least Concern under Criterion B1. The AOO has not been calculated according to IUCN guidelines (IUCN Standards and Petitions Committee), and so this species cannot be assessed against criterion B2.

Criterion C: The total population is estimated to be between 100,000 and 499,999 total individuals (BirdLife International, 2020), which would equate to roughly 67,000- 334,999 mature individuals. Even assuming that the true population size is at the lower end of this estimate, this is above the threshold (<10,000 mature individuals) to be considered threatened under this criterion. Yellow-bearded Greenbul may therefore be considered Least Concern under Criterion C.

Criterion D: The population size and range are too large to trigger the threshold for listing as threatened under Criterion D; thus Yellow-bearded Greenbul is assessed as Least Concern under this criterion.

Criterion E: To the best of our knowledge, no quantitative analysis has been carried out on this species. Therefore, we cannot assess this species against Criterion E.

We therefore suggest that Yellow-bearded Bulbul (Criniger olivaceus) be listed as Least Concern. 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.

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 (2020) Species factsheet: Criniger olivaceus. Downloaded from http://www.birdlife.org on 07/05/2020

Freeman, B.; Dami, F. D.; Molokwu-Odozi, M> (2018): Status of globally threatened birds of Sapo National Park, Liberia. Ostrich.

Gatter, W. 1997. Birds of Liberia. Pica Press, Robertsbridge, UK.

Global Forest Watch. 2020. World Resources Institute. http://www.globalforestwatch.org (Accessed 06 May 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

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.

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5 Responses to Archived 2020 topic: Yellow-bearded Greenbul (Criniger olivaceus): 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 6.5% 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.

  2. Ben Phalan says:

    The big unknown here is the effect of forest degradation (as for other species such as Gola Malimbe). Forests can maintain and even gain tree cover, while losing the largest trees to legal and illegal selective logging – this is widespread in West Africa. Loss of these large trees can result in habitat structure which is no longer suitable for specialist understory and midstory species.

    The one study I am aware of that quantifies population change for this species is that of Arcilla et al. (2015). They had a capture rate of 2.70 Yellow-bearded Greenbuls per 10,000 net-metre-hours in 1993-1995, and this fell to 0.00 (zero) in 2008-2010, following a period when logging activity increased by >600%. Mistnetting was carried out in large, forest fragments (100–524 km2), protected to varying degrees, which are the best of what remains of tropical forest in Ghana.

    The finding by Arcilla et al. is alarming, as it suggests that not only was this species already greatly depleted in the 1990s because of forest loss and degradation, but that it has continued to decline, and may have been extirpated from some sites. There is no reason to expect this decline to have slowed since, as logging continues to affect forests across the species’ distribution.

    I would suggest that the most precautionary approach for this species is to maintain it as Vulnerable under Criterion A. This is based on the limited evidence available, which indicates that even within large, protected blocks of forest, forest degradation from logging is causing declines much greater than those that would be expected given forest loss alone.

    Arcilla, N., Holbech, L. H., & O’Donnell, S. (2015). Severe declines of understory birds follow illegal logging in Upper Guinea forests of Ghana, West Africa. Biological Conservation, 188, 41–49.

  3. 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

  4. Red List Team (BirdLife International) says:

    Preliminary proposal

    Based on available information, our preliminary proposal for the 2020 Red List would be to list Yellow-bearded Greenbul as Vulnerable under Criterion A2bc+3bc+4bc.

    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

  5. Red List Team (BirdLife International) says:

    Recommended categorisation to be put forward to IUCN

    The final categorisation for this species has not changed. Yellow-bearded Greenbul is recommended to be listed as Vulnerable under Criterion A2bc+3bc+4bc.

    Many thanks for everyone who contributed to the 2020 GTB Forum process. 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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