Bangwa Warbler (Bradypterus bangwaensis): revise global status?

BirdLife species factsheet for Bangwa Warbler

Bangwa Warbler (Bradypterus bangwaensis) occurs in the highlands of south-eastern Nigeria and western Cameroon. It inhabits a variety of habitats, from the undergrowth of montane forest to forest edge, secondary growth and thickets (F. Dowsett-Lemaire in litt. 1999). In Nigeria, the species is found at elevations of 1,600-2,300 m, while in Cameroon it occurs in higher zones from 1,900 to 2,950 m (del Hoyo et al. 2006). The species is described as locally common or very common (Dowsett-Lemaire and Dowsett 1999, del Hoyo et al. 2006). It was found to occur at a density of 23 individuals/km2 (Santini et al. 2018) and may thus number 160,000 individuals in total, which roughly equates to 100,000 mature individuals. Bangwa Warbler is potentially threatened by habitat clearance due to small-holder cultivation and livestock farming, and timber logging (Stuart 1986).

The species has been considered Near Threatened, approaching the threshold for listing as threatened under Criterion B1ab(i,ii,iii) (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 known to Bangwa Warbler is habitat loss. However, deforestation data from between 2000 and 2012 (Tracewski et al. 2016; see also Global Forest Watch 2020) suggests that forest loss within the range has been negligible (<1% over ten years; see Bird et al. 2020)*. Therefore, in the absence of evidence for any declines or substantial threats, the species is assessed as being stable, and may be listed as Least Concern under this criterion.

Criterion B – The newly calculated Extent of Occurrence (EOO) for this species is 41,300 km2. This is too large to meet the threshold for Vulnerable under Criterion B1, and Bangwa Warbler 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 size is too large to warrant listing as threatened under Criterion C, and therefore Bangwa Warbler is considered Least Concern under this criterion.

Criterion D – The global population and range are too large to warrant listing as threatened under Criterion D, and therefore Bangwa Warbler is considered Least Concern under this criterion.

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 Bangwa Warbler (Bradypterus bangwaensis) 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: Bradypterus bangwaensis. http://www.birdlife.org (Accessed 26 February 2020).

del Hoyo, J.; Elliott, A.; Christie, D. 2006. Handbook of the Birds of the World, vol. 11: Old World Flycatchers to Old World Warblers. Lynx Edicions, Barcelona, Spain.

Dowsett-Lemaire, F.; Dowsett, R. J. 1999. Survey of birds and amphibians on Mt Manenguba, Mt Nlonako, north Bakossi and around Kupe in 1988-99.

Global Forest Watch. 2020. Interactive Forest Change Mapping Tool. http://www.globalforestwatch.org (Accessed 18 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.

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.

Santini, L.; Isaac, N. J. B.; Ficetola, G. F. 2018. TetraDENSITY: A database of population density estimates in terrestrial vertebrates. Global Ecology and Biogeography 27: 787-791.

Stuart, S. N. 1986. Conservation of Cameroon montane forests. International Council for Bird Preservation, Cambridge, U.K.

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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4 Responses to Bangwa Warbler (Bradypterus bangwaensis): revise global status?

  1. Alan Lee says:

    A recent publication on this species is just out, which generally indicates the species is common in its range and a habitat generalist, albeit facing loss of habitat due to forest loss.
    https://www.tandfonline.com/doi/abs/10.2989/00306525.2020.1723139

  2. 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 2.1% 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.

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

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