Black-headed Rufous-warbler (Bathmocercus cerviniventris): request for information.

Currently listed as Near Threatened, the Black-headed Rufous-warbler (Bathmocercus cerviniventris) is endemic to the Upper Guinea forest of West Africa (see BirdLife International 2018), where the species is predominantly threatened by habitat destruction and degradation. The species is listed as occurring in Guinea, Sierra Leone, Liberia, Côte d’Ivoire and Ghana, yet it is not widespread, instead being very locally distributed, and the last record (of 3) from Ghana was from 1900, suggesting it could be extinct there (Dowsett-Lemaire and Dowsett 2014).

Gatter (1997) estimated the population in Liberia alone to include 60,000 pairs, and overall the species was thought to be locally common. However, the species is now only seldom recorded. There are only very limited records on the Global Biodiversity Information Facility (Bathmocercus cerviniventris (Sharpe, 1877) in GBIF Secretariat, 2017) and on eBird (eBird 2018), although this of course could be more to do with a lack of reporting effort than a small population size. That said, recent surveys in eastern Liberia, where the species had been thought to be fairly common (Gatter [2017] states 4 males per 200m around Zwedru), found no individuals despite using playbacks (Phalan et al. 2013). And in addition to this, the population in the Nimba Mountains, in Liberia, may be severely threatened by mountain-top mining (Dowsett-Lemaire and Phalan 2013, B. Phalan in litt. 2018), to the extent that the site where individuals were seen was being completely destroyed (F. Dowsett-Lemaire in litt. 2016). Meanwhile, in Côte d’Ivoire one of the IBAs where the species was known to occur (Marahoue National Park) has been near-completely destroyed due to settlement following conflict (L. Fishpool per B. Phalan in litt. 2018).

Given this worrying evidence we have therefore reassessed the species here against all criteria.

 

Criterion A – The species is currently listed as Near Threatened under criterion A based on potentially moderately rapid declines due to habitat loss and degradation (see BirdLife International 2018). The current rate of decline, however, is uncertain. Gatter (1997) estimated the population in Liberia alone to contain 60,000 pairs, yet Phalan et al. (2013) failed to locate the species at all during surveys of Grand Gedeh County, even when using playback. This is despite Gatter (1997) saying that there could be as many as 4 males per 200m at Zwedru. Therefore, the species may have undergone a very rapid decline, assuming that previous population estimates were not too high.

However, the rate of forest loss within the currently mapped range for this species was listed as c.0.5% per year between 2000 and 2012 (Tracewski et al. 2016), which would roughly equate to c.4.9% over 10 years (the generation length for this species is too short to use a three generation length time period). Assuming that population change is proportional to habitat loss, the species would then no longer approach the threshold for Vulnerable under this criterion. However, this is based on the currently mapped range, which encompasses a large amount of habitat that is no longer considered to hold the species. Therefore, these values are not really representative of habitat loss within the species’s range, and it is very difficult to accurately assess the rate of population change.

 

Criterion B – A revision of the current map is underway, but at the moment the Extent of Occurrence (EOO) for the species is 381,000km2. This far exceeds the threshold for listing as Vulnerable under this criterion (20,000km2). Even with revisions, given that a Minimum Convex Polygon is required to calculate the EOO, it is highly unlikely that the species would warrant listing under criterion B1.

The species’s Area of Occupancy (AOO), however, is uncertain and could well be very small. This parameter does still require calculating per IUCN guidelines (IUCN Standards and Petitions Subcommittee 2017), and so clear comparison to IUCN thresholds is not possible currently. The species is thought likely severely fragmented and declining as a result of habitat loss, so if a calculation were to show that the species’s AOO meets IUCN thresholds, it would likely warrant listing under a threatened category. In the absence of this measured parameter, however, the species could be tentatively listed as Near Threatened under criterion B2ab(ii,iii,v).

 

Criterion C – The population size in Liberia alone had been estimated to be 60,000 pairs (120,000 mature individuals) (Gatter 1997). However, given the recent evidence of the loss of occupied habitat, and its disappearance from areas where it had been thought to be locally common, it is likely that the global population size is much smaller than this, or in fact previous estimates had been too high. The species is only sparsely recorded, and in fact the population size could be fairly small. Getting a clear and accurate population size estimate, however, is still difficult as the best population density estimates available appear to now be too high.

The fragmented nature of the species’s range means that the species would not warrant listing under Criterion C2a(ii), and while it is likely that the species is in decline, the data quality regarding actual rates of decline is not strong enough to assess the species against Criterion C1. The species is also not known to undergo extreme fluctuations, so it would not warrant listing under Criterion C2b. Therefore, the only option available is Criterion C2a(i). To be listed under this criterion, however, will require further information, insight or comment. Is the global population likely to approach or even meet the threshold population size for consideration as Vulnerable (2,500-9,999 mature individuals)? And is it plausible that no subpopulation contains >1,000 mature individuals?

 

Criterion D – As for Criterion C, the estimation of population size is extremely tentative, but it would likely not approach the threshold for Vulnerable under this criterion. Additionally, while the species has a restricted range, it is not thought to meet the thresholds and conditions for listing under criterion D2 either.

 

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

 

Therefore, we urgently request any further information or comments regarding potential sightings of this species, and estimates of its population size. In the absence of these it is very difficult to clearly assess the species against the IUCN criteria and their thresholds, to such an extent that it is plausible that the species could fall in any of the categories between Least Concern and Critically Endangered. Thus the species could potentially warrant listing as Data Deficient.

 

References

Bathmocercus cerviniventris (Sharpe, 1877) in GBIF Secretariat. 2017. GBIF Backbone Taxonomy. Checklist Dataset https://doi.org/10.15468/39omei accessed via GBIF.org on 2018-02-16.

BirdLife International. 2018. Species factsheet: Bathmocercus cerviniventris. Downloaded from http://www.birdlife.org on 16/02/2018.

Dowsett-Lemaire, F.; Dowsett, R. J. 2014. The Birds of Ghana: an atlas and handbook. Tauraco Press, Liège, Belgium.

Dowsett-Lemaire, F.; Phalan, B. 2013. Nimba Western Area Iron Ore Concentrator Mining Project, Liberia. Environmental and Social Impact Assessment Volume 4, Part 2, Appendix 2: Ornithological Surveys in the Nimba Mountains in October-November 2011, with special reference to East Nimba Nature Reserve and the West Nimba (Gba) Community-Managed Forest. Report to URS for ArcelorMittal.

eBird. 2018. eBird: An online database of bird distribution and abundance [web application]. eBird, Ithaca, New York. Available: http://www.ebird.org. (Accessed: February 16, 2018).

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

IUCN Standards and Petitions Subcommittee. 2017. Guidelines for Using the IUCN Red List Categories and Criteria. Version 13. Prepared by the Standards and Petitions Subcommittee. Downloadable from http://www.iucnredlist.org/documents/RedListGuidelines.pdf.

Phalan, B.; Fishpool, L. D. C.; Loqueh, E. M.; Grimes, T.; Molubah, F. P.; Garbo, M. 2013. Liberian Greenbul expedition 2013. Final Report. Unpublished report to African Bird Club and RSPB. Cambridge, United Kingdom.

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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