Pale-throated Barbet (Gymnobucco vernayi): request for information.

BirdLife species factsheet for Pale-throated Barbet: http://datazone.birdlife.org/species/factsheet/pale-throated-barbet-gymnobucco-vernayi

Endemic to highlands of west-central Angola, Pale-throated Barbet (Gymnobucco vernayi) was until recently considered conspecific with Naked-faced Barbet (G. calvus) (del Hoyo et al. 2018). Its habitat is described as similar to that of G. calvus (del Hoyo et al. 2018), and so it likely occurs in primary forest, as well as into secondary areas adjacent to such forest, as well as wooded grasslands and pastures (del Hoyo et al. 2002). As such, habitat loss – particularly the loss of nesting and roosting trees (del Hoyo et al. 2018) – is considered to be the key threat to the species.

The species is currently listed as Least Concern (BirdLife International 2018), but there have been noted declines including at Mt. Moco, where it has potentially gone extinct, and in Kumbira Forest (M. Mills in litt. 2018). Given its restricted range, and these observed declines we have therefore taken the opportunity to review the species’s status here against all criteria.

Criterion A – Extrapolating data from Tracewski et al. (2016) suggests that the area of forest within the species’s range is declining at a rate of approximately 8.6% over three generations (25.5 years). Therefore, if we assume that population trends are proportional to habitat trends, we can infer that the population is declining at a suspected rate of 8.6% over three generations. Survey work has shown the species to have disappeared from or declined at some locations (M. Mills in litt. 2018), which further supports the view that the species is in decline. However, a suspected rate of decline of 8.6% over three generations does not approach the threshold for Vulnerable under this criterion (30%), and even if we assume that the species is more susceptible to habitat loss and fragmentation, it is still unlikely that overall declines are at such a rate. As such, the species is considered Least Concern under Criterion A.

Criterion B – The species’s Extent of Occurrence is 38,600km2. Its Area of Occupancy has not been directly calculated, but the area of forest within its range, as calculated per Tracewski et al. (2016) is 10,900km2. Both of these values are too large to warrant listing under Criterion B. As such, the species is considered Least Concern under this criterion.

Criterion C – Up to now, the population size of this species has not been estimated. It was relatively frequently observed during the surveys of Mills et al. (2013), but from these observations we cannot calculate a global density estimate.

The species is considered to be in decline (although we can only suspect an overall rate), and it should likely be considered to be in one subpopulation per IUCN definitions (IUCN Standards and Petitions Subcommittee 2017). Therefore, the population size is key to whether the species warrants listing under this criterion. A population size of <15,000 mature individuals would mean the species warrants listing as Near Threatened, while a population size of <10,000 mature individuals would mean it warrants listing as Vulnerable. Assuming that only a proportion of its mapped range is occupied, density estimates would have to be <c.2.0 mature individuals/km2 to trigger a listing as Near Threatened, and <c.1.3 mature individuals/km2 to trigger a listing as Vulnerable under Criterion C2a(ii). Are these estimates plausible? If not, the population size of the species would be too large for listing under this criterion.

Criterion D – The species’s population size and range are too large to warrant listing under this criterion, and the species is assessed as Least Concern under Criterion D.

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

Therefore, we request any further information regarding population size estimates to see whether the species would warrant listing as Vulnerable or Near Threatened under Criterion C2a(ii). In the absence of any further information the species may warrant retaining as Least Concern. 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.

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

BirdLife International. 2018. Species factsheet: Gymnobucco vernayi. Downloaded from http://www.birdlife.org on 27/11/2018.

del Hoyo, J.; Collar, N.; Kirwan, G. M. 2018. Pale-throated Barbet (Gymnobucco vernayi). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/467447 on 26 November 2018).

del Hoyo, J.; Elliott, A.; Sargatal, J. 2002. Handbook of the birds of the world, Vol 7: Jacamars to Woodpeckers. Barcelona, Spain: Lynx Edicions.

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.

Mills, M. S. L.; Melo, M.; Vaz, A. 2013. The Namba mountains: new hope for Afromontane forest birds in Angola. Bird Conservation International 23: 159-167.

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 Pale-throated Barbet (Gymnobucco vernayi): request for information.

  1. Michael Mills says:

    A few comments which I hope will help
    1. Strictly speaking it is not endemic to the highlands, as it also occurs along the escarpment down to at least 900 m altitude
    2. I have surveyed every forest patch at Mt Moco and am certain as I can be that it is extinct there
    3. I have managed to find one group of them at Kumbira forest in May 2018 and they were still present last week. This is the only group I have seen in the past 5 years of the species. Before 2010 I saw it on every visit to Kumbira. After 2010 I have only seen it at one site in Kumbira forest.
    4. I spent 5 days in the last week exploring new and little-visited sites along the escarpment within the known range of the species and failed to find any Pale-throated Barbets
    5. Forest decline along the central escarpment is greater than that estimated. Based on continued observation at Kumbira Forest and a field trip to new sites along the Angolan escarpment last week, the rate of forest loss along the main central escarpment is several fold higher now than it was in 2000-2012. I would estimate that 80% of forest loss at Kumbira that has occurred since my first visit in 2003, has occurred after 2012. Commercial logging, commercial farming (especially bananas) and charcoal production have emerged as serious threats to remaining forest.
    6. I believe the only significant population of the species exists in the Namba mountains.
    7. Pedro Vaz Pinto also found it near Chongoroi a few years back, from where it had been previously collected
    8. We don’t have real population estimates, but based on what I have seen I would expect that there are fewer than 2000, possibly far less.
    9. Although the range is fairly large, the AOO is probably rather small
    9. The only sites from which it has been recorded according to Dean (2000) are Chongoroi [still present, confirmed Vaz Pinto], Mount Moco [extinct], Gabela [probably includes Kumbira; one group seen at one site in the last 5 years], Mombolo [a region which includes the Namba Mountains, which probably hold the only significant population]. In 2005 I found it at 6 sites along the central escarpment. Now I can only find it at one.
    10. Perhaps I have been looking in the wrong places, but I believe it to be one of the rarest and most threatened birds in Angola

  2. Rob Martin (BirdLife International) says:

    Many thanks for the highly valuable comment. It appears that the rarity of this recently recognised species has been overlooked, and the proportion of the range that is occupied appears far less than would be expected. An estimate of 2,000 individuals, coupled with an ongoing decline potentially warrants listing as Endangered, but further attention to subpopulation structure is needed. An assumption of a single subpopulation, as above, would indeed qualify: multiple subpopulations would require that the largest held <1000 mature individuals. It may be that the fragmentation of habitat has left separated populations, but with the largest area along the escarpment suspected to hold fewer than the Namba Mountains, this could also be reasonably assumed to be the case.
    But there remains a question over the population estimate. How much additional habitat is there that does seem suitable for the species, considering the note that it occurs down to 900 m? Chongoroi is a distance from Namba, could there be a significant number there?

  3. Rob Martin (BirdLife International) says:

    Preliminary proposal

    Based on available information, our preliminary proposal for the 2019 Red List would be to list Pale-throated Barbet as Vulnerable under criterion C2a(ii).

    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 2019 Red List categories will be published on the BirdLife and IUCN websites in December, following further checking of information relevant to the assessments by both BirdLife and IUCN.

  4. Michael Mills says:

    Regarding sub-populations sizes, in the Namba Mountains there is about 600 ha of suitable habitat (forest). Not sure if there is any information on densities of Gymnobucco barbets, but I would have thought 1 pair per 2 hectares, on average, would be a reasonable estimate, so around 600 adult individuals at the Namba Mountains. Being a colonial species makes this harder to evaluate. But I cannot imagine that there are more than 1000 adult individuals. I doubt there are any other areas that hold more than 100 adult individuals. The forests around Chongoroi are is small, isolated pockets and the total amount of forest is small.

    I’m pleased that it is at least being listed as Threatened, but I am concerned that the situation is more dire and that it should be treated as Endangered until we can prove that they are doing better than thought.

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