Archived 2016 topics: Large-billed Gerygone (Gerygone magnirostris) is being split: list Gerygone hypoxantha as Vulnerable?

This is part of a consultation on the Red List implications of extensive changes to BirdLife’s taxonomy for passerines

Lynx Edicions and BirdLife International will soon publish the second volume of the HBW-BirdLife Illustrated Checklist of the Birds of the World, building off the Handbook of the Birds of the World series, and BirdLife’s annually updated taxonomic checklist.

The new Checklist will be based on the application of criteria for recognising species limits described by Tobias et al. (2010). Full details of the specific scores and the basis of these for each new taxonomic revision will be provided in the Checklist.

Following publication, an open and transparent mechanism will be established to allow people to comment on the taxonomic revisions or suggest new ones, and provide new information of relevance in order to inform regular updates. We are also actively seeking input via a discussion topic here regarding some potential taxonomic revisions that currently lack sufficient information.

The new Checklist will form the taxonomic basis of BirdLife’s assessments of the status of the world’s birds for the IUCN Red List. The taxonomic changes that will appear in volume 2 of the checklist (for passerines) will begin to be incorporated into the 2016 Red List update, with the remainder to be incorporated into subsequent Red List updates.

Preliminary Red List assessments have been carried out for the newly split or lumped taxa. We are now requesting comments and feedback on these preliminary assessments.

Large-billed Gerygone (Gerygone magnirostris) is being split into G. magnirostris and G. hypoxantha, following the application of criteria set out by Tobias et al. (2010).

Prior to this taxonomic change, Gerygone magnirostris (BirdLife species factsheet) was listed as Least Concern on the basis that it did not approach the thresholds for listing as Vulnerable under any criteria. Previously G. hypoxantha was recognised by Birdlife International and assessed as Endangered under the criterion equivalent to B1ab(iii,v), with a population assessed as falling in the band 1,000-2,499 individuals, a range of 2,540km2 and suffering an ongoing decline due to habitat loss and degradation.

G. hypoxantha was considered a very poorly-known species, but in recent years it has been regularly seen on Biak including in secondary habitats (BirdtourAsia 2008, Hutchinson 2015, 2016). The newly calculated extent of occurrence (using a minimum convex polygon) for the species is 3,574 km2. Although Biak-Supiori is two islands, the minimal distance between the islands (separated only by a mangrove bordered channel) suggests that it is best to treat the species as being comprised of a single subpopulation. Forest on Biak is under heavy threat from logging and subsistence farming, but there are areas of forest remaining in the north and especially in interior Supiori, and the species is assumed to be present in the 110km2 Biak-Utara protected area, which comprises virtually impenetrable forested limestone areas (Dekker et al. 2000, Bishop 1982 in Stattersfield et al. 1998, Wikramanayake et al. 2002). A great deal of the forest clearance on Biak took place during the last century however, with the suggestion that further large scale logging is presently not economically feasible (Wikramanayake et al. 2002). Given an assumed generation length of 5.5 years, it is now considered unlikely that declines in the area of habitat are likely to exceed the thresholds for listing under inferred population reduction (category A). However, a slow decline is suspected to be occurring owing to on-going habitat destruction.

A key question arises around the number of locations* that there are for the species, on the basis of the principal threat being habitat loss. The number of sightings in new locations suggests that there are more than 5, and likely more than 10. As the population does not appear to be severely fragmented, this would mean listing under criterion B is inappropriate.

A revised population estimate of 2,500-9,999 individuals has been made based on an assessment of known records, descriptions of abundance and range size. This is consistent with recorded population density estimates for congeners, and the fact that only a proportion of the estimated Extent of Occurrence is likely to be occupied. This estimate is equivalent to 1,667-6,666 mature individuals, rounded here to 1,500-7,000 mature individuals. In conjunction with an inferred continuing decline and on the basis that all mature individuals present represent a single subpopulation the species is proposed to qualify as Vulnerable under Criterion C2a(ii).

If there is evidence that the population is clearly below 2,500 mature individuals then the species would qualify as listing as Endangered under the same criterion.

Comments are invited on these proposed categories and further information would be welcomed.

*Note that the term ‘location’ defines a geographically or ecologically distinct area in which a single threatening event can rapidly affect all individuals of the taxon present. The size of the location depends on the area covered by the threatening event and may include part of one or many subpopulations. Where a taxon is affected by more than one threatening event, location should be defined by considering the most serious plausible threat (IUCN 2001, 2012).

 

References:

BirdtourAsia 2008. ‘Exploratory trips, 2008’ http://www.birdtourasia.com/exploratory2008.html

Dekker, R.W.R.J., Fuller, R.A., and Baker, G.C. (eds.). (2000). Megapodes. Status Survey and Conservation Action Plan 2000–2004. WPA/BirdLife/SSC Megapode Specialist Group. IUCN, Gland, Switzerland and Cambridge, UK, and the World Pheasant Association, Reading, UK. vii + 39 pp

Hutchinson, R. 2015. West Papua, New Guinea 8th July – 2nd August 2015. BirdtourAsia tour report. http://www.birdtourasia.com/pdf%20Reports/Birdtour%20Asia%20West%20Papua%20July%202015.pdf

Hutchinson, R. 2016. ‘Easy’ West Papua, New Guinea 5th – 28th July 2016. BirdtourAsia tour report. http://www.birdtourasia.com/pdf%20Reports/Birdtour%20Asia%20West%20Papua%20July%202016.pdf

IUCN. 2001. IUCN Red List Categories and Criteria: Version 3.1. Gland, Switzerland and Cambridge, UK: IUCN Species Survival Commission.

IUCN. 2012. Guidelines for Application of IUCN Red List Criteria at Regional and National Levels: Version 4.0. Gland, Switzerland and Cambridge, UK: IUCN.

Stattersfield, A.J., Crosby, M.J., Long, A.J. and Wege, D.C. (1998) Endemic Bird Areas of the World. Priorities for biodiversity conservation. BirdLife Conservation Series 7. Cambridge: BirdLife International.

Tobias, J. A., Seddon, N., Spottiswoode, C. N., Pilgrim, J. D., Fishpool, L. D. C. and Collar, N. J. (2010) Quantitative criteria for species delimitation. Ibis 152: 724–746.

Wikramanayake, E., Dinerstein, E., Loucks, C., Olson, D., Morrison, J., Lamoreux, J., McKnight, M. and Hedao, P. 2001. Terrestrial ecoregions of the Indo-Pacific: A conservation assessment. Washington (DC): Island Press

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3 Responses to Archived 2016 topics: Large-billed Gerygone (Gerygone magnirostris) is being split: list Gerygone hypoxantha as Vulnerable?

  1. Guy Dutson says:

    I agree that this sp occurs at >10 locations.
    In six days on Biak in 2012, I recorded this species regularly in old-growth and secondary forest habitats, but noted that it was often silent and likely overlooked. Its population density was less than that of G. chrysogaster in lowland New Guinea but more than that of G. palpebrosa in lowland NG, which Bell (1982) estimated to occur at 40 birds / 10 ha and 1 bird / 10 ha respectively. I think that G. hypoxantha is most likely to have a population >10,000 birds, but would welcome other opinions.

  2. Rob Martin (BirdLife International) says:

    Preliminary proposals

    Based on available information, our preliminary proposal for the 2016 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 of 28 October, 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 2016 Red List categories will be published on the BirdLife and IUCN websites in early December, following further checking of information relevant to the assessments by both BirdLife and IUCN.

  3. Rob Martin (BirdLife International) says:

    Comments have been received from Burung Indonesia (in litt. 2016) supporting the listing of G. hypoxantha as Vulnerable.

Comments are closed.