Archived 2016 topics: New Britain Thrush (Zoothera talaseae) is being split: list Z. atrigena as Near Threatened?

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.

New Britain Thrush Zoothera talaseae is being split into T. talaseae and T. atrigena, following the application of criteria set out by Tobias et al. (2010).

Prior to this taxonomic change Z. talaseae (BirdLife species factsheet) was listed as Near Threatened under Criterion C1 + C2a(i), on the basis that it was thought to have a moderately small population on three islands and was suspected to be declining at a slow to moderate rate owing to predation by introduced mammals. The pre-split species was very infrequently observed, though was clearly highly secretive and was thought likely to have been under-recorded. The population was estimated to be between 2,500 and 10,000 mature individuals.

Following the split, Z. talaseae is restricted to the mountains of Umboi (one record) and New Britain, where it is probably widespread, while Z. atrigena is found on the island of Bougainville, where it has only been recorded in the Crown Prince Range (Dutson 2011).

However, accessing the habitat within the elevational range of Z. atrigena is particularly awkward, and the lack of records likely reflects a lack of adequate effort. Consequently it is thought likely that the species occurs at considerably more than 10 locations*, when the major threat is considered to be habitat loss. It is also considered (as noted above) that Z. talaseae is most likely also under-recorded and actually widespread at the correct altitudes, although still exceptionally hard to see.

The population of Z. talaseae is estimated to number 2,500-9,999 individuals based on an assessment of known records, descriptions of abundance and range size. This is consistent with recorded population density estimates for congeners or close relatives with a similar body size, and the fact that only a proportion of the estimated Extent of Occurrence is likely to be occupied (in this case the area above 1,000 m). This estimate is equivalent to 1,667-6,666 mature individuals, rounded here to 1,500-7,000 mature individuals.

The population of Z. atrigena is estimated to number 1,000-2,499 individuals based on an assessment of known records, descriptions of abundance and range size. This is consistent with recorded population density estimates for congeners or close relatives with a similar body size, and the fact that only a proportion of the estimated Extent of Occurrence is likely to be occupied (in this case the area above 1,000 m). This estimate is equivalent to 667-1,333 mature individuals, rounded here to 600-1,500 mature individuals.

The threat from introduced predators does not seem to be directly applicable, following observations of congeners on Makira persisting alongside a large feral cat population (G. Dutson in litt. 2016). In the absence of clear evidence of any current impacts directly affecting the species their populations are considered likely to be stable at present.

Z. atrigena is suggested to be listed as Near Threatened on the basis that it may approach the thresholds for listing as Vulnerable under criterion D1, as it appears to have a very small population. If there is evidence that the population is considerably larger than estimated, then the species would be considered Least Concern. On the other hand, if there was evidence of a population decline then the species may qualify as Vulnerable or Endangered under criterion C2a(ii).

Z. talaseae is suggested to be listed as Least Concern on the basis that it does not appear to approach the thresholds for listing as Vulnerable under the D criterion, and the population is considered likely to be stable. If there is evidence that the New Britain population is smaller than estimated here, closer to 1,000 mature individuals, then the species would warrant listing under criterion D1. If there was evidence that the population was declining then the species may be considered for listing as Near Threatened under criterion C2a(i).

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:

Dutson, G. 2011. Birds of Melanesia: Bismarcks, Solomons, Vanuatu and New Caledonia. Christopher Helm, London.

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.

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.

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One Response to Archived 2016 topics: New Britain Thrush (Zoothera talaseae) is being split: list Z. atrigena as Near Threatened?

  1. James Westrip (BirdLife) says:

    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.

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