Archived 2016 topics: Black-browed Triller (Lalage atrovirens) is being split: list Biak Triller L. leucoptera as Near Threatened or Least Concern?

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.

Black-browed Triller Lalage atrovirens is being split into L. atrovirens, L. moesta and L. leucoptera, following the application of criteria set out by Tobias et al. (2010).

Prior to this taxonomic change, L. atrovirens (BirdLife species factsheet) was listed as Least Concern, as it was not thought to approach the thresholds for listing as Vulnerable under any of the criteria. The pre-split species was characterised as fairly common in New Guinea and common in Tanimbar, and utilises a wide range of typically disturbed forest, scrub and gardens (Taylor 2016).

L. atrovirens (as defined following the taxonomic change) is found throughout northern New Guinea, including Waigeo, Salawati and Misool. L. moesta is found on the Tanimbar Islands and L. leucoptera is found on Biak Island, Geelvink.

Biak Triller L. leucoptera inhabits forest and edge on Biak (Pratt and Beehler 2015). A new extent of occurrence for the species has been calculated as 3,600km2. Given the tolerance for disturbed habitat the considerable habitat clearance that occurred on Biak during the later decades of the 20th Century may not have severely impacted the species.

The population is estimated to number 10,000-19,999 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. This estimate is equivalent to 6,667-13,333 mature individuals, rounded here to 6,000-15,000 mature individuals.

Given that the species is found in relatively disturbed habitat it is not clear that there is any evidence of a continuing decline in the population. Should there be evidence to suggest that there was a continuing decline, then the species could potentially qualify for listing as Vulnerable under criterion C2a(ii). On a precautionary basis, it is suggested that the species be listed as Near Threatened, on the basis that it is thought to approach this threshold for C2a(ii).

However, if it there was evidence that the population was stable or increasing, then the species would not approach this threshold and the species would be Least Concern.

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

References:

Taylor, B. (2016). Black-browed Triller (Lalage atrovirens). 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 http://www.hbw.com/node/57890 on 6 October 2016).

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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2 Responses to Archived 2016 topics: Black-browed Triller (Lalage atrovirens) is being split: list Biak Triller L. leucoptera as Near Threatened or Least Concern?

  1. Guy Dutson says:

    L. leucoptera is relatively common and tolerant of degraded habitats on Biak

  2. 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.

Comments are closed.