Archived 2016 topics: Eurasian Scaly Thrush (Zoothera dauma) is being split: list Z. major 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.

Sri Lanka Scaly Thrush Z. imbricata is being lumped into Z. dauma. Following this change, Z. dauma is being split into Z. dauma, Z. major and Z. aurea, following the application of criteria set out by Tobias et al. (2010).

Prior to this taxonomic change, Zoothera dauma (BirdLife species factsheet) was listed as Least Concern, on the basis that it did not approach the thresholds for Vulnerable under any criteria. Z. imbricata (BirdLife species factsheet) was listed as Near Threatened under criteria B1ab(ii,iii,iv,v) and C2a(ii), on the basis that the population was thought to be moderately small and the species was thought to be declining owing to the ongoing loss and degradation of evergreen forest within the Sri Lank Wet Zone. Z. dauma was previously described as uncommon to fairly common across much of its previous range; Sri Lanka Scaly Thrush was considered uncommon.

Following the split, Z. dauma is found from the west Himalayas east to Assam and south central China, N Myanmar, N and W Thailand and N Indochina, moving in the non-breeding season into the Himalayan foothills, the lowlands of the northeast Indian subcontinent, south China and south east Asia. Z. aurea breeds from eastern European Russia east to north Mongolia and the Sea of Okhotsk, Amurland and Sakhalin south to Korea, Japan and the south Kurils, then disjunctly in the hills of southwest India and in the hills of south west Sri Lanka and also the Greater and Lesser Sundas. Northern breeders are strongly migratory, wintering in northern south east Asia, southern China, Taiwan, Japan and north and west Philippines. Z. major Amami Thrush is restricted to the island of Amami in the northern Ryukyu Islands, it was also previously known from Kakeroma-jima adjacent to Amami-ooshima but is considered to have become extinct there.

Both Z. dauma and Z. aurea are not believed to approach the thresholds for listing as Vulnerable under any criteria, and are therefore considered Least Concern.

Z. major has previously been recognised as a separate species, and has previously been assessed as Critically Endangered due to a single, tiny declining population as a result of deforestation for timber, perhaps compounded through predation by an introduced mongoose. The population size at this time was reported as 58 mature individuals based on a 1996 estimate (Amami Ornithologists’ Club [AOC] 1997). Further surveys of singing birds resulted in an estimate of 74 pairs in 1999 (Khan and Takashi 2007) and subsequently the population appears to have been showing a gradual recovery with 502 singing birds recorded during surveys in 2013 (Mizuta et al. 2016). However no attempt was made to extrapolate to the full extent of suitable habitat, and doing so indicated that the population was actually 945-1,858 prior to 2012 (Mizuta et al. 2016). Detection rate (birds per point count) between 2007 and 2012 was roughly similar at approx. 1.2, but the 2013 survey returned 1.7 birds per point. The increased detection of birds in 2013 indicated the population may be 2,513 mature individuals in that year (Mizuta et al. 2016), though this appears a remarkable increase in the population within a single year. Efforts to control the introduced mongoose population caused a dramatic reduction in the density of mongoose, particularly after 2010, suggesting that predation may have been affecting the population (Mizuta et al. 2016). Several other endemic species on Amami were demonstrated to show increased populations as a result of decreased mongoose density following eradication efforts (Watari et al. 2013).

Therefore the population of Amami Thrush Z. major is not considered to be declining at present, and hence cannot be considered for listing under any criteria other than D. The estimated population size would indicate that it exceeds the thresholds for listing under this criterion. But the clear evidence of the impact of mongoose on the population previously indicates that until mongoose is eradicated from the island there remains the threat that the population could become threatened within a short time frame. Consequently it is proposed that Amami Thrush Z. major is listed as Near Threatened as it is believed to approach the thresholds for listing as Vulnerable under criterion D.

If the plausible future threat, the Small Indian Mongoose population, is entirely removed, then the species is likely to be eligible for listing as Least Concern, despite the restricted range.

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

References:

Mizuta, T., Takashi, M., Torikai, H., Watanabe, T. & Fukasawa, K. 2016. Song-count surveys and population estimates reveal the recovery of the endangered Amami Thrush Zoothera dauma major, which is endemic to Amami-Oshima Island in south-western Japan. Bird Conserv. Int. 1–13. doi:10.1017/S095927091600023X

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.

Watari, Y., Nishijima, S., Fukasawa, M., Yamada, F., Abe, S., Miyashita, T., 2013. Evaluating the “recovery level” of endangered species without prior information before alien invasion. Ecol. Evol. 3, 4711–21. doi:10.1002/ece3.863

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One Response to Archived 2016 topics: Eurasian Scaly Thrush (Zoothera dauma) is being split: list Z. major as Vulnerable?

  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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