Archived 2017 topics: Large-billed Reed-warbler (Acrocephalus orinus): list as Least Concern?

This enigmatic reed-warbler (BirdLife Species Factsheet) has confused many since the initial specimen was collected in 1867 in Himachal Pradesh. To date it has been considered that there is too little information to conduct a robust assessment of the threat status of the species, hence it is currently listed as Data Deficient. Following the rediscovery of the species in Thailand, diligent fieldwork and reassessment of museum specimens have greatly increased our understanding of the distribution and habits of the species, such that an assessment of its Red List status is possible.

There are now several reports of individuals trapped in apparent wintering areas, including five records in Thailand involving four individuals (Nimnuan and Round 2008, P. Round in litt. 2016). The first individual trapped at Laem Phak Bia in 2006 was remarkably retrapped at the same site in 2008; and there have subsequently been birds trapped in northern and central Thailand. Birds have additionally been recorded in northern Bangladesh (Round et al. 2014) and in mangroves in the Sundarbans in West Bengal, India (K.S. Ray and B. Das in litt. 2009). Therefore, although clearly scarce, it appears that the species winters in lowland wetland sites from the Sundarbans to Thailand and perhaps elsewhere in south-east Asia.

Regarding its breeding distribution, it has been established that the species currently breeds in the larger valleys of the western Pamir mountains in both Afghanistan and Tajikistan (Timmins et al. 2009, Ayé et al. 2010, Kvartalnov et al. 2013). Svensson et al. (2008) and Koblik et al. (2011) unearthed a total of 24 new specimens mislabelled as Blyth’s Reed-warbler, allowing the identification of the potential extent of the breeding range as being from western Afghanistan, Tajikistan, Kyrgyzstan, south and south-west Kazakhstan and adjacent eastern areas of Xinjiang, China. However, recent searches in Chinese valleys close to the Kyrgyzstan and Tajikistan border have so far failed to locate the species (R. Ayé in litt. 2016), and the most recent Kazakhstan record is from 1926 (Koblik et al. 2011). Thus it appears that the present breeding distribution is a limited to a relatively small area, and the present estimated Extent of Occurrence of 294,000 km2 is too large. Even so, it still likely exceeds the threshold for listing under the geographic range size criterion (<20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation).

Within the valleys that birds have been found it is reported to be common (Timmins et al. 2009, Ayé et al. 2010, Kvartalnov et al. 2013), and the widespread nature of the wintering records also suggests that the global population is not especially small, or at least larger than suggested from the limited number of historical records. However, estimating the population is difficult, especially as the actual extent of the suitable breeding habitat is unclear, and may be relatively restricted even within the valleys in which it occurs (Timmins et al. 2009). Genetic analysis has demonstrated considerable intraspecific variation from across the range suggesting declining or stable populations (Svensson et al. 2008), and the discovery of much of this variation within a relatively small breeding area suggests that there is now a single subpopulation that may have coalesced following the removal of previous geographic barriers (Koblik et al. 2011), e.g. glacial retreat. The haplotype analysis (Svensson et al. 2008) was based on specimens that span a time period in excess of a hundred years and no sudden decline events could be inferred: more likely the population has been largely stable with a slow decline over the whole period.

Habitat loss due the clearance and conversion of riverine areas for agriculture and livestock grazing, coupled with clearance for fuelwood have been identified as the most significant threats to the species within the breeding range (Timmins et al. 2009). However, much habitat remains, especially in the Wakhan Corridor and the impact of livestock and firewood collection on the species is unknown.

Despite the relatively restricted area in which the species is known to breed, the apparent high densities within suitable habitat and the likely suitable area indicates that the global population size exceeds the threshold for listing under Criterion C (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). Also, while there is evidence to suggest that the population is undergoing a slow decline, this is not thought likely to approach the thresholds for listing under the population trend criterion (>30% decline over ten years or three generations). As such, the present information suggests that the species does not approach the thresholds for Vulnerable under any of the Red List Criteria. It is therefore proposed to list Large-billed Reed-warbler as Least Concern.

We welcome any comments regarding this proposed listing. Any information that may imply that the species is likely to be severely impacted by current threats (such as changes to the hydrological regime within the breeding areas) may imply that the species is undergoing a more rapid decline than suspected. Additionally, if there is sufficient concern that the population may approach this threshold of 10,000 mature individuals the species may be best treated as Near Threatened; and any evidence that the global population is below 10,000 mature individuals would mean that the species would instead qualify as Vulnerable under Criterion C2a(ii).

 

References

Ayé, R.; Hertwig, S. T.; Schweizer, M. 2010. Discovery of a breeding area of the enigmatic Large-billed Red Warbler Acrocephalus orinus. Journal of Avian Biology 41(4): 452-459.

Koblik, E. A.; Red’kin, Y. A.; Meer, M. S.; Derelle, R.; Golenkina, S. A.; Kondrashov, F. A.; Arkhipov, V. U. 2011. Acrocephalus orinus: A Case of Mistaken Identity. PLoS ONE 6(4): e17716. doi:10.1371/journal.pone.0017716

Kvartalnov, P.; Abdulnazarov, A.; Samotskaya, V.; Poznyakova, J.; Ilyina, I.; Bannikova, A.; Solovyeva, E. 2013. Nesting of the Large-billed Reed Warbler Acrocephalus orinus: a preliminary report. Forktail 29: 37-42.

Nimnuan, S.; Round, P. D. 2008.  Further Thai records of Large-billed Reed Warblers. Acrocephalus orinus. BirdingASIA 9: 10

Round, P. D.; Ul Haque, E.; Dymond, N.; Pierce, A. J.; Thompson, P. M. 2014. Ringing and ornithological exploration in north-east Bangladesh wetlands. Forktail 30: 109–121.

Svensson, L.; Prys-Jones, R.; Rasmussen, P. C.; Olsson, U. 2008. Discovery of ten new specimens of Large-billed Reed Warbler Acrocephalus orinus, and new insights into its distributional range. Journal of Avian Biology 39(6): 605-610.

Timmins, R. J.; Mostafawi, N.; Rajabi, A. M.; Noori, H.; Ostrowski, S.; Olsson, U.; Svensson, L.; Poole, C. M. 2009. The discovery of Large-billed Reed Warblers Acrocephalus orinus in north-eastern Afghanistan. BirdingASIA 12: 42-45.

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3 Responses to Archived 2017 topics: Large-billed Reed-warbler (Acrocephalus orinus): list as Least Concern?

  1. Philip Round says:

    There is still relatively little information on Large-billed Reed Warbler. There are no reliable estimates of population size or of the extent of available breeding habitat. While non-breeding season records may be widespread, they are still few in number, given the increase in mist-netting in reedbed habitats in S and SE Asia. Actual regular wintering areas are still unknown. Additionally, good quality reedbed habitats themselves are few and highly fragmented in the possible winter range of the species and are being rapidly lost due to agricultural and aquacultural intensification. I oppose listing this species as “Least Concern”. It can surely be no less threatened than Acrocephalus tangorum (currently listed as “Vulnerable”).

  2. Andy Symes (BirdLife) says:

    Preliminary proposals

    Based on available information, our proposal for the 2017 Red List would be to pend the decision on this species and keep this discussion open until 2018, while leaving the current Red List category unchanged in the 2017 update.

    There is now a period for further comments until the final deadline of 4 August, after which the recommended categorisations will be put forward to IUCN.

    Final 2017 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. Raffael Ayé says:

    Sorry I wasn’t able to comment before the first deadline. I very much agree with Phil’s comment.
    We should not optimistically assume things that we don’t know. I do not think it is justified upon published information to call western Afghanistan or China part of the species’ breeding range. It may be more or less likely, but we don’t know.
    Based on the papers cited above, namely Timmins et al. 2009, Ayé et al. 2010, Koblik et al. 2011 and Kvartalnov et al. 2013, the area where the species has been recorded in the breeding season since the 1960ies (or the 1940ies, because there are no records in the 1940ies and 1950ies) is pretty small. It consists of two sites in far north-eastern Afghanistan (Zebak and Goz Khun) and a few places in western Gorno-Badakhshan Autonomous Region (GBAO) of Tajikistan. The easternmost places in GBAO are Langar (less than 2km from Goz Khun, just across the border) and Dehmiona or Marj/Mardz. The northernmost is in the Vanj valley. Connecting all these dots (=distribution range, B1) gives you an area far below 20’000km2.
    The suitable habitat is extensive in the sense that in some places there is 1km2 (and a little more) of continuous riverine bushland. However, the habitat is quite restricted in the sense that there are also stretches of maybe 40-50km of river with hardly any suitable habitat. As a consequence, also the effective area of occurrence (judged by the availability of prime habitat) is likely to be far below 2000km2 (and also below 500km2) on a 2x2km grid according to current knowledge.
    With these surfaces in mind, I am also not sure whether the population exceeds 10’000 mature birds.
    In consequence the criteria a to c of both B1 and B2 (and probably also C1 and C2) are relevant for the RL assessment. And I think we just do not have the data to decide on these criteria a to c. Maybe the species belongs in DD for the time being…

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