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).
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.