Justification of Red List Category
This species was formerly recorded frequently on its non-breeding grounds but there has been a rapid attrition of sightings, with only a single probable record (in 2019) since 2015. It is therefore suspected to have declined extremely rapidly over the past ten years and is assessed as Critically Endangered. The loss and degradation of habitat in its wintering grounds (and possibly too its unknown breeding grounds) is thought to be the principal cause of its decline. Its breeding grounds are in urgent need of discovery as is the protection of any remaining marshlands that may contain suitable habitat for the species in the Philippines.
The population size is very difficult to estimate given the poor quality of historic data and the paucity of recent sightings. In 2016, the population size was set at fewer than 1,000 mature individuals (250-999) based on the paucity of records. This attrition of sightings has continued, with only a single probable record (in 2019) between 2016 and 2021. The population may therefore now be very small (<50 mature individuals), although there remains substantial uncertainty in this inference given the lack of surveys on its (thus far undiscovered) breeding grounds. The population is therefore suspected to number 1-999, although it is acknowledged that this represents little more than a best guess.
The trends of this species are poorly known due to the very few recent records, but it appears clear that the population has declined rapidly over the past few decades. The breeding grounds remain wholly unknown, such that trends can be derived only from the passage and non-breeding parts of its range. On passage in north-east China (in and around Beijing), the species has declined substantially since La Touche (1920) described it as "very common every early autumn" near Qinghuangdao (Hebei). There were several scattered sightings between 2005 and 2015: Peking University campus, September 2007; Yuanmingyuan, Haidian, September 2008; Wild Duck Lake, October 2008; Summer Palace, Haidian, June 2009; Olympic Forest Park, May 2011; Yanqing, June 2014; Dayunhe Forest Park, Tongzhou, September 2014; Liaotieshan, Dalian, Liaoning, September 2015 (T. Townshend in litt. 2021). Since then, there has been only a single probable (and undocumented) record—of an individual at the Shunyi Patch on 6 October 2019—despite greatly increased coverage of the region by birdwatchers over the past decade (T. Townshend in litt. 2021).
In the Philippines (its only known wintering area), this attrition of sightings and abundance has been mirrored. At Dalton Pass, Luzon, 351 individuals were ringed on passage in 1965-1970, with yearly maxima of 141 in 1965 and 127 in 1966 (McClure and Leelavit 1972) and several birdwatchers and ornithologists recorded multiple individuals—and did not regard it as especially uncommon—on Luzon in the 1980s and 1990s (Collar et al. 1999). Since then, a precipitous decline has seemingly ensued, thought to have principally been caused by habitat degradation and destruction (Collar et al. 1999, BirdLife International 2001). There have been no confirmed sightings in the Philippines since the ringing of an individual at Candaba Swamp (the only known wintering site) in 2009 among 123 Acrocephalus warblers (Round and Allen 2010), despite increased visitation by birdwatchers (eBird 2021).
Based on historic records of passage the breeding grounds are assumed to lie in north-east China (Kennerley and Pearson 2010) however a targeted search for Acrocephalus and Locustella warblers in 2019 in a hitherto unsurveyed area in northern Jilin and Heilongjiang failed to detect the species at all, despite this area being considered a prime candidate for its discovery (with a climate that well matches the species' unusually late migration phenology; see Townshend 2019) and surveyors covering a range of feasible habitats over 31 days. They detected (and therefore identified new sites for) other poorly known species including A. tangorum and L. pryeri (Guillaumet et al. 2021). A single sighting of a singing A. sorghophilus along the Amur River, Russia, in 2004 has not been replicated, despite a return to the same area in 2007 (Kennerley and Pearson 2010).
Given these trends in spite of increased coverage, the population is suspected to have declined rapidly (80-99%) over the past ten years and it is not definite that the a population of the species even persists, although further surveys across north-east China at likely localities is needed.
Acrocephalus sorghophilus has occurred on passage in Liaoning, Hebei, Hubei, Jiangsu, Fujian and Beijing in eastern China, and Taiwan, China, where there are eight confirmed records (Yang Liu in litt. 2007). The species is believed to winter in the Philippines, but there have been no records since 2009 (D. Allen in litt. 2016, Allen 2020). There are also previous winter records from Taiwan, China. In the Philippines it occurred at Candaba and has been trapped Dalton Pass (Luzon), although there have been no records from Dalton Pass since 1970 and the species has been very scarce at Candaba since the mid-1990s (P. Round et al. in litt. 2008), despite increasing observer effort (D. Allen in litt. 2012). The species continued to be absent at Candaba Marsh during surveys in 2017, with no further records in 2019 (Allen et al. 2019). It is presumed to breed in north-eastern China (probably in Hebei, Liaoning, Jilin and Heilongjiang provinces) and adjacent areas of Russia, namely the Amur region (Kennerley and Pearson 2010). There was a record of a singing male at Muraviovka (Russia) (P. Fabien in litt. 2004), although a subsequent search of the site failed to find any individuals (P. Leader in litt. 2007) and recent intensive fieldwork at Muraviovka is yet to record the species (W. Heim in litt. 2016).
On passage, it has been recorded in millet crops and marshland, and in winter it occurs in reed and grass marshes, often near water. It potentially uses willow scrub and reedbeds in its breeding range (Kennerley and Pearson 2010). It probably feeds largely on invertebrates, and may also consume seeds, although this requires confirmation (D. Allen in litt. 2012). Spring passage in China is from late May to early June, with autumn passage from late August to early September. All records in the Philippines are from September to June.
Habitat destruction on the wintering grounds is likely to be causing a decline. At Candaba, almost all marshland has been destroyed through conversion to rice cultivation and fishponds. In addition, local people there burn reeds and other native vegetation to encourage new shoots for livestock to graze on (P. Round et al. in litt. 2008). Uncultivated areas that in the past served as refuges for many migratory species, as well as an important site for ecotourism has in recent years overgrown with floating vegetation due to high cost of clearing (Allen et al. 2019). Open water areas are now limited whilst other areas continue to be cultivated for rice. The banks of Laguna de Bay are being occupied by settlers and factories so that the reedbeds are becoming highly fragmented and greatly reduced in area and at Bukal, Laguna, most reedbed has been drained for conversion to poultry-processing factories. The conversion of wetlands for agricultural use in north-eastern China may also be contributing to a population decline (Kennerley and Pearson 2010). The impacts of the extensive use of insecticides to reduce mosquito and other invertebrate populations may have been to considerably reduce food availability at critical times for the species.
Trapping birds for food and for sale as cagebirds is frequently conducted at reedbed sites and may have severely affected this species, particularly given the potentially extremely limited number of suitable sites that remain in the wintering range.
Conservation Actions Underway
CMS Appendix II. All bird species are legally protected in the Philippines. Candaba Marsh has been proposed as a Ramsar Site and education material has been prepared; however, most habitat there has now been converted (D. Allen in litt. 2012). It is also an Important Bird Area (IBA) (Allen et al. 2019). Surveys by a team from the Wild Bird Club of the Philippines (WBCP) and the Wetland Trust were conducted at Candaba in late April 2008 and March 2009, with the aim of locating the species; however, it was not recorded and the survey documented a lack of extensive suitable habitat (Round 2008, P. Round et al. in litt. 2008). The species had been recorded a few days before the survey, representing the first confirmed record there for seven years. A mist-netting survey was carried out at Candaba and other wetland sites in March 2009 in which one individual of this species was trapped and subsequently appears to have been the first published record of moult in A. sorghophilus (Round and Allen 2010). No records were found at Candaba in surveys conducted in 2017, with no further records in 2019 (Allen et al. 2019). An expedition to Dalton Pass in October 2009 did not record the species (P. Round per Sykes 2009). WCBP have been working to encourage the local government of Candaba to raise awareness amongst local communities and stop the burning of vegetation (M. C. Lu per P. Round et al. in litt. 2008).
12-13 cm. Small, lightly streaked warbler. Pale buff upperparts with faint dark streaking on mantle and scapulars and bright buff rump, tinged rufous. Faintly streaked crown with prominent black lateral crown-stripe over broad creamy-buff supercilium. Buff-ochre underparts with whiter throat and belly. Similar spp. Black-browed Reed-warbler A. bistrigiceps has uniform olive-brown upperparts, lacking streaking on crown, mantle and scapulars.
Text account compilers
Berryman, A., Martin, R.
Allen, D., Bajarias, M., Benstead, P., Crosby, M., Fabien, P., Gilroy, J., Heim, W., Jensen, A., Johnson, R., Leader, P., Mahood, S., Peet, N., Round, P., Taylor, J., Townshend, T. & Yang, L.
BirdLife International (2022) Species factsheet: Acrocephalus sorghophilus. Downloaded from http://www.birdlife.org on 04/10/2022. Recommended citation for factsheets for more than one species: BirdLife International (2022) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 04/10/2022.