Data Zone
Justification of Red List category
This species has been uplisted to Vulnerable on the basis that increasing human disturbance, dam construction, waterbody management, and predation are causing a rapid population decline. Whilst the situation is more concerning in Southeast Asia despite its marginal occurrence here, localised declines also continue to occur across India, of which currently holds the majority of the population. Comprehensive monitoring is however required to limit uncertainty in population trends.
Population justification
A previous Asian Waterbird Census' observed 5,733 birds during 1987-1992, 5,558 during 1993-1997, 9,963 during 1998-2002, and 7,578 during 2003-2007 across some Indian and Pakistan sites (Li et al. 2009). Further surveys for the 2008-2015 period then observed across all of Asia and Australasia found an overall 29,577 birds across Myanmar (39 birds), Thailand (2), Bangladesh (74), India (17,776), Nepal (31), Pakistan (11,485) and Cambodia (170) (Mundkur et al. 2017). Under-counting is however thought to have been a major factor. Populations in the Southeast Asian part of its range are however very small and at higher risk of extinction (F. Goes in litt. 2020). In the whole Mekong basin for example, the population is currently estimated at less than 100 birds (F. Goes in litt. 2020). Using the best available information therefore, the population is thought to number at least 29,500 individuals, with the potential to reach a maximum of 100,000 individuals, assuming some surveys may be under-counted. Using a rounded population size of 30,000-100,000 individuals therefore, the population is placed in the band of 20,000-70,000 mature individuals.
Trend justification
Precipitous declines have occurred in parts of South-East Asia, whilst local increases and stability have been noted in a few areas of India, making the deciphering of past population trends difficult. Recent estimates in India show that the long-term trend of the species amounts to 41.2% decline (measured as an index of abundance in 2014/2015 relative to pre-2000 data; State of India's Birds 2020). An average annual change estimated at 5.2% (measured between 2014/2015 and 2018/2019; State of India's Birds 2020), additionally translates to over 70% decline over a 3-generation period (23.4 years; Bird et al. 2020). In China however, following historical declines, the species seems to be in recovery on at least one main site, although this is recognised as being only a small proportion of the species's overall range (W. Cheng in litt. 2020). Declines may however still be present on some riverine habitats, with certain sites known to be deteriorating and under continuous population decline (X. Zheng in litt. 2020). In Cambodia, the species is experiencing a continuing decline, with >80% reduction observed in the past 20 years (Claassen 2018, J. C. Eames in litt. 2020). It was also recorded to have undergone a strong decline during surveys in 2017-2019 at Ayeyarwady River between Myitkyina and Bagan, Myanmar (Zöckler 2020). Similarly on a tributary in the Chindwin River, a 90% decline had been observed since 2004 (C. Zöckler in litt. 2020). Reports to the International Waterbird Census (IWC) have been declining in the last decades, with an all-time maximum of 10,011 individuals in 2001, to 5,999 in 2010 and 3,944 in 2016 (IWC 2020, T. Mundkur in litt. 2020). This equates to a 76.6% decline over a 3-generation period, assuming the population had undergone an exponential decline since at least 2001.
However, based on recognition that previous methodologies in observing population trends for sandbank, riverine species as unreliable, primarily due to surveys being undertaken only in smaller wetlands (unrepresentative of larger areas), only in wintering seasons (excluding numbers at breeding sites), with large variability in coverage, and biased monitoring (S. Subramanya in litt. 2020, G. Sundar in litt. 2020, S. Quader in litt. 2020, S. Chowdhury in litt. 2020, Praveen. J in litt. 2020), the overall decline of the species is thought to be not as exaggerated as previously considered. The overall population also remains widespread and large (G. Sundar in litt. 2020). Nonetheless, precautionarily assuming that the species is declining across some sites in India (of which holds core breeding colonies and has undergone a long-term decline exceeding 40% per State of India's Birds [2020]), as well as declines in other peripheral sites across Myanmar, Cambodia, China, Nepal, and Bangladesh (A. Claassen in litt. 2020), it is considered that the population is undergoing an estimated rapid decline of 30-49% over a 3-generation period. The species is also expected to undergo a reduction over the next 3 generations, as threats, such as dam construction and disturbance of breeding sites, and predation, increase in prevalence.
This species occurs along river systems across a wide range in southern and south-east Asia, being found in Pakistan, India, Bangladesh, Myanmar, Thailand, Cambodia, and southern China (Yunnan), as well as Nepal, Bhutan, Laos, and Vietnam during the non-breeding season ([T. Mundkur in litt. 2020], del Hoyo et al. 1996), with vagrant records from Iran and Afghanistan, although it is generally resident over most of its range. Whilst the population in India, Bangladesh (albeit has become rare here), Pakistan, Myanmar and Cambodia are largely resident with some seasonal movement, the species will have additional local movement in India, often migrating to Nepal (considered rare) and Bhutan during the non-breeding season, with breeders from southwest China migrating to northern Laos (T. Mundkur in litt. 2020).
It has also reportedly declined in abundance in Thailand, where it is now considered very rare (del Hoyo et al.1996). The species has declined along the Mekong River, in Laos since the early 20th century (Thewlis et al. 1998), and was very close to being extirpated from the country (W. Duckworth in litt. 2011). It has also been undergoing a strong decline along the Mekong River system in Cambodia since 2003, with extirpation or sharp declines noted in the number of pairs throughout its breeding range since 2007 (Goes 2014, F. Goes in litt. 2016). Specific conservation actions, alongside nest protection and exclosure have however slowed down decline rates and stabilised the core Mekong population (Classern 2018, F. Goes in litt. 2020).
It however no longer breeds on the Tonle Sap, the Mekong below Kratie, or Sesan River (Goes 2014, Claassen 2018). In Cambodia the species has declined by >80% in the past 20 years and in 2018, the population was estimated to be 54-62 individuals whilst less than 50 birds are thought to exist currently (Claassen 2018, S. Mahood in litt. 2020). Moreover, only 2-3 pairs are thought to occur at Sekong River in Siem Pang (J. C. Eames in litt. 2020). Two nests were observed in May 2019 at Khang Lok Krau, albeit only one was observed 3 days later due to rat predation (Mittermeier et al. 2019). During the 2017-2019 period overall, further records showed only one pair that nested on a particular river stretch in Myanmar (with nesting deemed unsuccessful due to flooding in 2019), a decrease from 65-69 birds observed during the 2003-2004 period (C. Zöckler in litt. 2020).
In China, the species occurs along two river systems, including the Dayingjiang River in Southwestern Yunnan and the Yarlung Zangbo River in Southeastern Tibet (W. Cheng in litt. 2020). The species is described as uncommon along the Dayingjiang river in south-western Yunnan (Yang Liu in litt. 2011, Dasgupta 2019). The number of species present on the Dayingjiang River had declined from 30 birds to less than 5-13 between 2010-2019, with only 5 individuals recorded in early 2020; 11 individuals are however currently thought to reside on this river temporarily after one pair successfully raised two clutches, harbouring 3 juveniles each (X. Zheng in litt. 2020, W. Cheng in litt. 2020), suggesting temporary recovery of the population here. 3 individuals have additionally been sighted on a tributary of the Yarlung Zangbo River in 2018. It may also occur in small numbers on the Ruili River, between the border of China and Myanmar (H. Fen-Qi in litt. 2020). In Pakistan, the species is common throughout most small and large wetlands, subjected to only selective declines in particular sites (Z. Shaikh in litt. 2020).
The species has greatly declined to a few pairs that may breed at few sites on the Ayeyarwady and Chindwin rivers in Myanmar (Zöckler 2020, A. Claassen in litt. 2020), with only one breeding site in Bangladesh (Chowdhury et al. 2014). Recent sightings at Padma River in the Rajshahi Division are confirmed however, with 10-20 pairs observed continuously over the last decade on the Bangladeshi coast (S. Chowdhury in litt. 2020). Only one pair was observed near Bhamo on the Ayeyarwady River between Myitkyina and Bagan during 2017-2019 (Zöckler 2020). This is after 81 birds observed in the early 2000s. More than 40 birds were although recorded at Chindwin River in recent surveys (2018-2019; C. Zöckler in litt. 2020). In Nepal, the species is now a rare and very local visitor, with a maximum population of 20 individuals estimated in 2016, having rapidly declined since the 1990s (Inskipp et al. 2016). The species is only regularly recorded from two protected areas here; Bardia National Park and Koshi Tappu Wildlife Reserve (K. Bhusal in litt. 2020, R. Chaudhary in litt. 2020).
In contrast to declines noted in South-East Asia, the species is now more regular in southern India than was once thought, having probably benefited from the development of reservoirs (Praveen J. in litt. 2012). At Pong Dam hydro-electric power station for example, 384 nests were observed in 2006 (with 126 nests observed on Ranseer Island), 400 nests in 2015, with an average sighting record of 800-1,000 birds every winter (S. Balachandran in litt. 2020). 22 nests were recorded in 2016 at a sandbar near Mundali Dam, downstream of the Mahanadhi (Odisha state; S. Balachandran in litt. 2020). Likewise, the species is described as having increased in Andhra Pradesh over the past 10 years (S. Riyazuddin in litt. 2012).
Encroachment of wetlands by cities and developments have however led to drastic declines in altered wetlands since the late 1990's (S. Subramanya in litt. 2020). At Chilika Lake (Odisha state), despite previously harbouring c. 1,500 birds and particularly 300-400 birds on the Nalabana Island (located within the lake), the species has seen marginal declines here for example; 540 nests were found in 2002, with declines beginning in 2003, leading to only 200 nests in 2008, and subsequently below 50 nests in 2010, and complete abandonment of the island by 2011 (S. Balachandran in litt. 2020). 20-30 nests have since been observed on a nearby, unprotected island (Panchgudhi; S. Balachandran in litt. 2020). Declines (likely attributed to fishing camps) have also been observed in sandy islands and tracts in Brahmaputra, Subansiri, Lohit and Dibang (A. Choudhury in litt. 2020). Alternatively, 11,000 nests were recently found at Almatti Backwaters in Bagalkot, Karnataka over 12 exposed sandy islands (S. Subramanya in litt. 2020). Despite some localised declines therefore, the species is overall considered to be widespread in large numbers across many artificial and natural wetlands in India (G. Sundar in litt. 2020).
It inhabits rivers and freshwater lakes, also occurring rarely on estuaries, and breeds on sandy and rocky islands, especially along river banks (Mundkur 1991, del Hoyo et al. 1996). The species is known to be breed in high concentrations in large reservoirs and rivers, as well as natural and artificial wetlands in India (G. Sundar in litt. 2020). Some sandbanks may also be used for nesting due to temporary exposure after irrigation and water management from nearby reservoirs (S. Subramanya in litt. 2020). It has been recorded up to 600 m in Nepal. It feeds predominantly on fish, small crustaceans, insects, and frogs (Mundkur 1991). Breeding occurs mainly in February-May, although the season may extent from November to May (del Hoyo et al. 1996, Mundkur 1991).
Nesting areas are vulnerable to flooding, predation and disturbance (del Hoyo et al. 1996). In a 2017-2019 study in Odisha, India, flooding, predation and cyclones were the main threats (Debrata 2019, Kar et al. in prep). Local fishermen also report that children collect eggs and chicks from sandbanks, which may pose a substantial threat to ground-nesting birds. Strangling of chicks in discarded fish nets is being recorded at different areas and may pose a growing risk (Narwade and Fartade 2013, Reshamwala 2017). Sand and gold mining is an emerging threat for breeding colonies of Indian Skimmer, with which the River Tern nests and is thought to have a long-term effect on nesting-island formation (Das 2015, Shaikh et al. 2018, Debata et al. 2019, C. Zöckler in litt. 2020). Poor water level management along rivers are additionally affecting the stability of colonies (T. Mundkur in litt. 2020). In several places in India, human disturbance, encroachment of wetlands by cities, disturbance by photographers and birdwatchers, egg and chick trampling by domestic water buffalo and Chital deer is common (Amoghavarsha 2013, S. Subramanya in litt. 2020). Future plans for water transportation routes on the Ayeyawady, and dredging on the Chindwin River for commercial and passenger transport may further create localised threats (C. Zöckler in litt. 2020). Predation of eggs and chicks by a variety of terrestrial animals including domestic dogs, Lesser Bandicoot Rat (Bandicota bengalensis), Monitor Lizard (Varanus salvator; Mundkur 1991), and avian predators including House Crow (Corvus splendens; Siddiqui et al. 2007), Black-headed Ibis (Threskiornis melanocephalus; Parimala 2013, Reshamwala 2017), Pallas’s Fishing Eagle (Haliaeetus leucophus; Lowther 1949) and Bonelli's Eagle (Aquila fasciata; Belur 2014) may be taking their toll. Invasive plants such as Mimosa pigra may also reduce nest success and chick survival (Claassen et al. 2018).
The negative population trend in Laos is probably due mainly to excessive human disturbance on sandbars (Thewlis et al. 1998). In Nepal, it is threatened by food shortage, illegal and over-fishing in protected areas, destruction of breeding habitats on rivers, disturbance and hunting (K. Bhusal in litt. 2020). In Cambodia, a study of breeding colonies in 2010-2015 found that egg collection, predation by rodents, crows and domestic dogs and trampling by domestic water buffalo were responsible for low breeding success (Claassen et al. 2017, J. C. Eames in litt. 2018). The study found very little evidence of direct hunting of the species, although a snare was once found at a nest (A. Claassen in litt. 2016). The multitude of dam construction projects completed, underway or planned in South-East Asia (e.g. along the Mekong river [F. Goes in litt. 2011]) may also threaten the species through changes to flow regime and flooding of nest-sites. Dams along the Sesan river in Vietnam have precipitated the decline and led to near-extirpation of the species downstream in Cambodia (F. Goes in litt. 2020). Its habitat may be threatened by the construction of dams in the Dayingjiang region of south-western Yunnan (Yang Liu in litt. 2011, Dasgupta 2019). In Nepal it is severely threatened by food shortages owing to illegal fishing inside protected areas and overfishing outside protected areas, it is also threatened by destruction of breeding habitat, disturbance and hunting (Inskipp et al. 2016). In Myanmar, planned developments may potentially threaten the species in the future (C. Zöckler in litt. 2020). Breeding sites along the Indus River (passing between the plains of Punjab and Sindh provinces) are threatened with encroachment and conflict, as well as agricultural expansion leading to increases in human settlements and live stock (M. Ali in litt. 2020).
Conservation Actions Underway
The species is protected under Schedule-IV of the Indian Wildlife Protection Act of 1972 (State of India's Birds 2020). The species is considered At Risk in Laos, Critical in Thailand, Critical in Cambodia (Goes 2014) and Critically Endangered in Nepal (Inskipp et al. 2016). The species occurs in a number of protected areas. In Cambodia, nest protection programmes have been implemented by the World Wildlife Fund on the Mekong River and by the Royal University of Phnom Penh on the Sekong and Sesan rivers (A. Claassen in litt. 2016). These programmes involve education, employing local people as nest guards and the use of predator exclosures (A. Claassen in litt. 2016). Local conservation efforts is thought to enable temporary recover of the species on Dayingjiang Rive (W. Cheng in litt. 2020). In Cambodia, a Species Action Plan has been prepared for 2018-2028 (Claassen 2018), however this currently lacks funding to be implemented properly (F. Goes in litt. 2020). In Myanmar, a nest protection programme is being implemented on the Ayeyarwady and Chindwin rivers by the Wildlife Conservation Society (A. Claassen in litt. 2020). Community Conservation Areas (CCA's) are also proposed to maintain the last remaining sites along the Ayeyarwady River (Zöckler 2019). In China (Yunnan), nest protection is being led by the Kandoorie Farm and Botanic Garden (A. Claassen in litt. 2020, X. Zheng in litt. 2020). Direct payments have additionally proven successful in implementing nest protection, however require continued use of monitoring, community relations and exclosures (Claassen et al. 2017). Implementation plans are additionally in place in order to declare Almatti, India as a Conservation Reserve (S. Subramanya in litt. 2020).
Conservation Actions Proposed
Carry out regular surveys to monitor the population and seasonal movements throughout its range. Implement appropriate counting methods in larger waterbodies to avoid bias and unreliable surveys (S. Subramanya in litt. 2020). Conduct education activities to help alleviate human pressures on river and lake habitats. Lobby against high-risk dam projects, especially in South-East Asia. Maintaining sufficient water levels around breeding sites or restoring the flow regimes to prevent loss of nesting islands during the breeding season (as rivers in India are under tremendous pressure of water demand; hence all future water-supply projects are subjected to cumulative impact assessments, including their impact on River Tern and other riverine ground nesting waterbirds; T. Mundkur in litt. 2020). Introduce nest protection measures where nesting sites are vulnerable to disturbance. Strengthen the nest protection program in Cambodia and secure sufficient funds to effectively conserve the core population of the Mekong area (F. Goes in litt. 2020). Protect river watersheds, minimise pollution to wetlands, and regulate fishing practices whilst promoting sustainable harvests (C. Inskipp and H. S. Baral in litt. 2020). Conduct research on the foraging ecology of River Tern in order to understand possible competition with local fishermen (T. Mundkur in litt. 2020).
Text account compilers
Fernando, E.
Contributors
Ali, M., Ashpole, J, Balachandran, S., Baral, H.S., Bhusal, K., Butchart, S., Chaudhary, R., Cheng, W., Choudhury, A., Choudhury, S., Claassen, A., Duckworth, W., Eames, J.C., Ekstrom, J., Fen-qi, H., Goes, F., Inskipp, C., Mahood, S., Mundkur, T., Praveen, J., Quader, S., Riyazuddin, S., Shaikh, Z., Subramanya, S., Sundar, G., Taylor, J., Thewlis, R., Yang, L., Zheng, X. & Zöckler, S.
Recommended citation
BirdLife International (2024) Species factsheet: River Tern Sterna aurantia. Downloaded from
https://datazone.birdlife.org/species/factsheet/river-tern-sterna-aurantia on 23/12/2024.
Recommended citation for factsheets for more than one species: BirdLife International (2024) IUCN Red List for birds. Downloaded from
https://datazone.birdlife.org/species/search on 23/12/2024.