River Tern (Sterna aurantia) breeds on sandbars and islands along large rivers and reservoirs across south and south-east Asia. The global population is estimated at 50,000-100,000 individuals (Wetlands International 2012), which roughly equates to 30,000-70,000 mature individuals. River Tern is facing a multitude of threats: its nesting areas are susceptible to flooding and cyclones (del Hoyo et al. 1996, Debata 2019). Eggs and chicks are predated by terrestrial animals and avian predators (Mundkur 1991, Siddiqui et al. 2007, Reshamwala 2017), or are caught in abandoned fish nets (Narwade and Fartade 2013).
The species is currently listed as Near Threatened under Criterion A3c (BirdLife International 2020), due to an expected moderate decline driven by increasing human disturbance, sand mining and dam projects over the following three generations. Alongside other species breeding in riverine habitats across Asia, such as Indian Skimmer (Rynchops albicollis), there is concern that the rate of population reduction has been greater than previously suspected and that the species is now declining at a rapid rate. Declines have been reported across much of the range, including predicted imminent extinction in Cambodia (Claassen 2018) and reductions to small numbers in Myanmar (Zöckler 2019, A. Claassen in litt. 2020), Bangladesh (Chowdhury 2014) and China (Dasgupta 2019). As such, the species may warrant listing at a higher threat category, and is therefore reassessed here against all Red List Criteria.
Criterion A – The species, in common with other sandbar nesting riverine species in Asia, is undergoing rapid declines. In Cambodia, the population has declined by more than 80% in the past 20 years to a population estimated at just 54-62 individuals (Claassen 2018). In Myanmar, only a single pair was recorded along the Ayeyarwady in recent years, in contrast to more than 60 little more than a decade ago (Zöckler 2019, C. Zöckler in litt. 2020). Further declines have been recorded in Thailand, Laos, China and Nepal (del Hoyo et al. 1996, Thewlis et al. 1998, W. Duckworth in litt. 2011, Yang Liu in litt. 2011, Inskipp et al. 2016, Dasgupta 2019).
Numbers reported to the International Waterbird Census (IWC) database declined from 10,011 individuals in 2001 to 5,999 individuals in 2010 and to 3,944 individuals in 2016 (T. Mundkur in litt. 2020). Assuming an exponential decline, this equates to a reduction of 76.6% over three generations (23.4 years; Bird et al. 2020)*. If we take observational records to the IWC database as a proxy of the population size, the population would be declining at a rate of 70-79% since at least 2001. If we further assume that population declines continue at a similar rate, River Tern may be listed as Endangered under Criterion A2bcd+3bcd+4bcd.
Criterion B – The Extent of Occurrence (EOO) is estimated at 9,730,000 km2. This does not approach the threshold for listing the species as threatened under Criterion B1. River Tern is therefore assessed as Least Concern under this criterion. The Area of Occupancy (AOO) has not been quantified according to IUCN Guidelines (see IUCN Standards and Petitions Committee 2019), and thus River Tern cannot be assessed against Criterion B2.
Criterion C – The species’s population size has been estimated at 50,000-100,000 individuals, roughly equivalent to 30,000-70,000 mature individuals.This does not approach the threshold for listing as threatened under Criterion C. River Tern is therefore assessed as Least Concern under this criterion.
Criterion D – The population size and range are too large to warrant a listing as threatened under Criterion D, and thus River Tern is considered Least Concern under this criterion.
Criterion E – To the best of our knowledge no quantitative assessment of the probability of extinction has been conducted for this species, and so it cannot be assessed against this criterion.
Based on the above assessment, it is proposed to list River Tern (Sterna aurantia) as Endangered under Criterion A2bcd+3bcd+4bcd. We welcome any comments on the proposed listing and specifically ask for information on the timeframe of declines: Data from IWC suggest a steady decline at a rate of 50-79% over three generations since at least 2001. Has the rate of decline increased gradually before that? Was the population stable or decreasing at <25% over three generations during 1988-1992? Have declines increased to 25-29% over three generations during 1992-1996, qualifying the species for listing as NT, and then increased to 30-49% over three generations during 1996-2000, qualifying for a listing as VU? Alternatively, has the rate of decline increased suddenly during the period 1996-2000, so that the species would have qualified for uplisting from NT straight to EN?
Please note that this topic is not designed to be a general discussion about the ecology of the species, rather a discussion of the species’ Red List status. Therefore, please make sure your comments are relevant to the species’ Red List status and the information requested. By submitting a comment, you confirm that you agree to the Comment Policy.
*Bird generation lengths are estimated using the methodology of Bird et al. (2020), as applied to parameter values updated for use in each IUCN Red List for birds reassessment cycle. Values used for the current assessment are available on request. We encourage people to contact us with additional or improved values for the following parameters; adult survival (true survival accounting for dispersal derived from an apparently stable population); mean age at first breeding; and maximum longevity (i.e. the biological maximum, hence values from captive individuals are acceptable).
An information booklet on the Red List Categories and Criteria can be downloaded here and the Red List Criteria Summary Sheet can be downloaded here. Detailed guidance on IUCN Red List terms and definitions and the application of the Red List Categories and Criteria can be downloaded here.
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