Animal migration is a global phenomenon (Dingle and Drake, 2007). Migratory animals, particularly birds, play indispensable ecological roles across multiple ecosystems worldwide (Bauer and Hoye, 2014). Their remarkable journeys, spanning vast distances across large spatial scales and often crossing diverse habitats, contribute to the interconnectedness and functioning of various ecosystems. Throughout their annual cycles, avian species require a sequence of connected sites along their migration routes where they can rest, forage, and accumulate energy reserves (Alerstam and Lindström, 1990). The presence of intact fueling sites along migratory flyways is of paramount importance for migratory birds, as they are essential for refueling and replenishing energy reserves. As among the largest of 9 global flyways, the East Asian-Australasian Flyway (EAAF) stretches at a continental scale from the Arctic Circle across East and Southeast Asia, to Australia and New Zealand and supports over 50 million migratory waterbirds from more than 250 different populations (access by June 25, 2023; https://www.eaaflyway.net). Waterbirds, particularly shorebirds (order Charadriiformes), strongly depend on coastal wetlands along their migratory routes as critical habitats. Unfortunately, these waterbird populations are facing alarming declines primarily attributable to a multitude of threats, such as hunting (Gallo-Cajiao et al., 2020), pollution (Ma et al., 2022), and habitat loss (Zhang et al., 2018) and degradation (Ma et al., 2014; Studds et al., 2017) and other forms of unsustainable use such as wetland reclamation (Yang et al., 2020). The imperative to ensure the conservation of migratory bird populations and effectively address the challenges faced by transboundary migrants calls for the urgent establishment of a comprehensive continental network of wetland sites. Relying solely on the protection of a few selected key sites proves insufficient in providing an adequate framework to safeguard these avian species and their habitats (Runge et al., 2014; Dhanjal-Adams et al., 2017; Xu et al., 2020).

The identification and protection of important wetlands for waterbirds will ensure an effective conservation ‘safety net’ for many species and the long-term survival of migratory waterbirds within the EAAF (Dinerstein et al., 2023). The Convention on Wetlands of International Importance, also known as the Ramsar Convention, provides internationally recognized criteria to identify wetlands of international importance (Ramsar sites) during the past 50 years (Stroud and Davidson, 2022). Further, some of the sites are already inscribed on the World Heritage List (UNESCO) due to Outstanding Natural Values (e.g., https://whc.unesco.org/en/list/1606/multiple=1&unique_number=2304), as well as the sites identified by the Convention of Migratory Species (CMS) and other international agreements and institutional arrangements (i.e., East Asian–Australasian Flyway Partnership, hearafter as ‘EAAFP’), base recognition on the 2 criteria specifically related to waterbird populations. So far, 152 sites, spread over 19 countries, have been included in this Flyway Site Network of EAAFP (access by June 25, 2023; https://www.eaaflyway.net). In particular, priority sites along the EAAF, such as those located within the Yellow/Bohai Sea and Australia, and their ecological importance to migratory waterbirds are relatively well studied, profiled, and monitored (Barter, 2002; China Coastal Waterbird Census Group, 2011; Clemens et al., 2016; Wang et al., 2022), and in 2019, several wetland sites such as Yancheng and Dafeng Milu National Nature Reserves were inscribed as UNESCO World Heritage Sites (UNESCO, 2019). Yet, the comprehensive conservation of migratory waterbirds hinges upon the adequate protection of interconnected sites spanning the entire EAAF, including regions such as southern China and Southeast Asia. Despite the importance of these regions, they have often received relatively less attention from conservationists and researchers (Chan et al., 2019; Yong et al., 2022).

The Beibu Gulf (also known as the Gulf of Tonkin, Vịnh Bắc Bộ as it is known in Vietnam; 北部湾 in Chinese) spans the frontiers of China and Vietnam and serves as one of the most important migration stopover and wintering areas for many waterbirds at the interface of Southeast Asia and south China along the EAAF, including critically endangered species such as spoon-billed sandpiper Calidris pygmaea (Pedersen et al., 1996; Chang et al., 2020; Nguyen et al., 2021). It is a large, semi-enclosed bay located northwest of the South China Sea, drained by several major rivers such as the Nanliu in China and the Red River in Vietnam. In China, it is bounded by the provinces of Guangxi and Guangdong and the island province of Hainan. In Vietnam to the south-west of the Bay, it is bounded by several provinces of northeast and central Vietnam from Quang Binh to Quang Ninh. With coastal habitats such as bare tidal flats, salt marshes, mangroves, reefs, and seagrasses, the Beibu Gulf is rich in marine biodiversity, supports key fisheries and aquaculture activities, recreation and tourism, ports, as well as a wide range of ecological services, e.g., storm surges (MacKinnon et al., 2012; United Nations Environment Programme (UNEP), 2014; Teng et al., 2022). More specifically, this region provides nursery habitats for “living fossils”, namely horseshoe crabs (Kwan et al., 2021), and critical habitats for migratory species such as black-faced spoonbill Platalea minor, spoon-billed sandpiper and spotted greenshank Tringa guttifer (Nguyen et al., 2021; Nguyen et al., 2023). Despite the ecological importance of the Beibu Gulf, research on migratory waterbirds in this part of the EAAF remains relatively limited. Based on the ringing and flag resighting records in Guangxi, Yi et al. (2023) demonstrated the migration connectivity between the Beibu Gulf and Tasmania in Australia, Chukotka in Russia, and Jiangsu in China. In Vietnam, Xuan Thuy National Park (NP) has been well recognized for its importance to the black-faced spoonbill and spoon-billed sandpiper and has been regularly surveyed since the 1990s (Pedersen and Nielsen, 1998). Besides, there are substantial knowledge gaps in Southeast Asia, including Vietnam (Choi et al., 2016; Yong et al., 2022; Nguyen et al., 2023). Vietnam has a well-established network of protected areas, but very few coastal wetlands are included (Rambaldi et al., 2001). According to Yong et al. (2022), Vietnam has 13 coastal Important Bird and Biodiversity Areas (IBAs), supporting 12 threatened bird species. Globally threatened species, such as Saunders’s gull Saundersilarus saundersi, have been recorded in the Red River Delta of north Vietnam (Pedersen and Nielsen, 1998), while great knot Calidris tenuirostris and spoon-billed sandpiper were recorded in the Mekong Delta, Southern Vietnam (Zöckler et al., 2016; Nguyen et al., 2023).

The densely populated Beibu Gulf has historically played a vital role in the economies of China and Vietnam, being particularly important for the production of seafood for both countries while supporting important biodiversity, including waterbirds (Qiu et al., 2008; Teng et al., 2022). Given an accelerating pace of development in both countries, particularly with the implementation of the Beibu Gulf Economic Rim, there is an urgent need to synthesize the distribution and abundance of waterbirds across the Gulf based on emerging knowledge and data and expand the identification of priority sites in the Beibu Gulf to guide effective conservation actions. Although 4 sites within the Gulf have been recognized as IBAs or Ramsar sites in China and several more in Vietnam (see Pedersen et al., 1996), recent bird surveys have indicated or discovered that additional sites may be important, and these sites should be included in the protected area network. Thus far, the absence of systematic waterbird survey data poses a significant challenge in accurately demarcating and defining the important coastal wetlands within the Beibu Gulf. This data gap hampers the implementation of effective conservation plans and policies to preserve these ecologically significant areas. Compounding the issue is the limited accessibility of related publications, which are predominantly available in vernacular languages. This language barrier impedes the smooth exchange of information and hinders effective communication and collaboration among international stakeholders (Amano et al., 2023).

In this study, we compile survey data from multiple sources of citizen science datasets across both countries and apply these data against internationally recognized criteria of the Ramsar Convention and the East Asian-Australasian Flyway Partnership. We delineate several new sites of significance to waterbird conservation. Based on all citizen science and open-access records collected from China and Vietnam, we aim to: 1) review the priority sites for waterbird conservation based on waterbird abundance and distribution; 2) identify the threats and conservation status of these sites in the region.

Coastal wetlands surrounding the Beibu Gulf are important wintering and stopover areas for threatened migratory waterbird species, particularly the spoon-billed sandpipers. The Beibu Gulf, along with the Leizhou Peninsula (Leung et al., 2022), represents the northernmost wintering region for the spoon-billed sandpipers. Notably, 4 flagged spoon-billed sandpipers were repeatedly re-sighted at Xichang between 2016 and 2019, indicating the importance of this site in connecting with birds in Chukotka in Russia and Jiangsu in China where the birds are usually tagged (Yi et al., 2023). Consistent with this finding, Xichang supported >1% population of spoon-billed sandpipers for their stopover. The Beibu Gulf region also supports the wintering or stopover populations of three other endangered shorebird species, namely Far Eastern curlew (present in at least 2 sites), great knot (9 sites), and spotted greenshank (6 sites). The region is probably the most north-easterly wintering region for all these species and probably a newly identified wintering region for Far Eastern curlews (Higgins and Davies, 1996; Zöckler et al., 2018). Furthermore, the Beibu Gulf is a well-known, important wintering region for the endangered black-faced spoonbills, with records from 11 sites. This region is considered one of the southernmost wintering regions for this species. Given its location at the northern and southern limits of the wintering ranges for several migratory waterbird species, continuous long-term monitoring of these waterbirds in the gulf can provide insights into their responses to climate change (Lehikoinen et al., 2021). The conservation of these endangered waterbird species and their habitats within the Beibu Gulf is of utmost importance for their long-term survival, in parallel with ongoing work to understand the impacts of environmental changes on migratory waterbird populations.

Following the criteria for wetlands of international importance defined by the Ramsar Convention Secretariat (2016), we identified 25 sites that met the criteria of significance to waterbird conservation along the Chinese and Vietnamese coasts of the Beibu Gulf through the integration of citizen science data. Over three-quarters of these important sites were located outside the existing Ramsar or protected areas. In 2014, China introduced the Ecological Conservation Redline (ECRL) policy, which holds exceptional national significance and has a proven history of effective enforcement by regional and local governmental authorities (Bai et al., 2016). Examining the implications of the ECRL, it becomes evident that 13 sites fall within or near the designated redline areas. This indicates the potential for these sites to receive protection under the ECRL policy, as development activities are prohibited in ECRL. The ECRL policy provides a framework for safeguarding these sites and underscores their importance in conserving ecological integrity and promoting sustainable development practices. In 2022, Vietnam passed the directive, ‘urgent tasks and solutions to conserve wild and migratory birds’, which provides new and important guidance to address conservation issues for migratory species such as hunting, and strengthening the national framework of policies and legislation addressing the protection of threatened species (Directive 4/CT-TTg, 2022; Nguyen et al., 2023).

Our results reinforce the need for urgent protection of important waterbird habitats in the Beibu Gulf to avert further declines of waterbird populations along the EAAF. Increasingly efficient harvesting tools and approaches for seafood products cause greater harvests of invertebrates that may lead to reduced food availability and increased energy cost for waterbirds due to disturbances, while heightening the risk of bycatches (Verhulst et al., 2004; Yasué, 2005; Yasué, 2006). Many areas of intertidal flats are actively being used for the fishery (Verhulst et al., 2004), therefore putting migratory birds in direct conflict with fisheries in both China and Vietnam. Livestock, poultry, and aquaculture were identified as dominant sources of pollutants, e.g., antibiotics in the Beibu Gulf (Wu et al., 2022). Furthermore, other pollutants, including heavy metals and organic contaminations are reported in this region, posing an ecological risk to waterbird species foraging here (Gan et al., 2013; Li et al., 2015). Besides, to reduce the dependency on imported fossil fuels in the Beibu Gulf, wind energy infrastructures can be expected to become more common in the near future. In addition, coastal wetlands along Guangxi province were reclaimed and caused significant habitat fragmentation due to infrastructure needs (e.g., port), commercial developments and coastal engineering (Liu et al., 2023). Similar changes are ongoing in An Hai, Vietnam (Nguyen et al., 2023). Therefore, a sustainable economic development policy balancing environmental protection is urgently needed (Chen et al., 2020).

The protection of the coastal wetland ecosystem and bird habitats in the Beibu Gulf is of utmost importance and requires collaborative efforts from governments, agencies, businesses, and citizens. Strengthening cooperation among these stakeholders is crucial to effectively conserve and manage the ecological environment of the region (Yong et al., 2018). Recent studies utilizing satellite tracking methods have initiated efforts to address these research gaps (Chan et al., 2019). These studies serve as a valuable complement to ground-based field surveys (Putra et al., 2019; Nguyen et al., 2021). However, there is an ongoing and pressing need for further field-based research to address several critical aspects of waterbird conservation in Beibu Gulf. Firstly, understanding the habitat use and local movement by various bird species in different sites is crucial. Secondly, investigating migratory routes and establishing knowledge about migratory connectivity among waterbird populations is imperative. Such research outcomes can guide the identification of priority sites, including IBAs, and inform the planning and establishment of protected areas. Continued field-based investigations are therefore necessary to address these research objectives effectively. Lastly, sustainable development practices that prioritize the preservation of intertidal and supratidal habitats (Jackson et al., 2020) in the Beibu Gulf and ensure the safety of bird populations should be considered and implemented. This includes measures to minimize habitat degradation, pollution, and disturbance. Additionally, efforts should be made to promote the diversity of bird species by conserving their habitats and addressing any potential threats they may face. By fostering collaboration and implementing environmentally friendly practices, it is possible to strive towards the long-term protection and sustainable management of the coastal wetland ecosystem and bird habitats in the Beibu Gulf.

It is important to note that relatively few surveys were conducted in Vietnam (60 sites in China; 6 sites in Vietnam were surveyed). Additionally, some sites, such as IBAs of Tien Hai and Tra Co, were only surveyed once, thus, our limited datasets due to low survey capacity could be a reason why some former important sites appeared to support insignificant waterbird congregations. On the other hand, sites such as Sanning Bay met the important wetland criteria only once, thus, long-term monitoring is needed to see if the record of large number of waterbirds was a rare incident or the site indeed supports a substantial number of waterbirds regularly. Furthermore, the data used in this study encompassed a combination of citizen science observations, publicly available bird records, and unpublished data from local observers. The quality of the data is influenced not only by the resources and capabilities but also by the skills and expertise of the observers. As information about the observers was often lacking, the verification of certain data points may have been challenging, potentially impacting the overall data quality. To address these limitations, future research, monitoring, and conservation effort should consider providing training and learning opportunities for local surveyors, enhancing the resources available to conservation and research organizations in both countries, and promoting transboundary collaboration between researchers, managers, and local observers. Moreover, the information collected in local languages, predominantly Chinese and Vietnamese, should be translated, analyzed, and published to facilitate accessibility and foster effective transboundary conservation efforts for migratory waterbirds (Liu et al., 2020; Amano et al., 2021; Fuller et al., 2021).

In conclusion, the Beibu Gulf, situated along EAAF and shared between China and Vietnam, is a vital stopover and wintering region for migratory waterbirds. Our study, which synthesized citizen science datasets and published records from wetland sites in both countries, revealed the presence of at least 97 waterbird species in the Gulf during the migration period. Among these species, some are highly threatened, such as the spoon-billed sandpiper and the black-faced spoonbill, are highly threatened and face significant conservation challenges. In addition, our research identified 25 sites within the Beibu Gulf that qualify as internationally important wetlands, including many for the first time and therefore underscoring the region’s significance for waterbird conservation. However, it is concerning that only a small fraction of these sites receive adequate protection, with less than a quarter benefiting from national or international protection and recognition. This study also shed light on localized threats faced by these waterbird habitats, including aquatic resource harvesting, hunting, and aquaculture/fisheries, which are widespread in the region. Addressing these threats is essential to ensure the survival and well-being of the diverse waterbird species that rely on the gulf’s wetlands. With the findings from this study, we emphasize the urgent need for continued waterbird monitoring, effective site prioritization, and the development of comprehensive habitat management plans. By bridging knowledge gaps and implementing conservation measures, stakeholders in both China and Vietnam can work collaboratively to protect and preserve the critical waterbird habitats in the Beibu Gulf, securing the future of these migratory species and maintaining the ecological integrity of this important coastal region.

The original contributions presented in the study are included in the article/ Supplementary Material . Further inquiries can be directed to the corresponding authors.

C-YC and YM contributed to the conception and design of the study. CA, CC, GL, FL, HN, QN, TL, SW, and TH participated in data collection through fieldwork in their respective countries. NT, SL, YY, LL, and PQ organized and analyzed the database. YM performed the statistical analysis. NT and YM wrote the first draft of the manuscript. DY and C-YC edited the manuscript. All authors contributed to the manuscript revision, read, and approved the submitted version.