Introduction
Migratory shorebirds with long-distance migration and complex phylogeny are an essential component of global biodiversity and include the endangered Spoon-billed sandpiper and Nordmann’s greenshank (Shumway et al. 2022). Recently, the migratory routes of many shorebirds, such asPhalaropus lobatus and Calidris tenuirostris, have been described by satellite or other techniques, which indicate that mazy migratory networks are formed by different species or subspecies (Gill et al. 2009, Lisovski et al. 2016, Mu et al. 2018). However, with rapid declines in populations of shorebirds due to diverse environmental threats, effective conservation requires improved understanding of the migration strategies and population structures of subspecies in key stopover sites (Warnock 2010, Sutherland et al. 2012, Melville et al. 2016, Studds et al. 2017, Ma et al. 2022). Identification of subspecies is crucial to determine where and when special subspecies populations are migrating (Boulet and Norris 2006, Gill et al. 2013).
Stopover sites are essential components of the flyways of migratory bird populations that link breeding sites with nonbreeding areas (Anderson et al. 2019, Wang et al. 2022). Migration stopover sites are vital in the whole migratory process by providing large quantities of food to help mixed populations of species or subspecies replenish energy reserves for continued migration (Moore and Simons 1990, Dunn 2001). The region of the southern Yellow Sea in Jiangsu Province, China, is important for migrating shorebirds, and many vulnerable shorebirds preferentially select the area as a refueling stop (Chen et al 2015, Ma et al. 2013, Tong et al. 2012). Overall, ring recoveries, geolocators, and morphological comparisons indicate that at least four subspecies of the dunlin Calidris alpina migrate along the East Asian–Australasian Flyway (EAAF), includingC. a. arcticola , C. a. sakhalina , C. a. kistchinski , and C. a. actites (Lagassé et al. 2022). Moreover, two additional dunlin subspecies, C. a. alphina and C. a.centralis , may stage at the Jiangsu stopover.
Morphological, genetic, ringing, and leg-flagging methods are often used to determine migratory connectivity and population structure (Verkuil et al. 2012, Weston et al. 2020). Ring recoveries, recaptures, and resightings are valuable in providing accurate records of the links between breeding, stopover, and wintering sites for dunlin (Evans 1984, Gill et al. 2013, Lagassé et al. 2020). Studies of genetic and morphological variation also provide an outline of the phylogeography and migratory patterns (Greenwood 1984, Wennerberg et al. 1999, Popovic et al. 2019). Morphological characters, especially bill, wing, and tarsus lengths, have also been used to identify the origins of dunlins from Alaska and the Russian Far East region (Greenwood 1986), but sexual dimorphism in size requires that birds be sexed for accurate analysis of morphological data. Analyses of dunlin mitochondrial DNA (mtDNA) can indicate breeding origins and migratory patterns of birds from different breeding sites as well as genomic homogeneity among overwintering populations (Wenink et al. 1996, Wennerberg 2001, Popovic et al. 2019).
The dunlin is a typical example of a polytypic wader with an extensive distribution for which analysis of phylogeographic patterns (Marthinsen et al. 2007) is suitable to show spatial and temporal patterns at a subspecies level. Five lineages of dunlin may occur in the Palearctic region, namely the European, Siberian, Beringian, Alaskan, and Canadian lineages (Wenink and Tilanus 1996). Moreover, in reviews of the systematics, dunlin is divided into 10 subspecies (Greenwood 1986). Dunlin subspecies from various breeding populations use different Palearctic migration flyways (Gromadzka 1989) and exhibit regional segregation and high site fidelity along a flyway (Lagassé et al. 2020). For example, two subspecies of dunlin, C. a. arcticola andC. a. sakhalina , are dominant in different regions along the EAAF (Lagassé et al. 2022).
Approximately 570,000 dunlins are estimated to use the EAAF to migrate from breeding to wintering areas (Jing-Gong et al. 2009). Although the migratory routes of different dunlin subspecies have been described (Lagassé et al. 2020), it is difficult to determine which subspecies of dunlin stopover at special key sites, which hinders understanding of subspecies migration. How many dunlin subspecies are present during key migratory periods is uncertain, and additional detailed evidence is needed to detect migratory origins and population composition of dunlins at key stopover sites (Webster et al. 2002). Coastal mudflats in southern Jiangsu Province are always important for mixed populations of different dunlin subspecies migrating along the EAAF. The aim of this study was to identify the subspecies of dunlin and their proportions using the stopover site in Jiangsu Province. Of particular concern wasC. alpine arcticola , which is considered a priority species for the Arctic Migratory Birds Initiative conservation efforts (Weiser et al. 2018). To analyze the subspecies of dunlin migrating on the EAAF, an integrated approach was used that combined morphometry, ringing recovery, and genetic phylogeny methods.