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.