Discussion
The study provided new insights into the origins of dunlins migrating along the EAAF and a different perspective on the subspecies composing Dunlin populations on the flyway. In previous studies, how many Dunlin subspecies migrate along the EAAF was not rigorously determined. The combined results of ring recoveries and flag resightings from this study and previous research (Lagassé et al., 2020; Lagassé et al., 2022) indicated that the subspecies C . a. sakhalina , C. a. actites , C. a. kistchinski , and C. a. arcticola staged at Jiangsu in autumn. The result of recoveries and resightings suggested that compared with other subspecies, more individuals from C. a. kistchinski and C. a. actites were staging in southern Jiangsu Province, which is an important site to replenish energy reserves for those individuals from the Kamchatka Peninsula and Sakhalin Island. In addition, recoveries and resightings also provided general indications of the routes of dunlin subspecies along the EAAF (Supporting information Fig. S1).
Identification of C. a. arcticola and C. a. actiteswas important, because a recent study showed that survival is relatively low for the arcticola subspecies and that the population size ofC. a. actites is relatively small (Weiser et al. 2018). The population of subspecies C. a. actites is thought to number less than 1000 individuals, compared with over 500,000 for the other subspecies (Johnston et al. 2015). Thus, the study indicated that the coastal intertidal flats in the study area are important to the conservation of priority dunlin subspecies.
The application of multiple approaches can improve the effectiveness of population identification and studies of migration in most cases. Because most birds and subspecies of birds have highly similar morphology, it is difficult to accurately determine differences among species or subspecies by one specific method. Resighting and recovery data are one of the many types of data obtained from long-term fieldwork and represent a large investment. Although molecular methods cannot designate all individuals to one lineage or one subspecies, phylogenetic analysis can substantially increase understanding of dunlin population structure and help to realize approximate migratory phenology. Genetic data and phenotypic characters are both widely used in studies of shorebird phylogeny and taxonomy (Popovic et al. 2019). Phenotypic characters may be useful in probing the composition of populations (Rheindt et al. 2011). Hence, this study combined data of resighting or recovery and morphological and molecular analyses to comprehensively delineate dunlin population structure and migratory connectivity.
The analysis of mtDNA sequence data provided detailed information on the population composition and migratory connectivity of dunlin subspecies at Jiangsu. The haplotypes from Jiangsu had sister relationships with different dunlin subspecies and shared some haplotypes with those subspecies. Haplotypes from Jiangsu suggested the occurrence of six subspecies grouped into two lineages: Alaskan and Beringian. This result indicated that the dunlin population in Jiangsu was composed of different subspecies and that the nonbreeding ranges of those subspecies overlapped substantially in Jiangsu. Similar observations are reported in Portugal (Lopes et al. 2006).
The morphological difference between Alaskan and Beringian lineages is consistent with that among subspecies from the Far East and Alaska. Therefore, individuals in the Alaskan lineage included C.a. arcticola and C. a. pacifica and individuals in the Beringian lineage included C . a. actites , C .a. kistchinski , and C . a. sakhalina. Similar to the descriptions of dunlin subspecies (Greenwood 1984, 1986), males of subspecies from Alaska had shorter bills and longer wings than those from the Far East, whereas females of subspecies from Alaska had longer bills and wings than those from the Far East.
Although the six subspecies could not be accurately separated, it is plausible that at least four dunlin subspecies previously thought to share winter ranges along the central coast of eastern Asia far-stage in China (C . a. actites , C . a. kistchinski ,C . a. sakhalina , and C. a. arcticola ) (Bentzen et al. 2016, Choi et al. 2010, Miller et al. 2014, Tomkovich 1986). The subspecies C. a. hudsonia and C. a. pacifica from breeding grounds in North America were unexpected, because they were not thought to stage on the coast of eastern China (Miller et al. 2014). However, in this study, the two subspecies most likely occurred in Jiangsu.
Because of changes in populations, different populations of dunlin might be represented to a different extent in the 2 years. However, distinct clades were not detected between the 2 years, and only 10 haplotypes were shared between the sample periods (Supporting information Fig. S2). This may be due to the indistinct phylogenetic relationships among the six subspecies (Miller et al. 2014). By combining size measurements and molecular genetic analyses, the results of this study highlight the need to reassess the composition and migratory routes of subspecies at the level of subspecies. Morphology can help with an intuitive feeling for the subspecies of dunlin. The morphological data from 2015 and 2016 indicated that bill and wing lengths differed between the 2 years, but those differences were in opposite directions for the two sexes. Dunlin were captured considerably later in autumn 2016 (26 September to 6 October) than in autumn 2015 (12–20 September), so apparent annual differences might be spurious and only represent differences in migration timing among subspecies.
Previous studies demonstrate consistent migration timing among breeding populations, which correlates with geographical locations of breeding grounds (Grattarola et al. 1999, Jahn et al. 2013). In addition, significant differences in morphological characters were observed between Alaskan and Beringian lineages, which can help determine breeding origin. Moreover, compared with the number of Beringian and Canadian individuals, the number of Alaskan individuals was greater in 2016 compared to 2015. This difference might be a possible reason for the difference in characters between the two years. However, the differences between the two lineages could not entirely explain the differences in individuals between the sample periods. This result might be explained by variable proportions of different lineages or a difference in the timing of migration of subspecies.
The above findings provided additional evidence on the importance of the Jiangsu mudflats in providing migratory connectivity for subspecies and strengthened the case that protective measures are needed. There is some evidence that populations of some priority dunlin subspecies that stage at Jiangsu, such as C. a. arcticola , C. a. actites , andC. a. sakhalina , are declining (Johnston et al. 2015, Weiser et al. 2018). The conservation of stopover sites to maintain the connectivity of migration is important for long-term population persistence of subspecies from the Beringian realm (Soikkeli 1967). Conversion of intertidal shorebird habitats to land for industry and farming, construction of China’s new great wall along the seacoast, and invasion of mudflats by exotic cordgrass (Spartina sp.) all contribute to rapid losses in the extent of intertidal shorebird habitats (Li et al. 2022, Ma et al. 2014). Although measures to protect the Jiangsu mudflats and their enormous contribution to the future of shorebird populations should be implemented, additional conservation action is still required. This study draws attention to the huge geographical area from which migrating dunlin subspecies using the Jiangsu coast are drawn. Further research of this type is needed that focuses on numbers of individuals and population trends of different dunlin subspecies staging in Jiangsu and the timing of their migrations. The approach used in this study should also be extended to other shorebird species using this important stopover site.
Funding This work was supported by the Basic Scientific Research Program of National Nonprofit Research Institutes (Nos. ZX2022QT025 and GYZX210405), migratory shorebirds study program from the Jiangsu Forestry Bureau, and the Jiangsu Academy of Forestry Youth Foundation (JAF-2022-01).
Conflicts of interest No conflicts of interest or competing interests.
Permits Dunlin capture, tagging, and sampling were conducted under a special methods permit from the Jiangsu Forestry Bureau and National Bird Ring Center.