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Additional file 1: Table S1. The fieldwork that generated the data analysed here came from 17 lakes and 501 points in the YRF from 2016 to 2019. Table S2. Photographic information relating to the 10 waterbird species involved in the juvenile ratio survey of Yangtze River, illustrating the salient plumage differences and other features that are characteristic of adult and young birds of all species (Brazil 2009). Table S3. Autumn and spring migration schedules of eight large waterbird species in China based on published tracking studies. Table S4. The 1% biogeographical flyway population levels for each of the 10 large-bodied waterbirds wintering in the Yangtze River Floodplain analysed here. Table S5. The sample sizes and juvenile ratio from each of the 10 large waterbird species surveyed in each year from 2016 to 2019 (see ���Methods��� ��� Data analysis). Table S6. The percentage of each large waterbird species counted in a given year at six sites of major flyway importance in the Yangtze River Floodplain. Table S7. The error rate between predicted juvenile ratio and YRF sampled juvenile ratio of 10 large-bodied waterbirds in Yangtze River Floodplain from 2016 to 2019. Table S8. The error rate between predicted juvenile ratio and Yangtze River Floodplain sampled juvenile ratio of 10 large-bodied waterbird species in sites of major flyway importance from 2016 to 2019. Table S9. The error rate between sampled juvenile ratio in November and sampled juvenile ratio in December of 10 large-bodied waterbirds in the same lake from 2016 to 2019. Table S10. The theoretical error, real error and T2_Weight results of 10 large-bodied waterbird species in the Yangtze River Floodplain based on statistical independence test analysis. Table S11. The sqrt_sum_Weight results of 10 large-bodied waterbird in YRF from 2016 to 2019 based on statistical independence test analysis. Table S12. Cross-comparison table showing juvenile ratios generated (j/N, where j is the number of juveniles in sample N) from each annual sample (sample size N) for each of the 10 large-bodied waterbird species in each year, 2016 to 2019.

The Lesser White-fronted Goose Anser erythropus, which breeds across northern Eurasia from Norway to Chukotka, is globally threatened and is currently classified as Vulnerable by the International Union for Conservation of Nature. The Eastern Palearctic population of the species was thought to breed in arctic Russia, from east of the Taimyr Peninsula to Chukotka, and to winter in East Asia, but its precise status, abundance, breeding and wintering ranges, and migration routes were largely unknown, reducing the effectiveness of conservation efforts. In this paper, we combined results from satellite tracking, field surveys, a literature review and expert knowledge, to present an updated overview of the winter distribution and abundance of Lesser White-fronted Geese in the Eastern Palearctic, highlighting their migration corridors, habitat use and the conservation status of the key sites used throughout the annual cycle. Improved count coverage puts the Eastern Palearctic Lesser White-fronted Geese population at c. 6,800 birds in 2020, which represents a rapid and worrying decline since the estimate of 16,000 in 2015, as it suggests at least a halving of numbers in just five years. East Dongting Lake (Hunan Province) in China is the most important wintering site for the species in East Asia, followed by Poyang Lake (Jiangxi Province) and Caizi Lake (Anhui Province), with one key wintering site in Miyagi County in Japan. Satellite tracking showed that eight individuals captured during summer on the Rauchua River, Chukotka, Russia wintered in the middle and lower reaches of the Yangtze River floodplain in China. Their migration speed was slower in spring than in autumn, mainly because of longer stopover duration at staging sites in spring. The tracked geese mainly used cultivated land on migration stopovers (52% in spring; 45% in autumn), tundra habitat in summer (63%), and wetlands (66%) in winter. Overall, 87% of the GPS fixes were in protected areas during the winter, far greater than in spring (37%), autumn (28%) and summer (7%). We urge more tracking of birds of differing wintering and breeding provenance to provide a fuller understanding of the migration routes, staging sites and breeding areas used by the geese, including for the birds wintering in Japan. The most urgent requirement is to enhance effective conservation and long-term monitoring of Lesser White-fronted Geese across sites within China, and particularly to improve our understanding of the management actions needed to maintain the species. Collaboration between East Asian countries also is essential, to coordinate monitoring and to formulate effective protection measures for safeguarding this population in the future.

East Asian Greater White-fronted Goose Anser albifrons populations are less well defined and monitored than those in Europe and North America. Here, we combine historical and newly-reported telemetry data, wintering waterbirds surveys and expert advice to synthesis and update our knowledge of movements between the breeding and wintering distributions for Greater White-fronted Geese in East Asia. These sources suggest the existence of two biological flyway populations with contrasting population trends. The first consists of birds breeding on the Russian arctic, extending from the Khatanga River to east of Svyatoy Nos Cape (near Yana Bay, Yakutia), which migrates to winter in China where it is now almost totally confined to the Yangtze River floodplain. In recent years, this population has numbered between 30,000–55,000 individuals (compared to 140,000 in the 1990s), with > 70% concentrated at Poyang Lake in winter. The number of key sites identified for the species in China has increased with improved survey coverage since 2004. Birds from the second population also breed in arctic Russia, from east of Svyatoy Nos Cape to the Anadyr River, and winter in Japan and South Korea where 224,000–242,000 (in 2017/18 and 2018/19) and 178,000–182,000 (2018/19 and 2019/20) occur respectively, compared to a total population size of 60,000 reported in the late 1990s. Although the telemetry studies provide single examples of tracked birds moving between the two populations, suggesting some permeability, we contend that the populations are relatively discrete, but recommend retaining three management units (for China-, Japan- and Korea-wintering birds) because of the count, management and legislative logic of doing so. The results given here provide a robust assessment of the current status of these populations, but between-year differences in count totals underline the need for continued improvement of the count system in China. They also build a stronger basis for the effective conservation for this species in the region, highlighting the need for improved monitoring and management for the declining numbers wintering in China.

Our ability to define the population status, migration routes and seasonal distribution of Bean Geese Anser fabalis throughout the annual cycle in East Asia is severely compromised by the presence of two subspecies (Eastern Taiga Bean Goose A. f. middendorffii and Eastern Tundra Bean Goose A. f. serrirostris), which are difficult to differentiate in the field. In this analysis, using tracking data from telemetry-tagged geese, count survey data and expert knowledge, we attempt to update existing knowledge of the ranges covered by both subspecies of Bean Goose in East Asia. We suggest that, in summer, the Eastern Tundra Bean Goose ranges from the Taimyr Peninsula in the west to the Anadyr River in the east. Taiga Bean Geese breed further south in the taiga zone, and results indicate that they occur in north-western Mongolia, Yakutia and the Kamchatka Peninsula during the summer months. The winter distribution of both subspecies extends through China, Japan and South Korea. Tracking data from 154 individuals revealed a major overlap in the migration routes of Tundra Bean Geese wintering in China, South Korea and Japan, but discrete flyways for Taiga Bean Geese wintering in different regions. Long-term ground surveys carried out in the wintering range showed that numbers of Bean Geese in China and South Korea have increased significantly, to number 253,100 and 88,300 individuals respectively, of which roughly 10% are considered to be Taiga Bean Geese, about which subspecies we need to know more. Numbers of Japanese-wintering Bean Geese are slowly rising, currently numbering c. 10,300 (c. 900 Tundra Bean Geese and c. 9,400 Taiga Bean Geese). On the basis of these national and flyway estimates, derived from counts over the last five years, we identify new key wintering sites for the species in East Asia. Distributional changes at sites in China showed that wintering Bean Geese (most likely of the Tundra form) have become more widespread and numerous in the Yangtze River floodplain since the early 2000s. We argue for future strengthening of international cooperation to continue tracking and monitoring of Bean Geese, to provide a sound scientific basis for the effective management and protection of the flyway populations of both Bean Goose subspecies throughout East Asia.

Two of the most fundamental ecological questions about any species relate to where they occur and in what abundance. Here, we combine GPS telemetry data, survey data and expert knowledge for the first time to define two distinct flyways (the East Asian Continental and West Pacific flyways), migration routes and abundance for the Eastern population of Bewick’s Swan Cygnus columbianus bewickii. The Eastern population is the largest flyway population, supporting c. 77% of Bewick’s Swan numbers globally. GPS telemetry data showed that birds breeding in the Russian arctic from the Yamal Peninsula to c. 140°E (including the Lena and Yana Deltas), winter in the middle and lower reaches of the Yangtze River in China (which we label the “East Asian Continental flyway”). Bewick’s Swans breeding from the Indigirka River east to the Koluchin Bay winter in Japan, mostly in Niigata, Yamagata and Ishikawa Prefectures (the “West Pacific flyway”). There was no overlap in migration routes used by tagged individuals from the two flyways. Counts of Bewick’s Swans in the East Asian Continental flyway during the 21st century have shown wide between-year variations, reflecting incomplete coverage in earlier years. Bewick’s Swans in this flyway currently numbers c. 65,000 birds based on extensive wintering survey coverage, compared to c. 81,000 in the early 2000s, based on less complete coverage. Chinese-wintering swans now concentrate mainly (c. 80%) at Poyang Lake in Jiangxi Province and Hubei Lakes (mostly in Longgan Lake), compared to a more widespread distribution both within Poyang and throughout the Auhui Lakes in 2004 and 2005. In contrast, Bewick’s Swans of the West Pacific flyway now numbers c. 40,000, compared to just 542 in 1970. This population has shown no significant overall change since 2004, when it numbered c. 45,000 birds. Small numbers within this population probably also winter in South Korea. These results provide our first basic understanding of the winter distribution of Chinese- and Japanese-wintering Bewick’s Swans in relation to their breeding areas, confirming the need to coordinate future research and monitoring in the two flyways, as well as the need for more information on swans wintering in South Korea.

The Falcated Duck Mareca falcata, which is considered Near Threatened by IUCN, winters almost exclusively in East Asia, with greatest numbers occurring in the Yangtze River floodplain and along the coasts of eastern China. Given gaps in knowledge of its distribution and population status, we combined tracking studies, winter surveys and expert knowledge, to revise the geographical definition of its summer range in eastern Russia, Mongolia and Japan, and the main wintering range in China, South Korea, Japan and India. Data from six tagged individuals tracked on migration between their wintering and breeding grounds, combined with a single ring recovery, indicated that Falcated Ducks wintering in the Yangtze River migrate via the Northeast China Plain to breed over an extensive area in eastern Russia and northeast China. However, in the absence of telemetry data from birds wintering in the Indian sub-continent, Japan and the Korean Peninsula, we cannot yet define a flyway structure for the species. Mid-winter field surveys of Yangtze River floodplain wetlands in January 2004/05, 2015/16 and 2019/20 showed Falcated Ducks to be widely distributed, with 17 key sites supporting > 93% of the total wintering numbers counted in the region. Falcated Ducks in the Yangtze River floodplain increased from c. 15,000 counted in winter 2004/05, to c. 40,000 in 2015/16, and 88,000 in 2019/20, ascribed to a combination of improved survey coverage as well as increases in local abundance (based on sequential counts at key sites in 2004/05, 2015/16 and 2019/20). Coastal surveys undertaken in eastern China in winters 2003/04–2006/07 identified two key sites, which together supported 84.9% of the 14,904 individuals recorded. Between 2015/16 and 2019/20, abundance at these two sites had declined by 78.8% and 99.4% respectively, likely attributable to habitat loss through land reclamation during that period. More telemetry research, combined with comprehensive surveys of wintering, stopover and breeding sites is required to improve our understanding of migratory connectivity and the major migration routes of this species. The wide distribution of Falcated Duck also requires further comprehensive surveys in other wintering areas (e.g. central China and along the eastern coasts of China) to track changes in local abundance within its winter quarters, which in turn should confirm whether increases in abundance in the Yangtze River floodplain reflect an increase or a redistribution of the population. Long-term monitoring and sympathetic management of key staging and wintering sites used by the Falcated Duck in China, and across the rest of its range, are also required to ensure the efficacy of conservation measures for the long-term survival of the species.