INTRODUCTION

The Whooper Swan (Cygnus cygnus) is a large-bodied migratory waterbird that breeds across the tundra regions of northern Eurasia and migrates to East Asia, Europe, and the Middle East during the winter. In Korea, the species holds both ecological and cultural significance and is designated as a Natural Monument and listed as Class II Endangered Wildlife under national law (CHA, 2020). Major wintering sites in Korea include the Nakdong River Estuary, Junam Reservoir, and Upo Wetlands. The national wintering population increased from approximately 5,590 individuals in the 1990s to over 39,000 in the early 2000s, with numbers stabilizing or slightly declining in recent years (Choi et al., 2018;Lee et al., 2022). Among these, the Nakdong River Estuary supports approximately 20–26% of the national population and provides a rich mosaic of wetlands essential for foraging, roosting, and migratory preparation (Kim et al., 2017b;Lee et al., 2025).

Habitat selection by Whooper Swans is strongly influenced by environmental factors such as water depth, aquatic vegetation density, and levels of human disturbance (Chisholm and Spray, 2002). In the Nakdong River Estuary, key determinants of habitat use include bridge spacing, salinity gradients, and the distribution of lotus (Nelumbo nucifera Gaertn.) beds (Hong, 2016;Hong, 2020). Expansion of lotus beds has been positively associated with food availability and local swan density, while increased human activity including infrastructure development, tourism, and domestic animal presence can reduce habitat suitability and lead to local displacement.

Morphologically, the Whooper Swan is easily identified by its distinctive yellow bill base extending to the nostrils, sharply contrasting with the black tip (Mitchell and Wass, 1996). The species primarily feeds on aquatic plant roots and stems, invertebrates, and agricultural grains (Cramp and Simmons, 1977).

Understanding the migratory behavior and wintering ecology of waterbirds is essential for species conservation, wetland management, disease surveillance, and biodiversity planning (Sutherland et al., 2012). Recent advancements in satellite telemetry have enabled high-resolution, real-time tracking of avian movements. In Japan, satellite-based studies have identified both northward and southward migratory routes of Whooper Swans (Shimada et al., 2014), while research in Mongolia has revealed individual variation in migration strategies and stopover site use (Lee et al., 2023). In northwestern China, habitat selection patterns have been investigated at multiple spatial scales along the East Asian–Australasian Flyway (Han et al., 2024).

In Korea, satellite telemetry has increasingly been applied to species such as the Eastern Spot-billed Duck (Anas poecilorhyncha), Mallard (Anas platyrhynchos), Whitenaped Crane (Grus vipio), Swan Goose (Anser cygnoides), and Black-faced Spoonbill (Platalea minor) to study home range dynamics, habitat connectivity, and seasonal movement patterns (Baek, 2020;Son et al., 2020;Jang et al., 2022;Lee et al., 2023). The National Institute of Biological Resources (2018) tracked 112 individuals from 19 species of ducks and swans, revealing key migratory routes and stopover sites both domestically and internationally. However, the spatial ecology of the Whooper Swan remains understudied in Korea, particularly using real-time telemetry data.

Previous studies have focused primarily on population size and distribution patterns, with limited quantitative analysis of individual space use, such as home range size and seasonal variation (Lee et al., 2022;Hong, 2020). A more detailed understanding of the spatial behavior of wintering Whooper Swans is urgently needed to inform targeted conservation strategies.

Home range analysis is a valuable tool for identifying space use patterns and core habitats. Techniques such as Minimum Convex Polygon (MCP) and Kernel Density Estimation (KDE) are widely applied in avian movement ecology and have proven effective for habitat conservation, disease monitoring, and the designation of protected areas (Harris et al., 1990;Seaman et al., 1999).

This study aimed to examine the winter home range dynamics and monthly habitat use of 12 GNSS-tagged Whooper Swans wintering in the Nakdong River Estuary. By analyzing the real-time tracking data, we sought to enhance understanding of their spatial ecology and provide scientific insights to guide future conservation planning.

MATERIALS AND METHODS

1. Study Area

This study was conducted in the Nakdong River Estuary in Busan, South Korea, one of the country’s most ecologically important habitats for migratory birds. Designated both as a Migratory Bird Sanctuary and a Natural Monument, the estuary spans approximately 37.7 ㎢ and Comprises a dynamic mosaic of freshwater and brackish ecosystems.

The study area encompasses a network of key wetlands and ecological zones, including:

These habitats provide a diversity range of foraging grounds and roosting sites, characterized by extensive mudflats, shallow open waters, sandbars, and aquatic vegetation beds dominated by lotus (Nelumbo nucifera Gaertn.) and common reeds (Phragmites australis (Cav.) Trin. ex Steud.). Seasonal fluctuations in water level, salinity, and food availability combined with anthropogenic factors such as bridges, recreational activities, and land reclamation strongly influence habitat use by overwintering waterbirds.

The estuary experiences relatively mild winters (average temperature 0–5°C), and surface waters rarely freeze, making it a reliable and stable overwintering refuge for large-bodied waterbirds such as the Whooper Swan. Ongoing habitat restoration projects and seasonal access restrictions in critical zones further enhance the area’s ecological value (Figure 1).

2. Monitoring

Systematic bird monitoring was conducted monthly from October 2024 to April 2025 throughout the Nakdong River Estuary. Surveys followed the standard protocols (National Institute of Biological Resources, 2011;Wetlands International, 2020) and employed both line transect and point count methods. The line transect method involved observing birds while walking or driving along the riverside sandbars, mudflats, agricultural fields, and wetlands, using binoculars (8×30, Nikon) and a spotting scope (20–60×, Swarovski) to record species, abundance and behavior.

Point counts were conducted at fixed vantage points within the study area, During each 20-30 minute session observers recorded flock composition, movement direction, and behavior. The geographic coordinates of each observation site were recorded using a handheld GNSS device. All field data were entered in a standardized format to assess seasonal changes in abundance, behavior, and habitat use, allowing for temporal comparisons across months and years.

3. Device Development and Tracking Summary

On 17 December 2024, 12 adult Whooper Swans were captured near Eulsukdo using a cannon net. Each individual was fitted with a backpack-style GNSS tracking device (WT-300, KoEco, South Korea), weighing approximately 65 g less than 1% of the bird’s body mass. Following the protocol of Kenward (1985), attachment procedures were completed within five minutes to minimize handling stress, after which the swans were released at the capture site. Location data were recorded at hourly intervals and transmitted to a server twice daily (09:00 and 16:00) via a mobile communication network. For quality control, fixes with a Dilution of Precision (DOP) value ≥ 5 were excluded due to moderate or poor accuracy (Tahsin et al., 2015). Additional filtering removed biologically implausible values, such as movements exceeding 100 ㎞/h. After filtering, 21,721 valid GNSS locations were retained. Individuals tracking durations ranged from 56 to 91 days, with a mean of 79.2 days.

4. Home Range and Pre-Migratory Spatial Use Analysis

GNSS-based tracking data were used to analyze the winter home range and pre-migratory inland movements of Whooper Swans in the Nakdong River Estuary. Spatial analyses were performed using QGIS (version 3.34.x) and R (version 4.x). We evaluated individual movement radii, core-use areas, pre-migratory dispersal, and stopover site use (Kauhala and Auttila, 2010).

Home range estimation employed Minimum Convex Polygon (MCP) and Kernel Density Estimation (KDE) methods. The 100% MCP was used to quantify the total spatial extent of individual movement (Murphy and Dowding, 1995;Walton et al., 2001), while the 95% and 50% KDE contours were used to identify high-use and core areas, respectively (Seaman et al., 1999). Seasonal differences in space use and intensity were compared based on these estimates.

Stopover sites were defined as areas where a bird stayed for at least 48 hours within a 3 ㎞ radius (Nuijten et al., 2014;Lee et al., 2023). For each stopover event, we calculated the duration of stay, geographic coordinates, and KDE-based area. Pre-migratory dispersal strategies were classified according to the framework proposed by Lee et al. (2023).

RESULTS

A total of 21,721 GNSS location records were analyzed from 12 Whooper Swans (Cygnus cygnus) that were captured and tagged in the Nakdong River Estuary between December 2024 and March 2025. Individual tracking durations ranged from 56 to 91 days. The majority of location data were collected in December 2024 and January 2025, during which all individuals were consistently located within the wintering areas of the Nakdong River Estuary. Beginning in mid-March, some individuals initiated pre-migratory movements toward inland wetlands, including Junam Reservoir, and the Joman River in Gimhae (Table 3, Figure 1).

1. Movement within wintering areas

All 12 individuals remained within the specific sites in the Nakdong River Estuary throughout the wintering period, primarily utilizing areas such as Eulsukdo, Maekdo Ecological Park, Baekhapdeung, and Samnak Ecological Park. Until January, most swans concentrated their activities around Eulsukdo, but their movement ranges appeared to contract in February. In March, a few individuals (e.g., ke2404 and ke2410) exhibited pre-migratory inland shifts toward wetlands such as Junam Reservoir in Changwon and the Joman River basin in Gimhae. These inland sites were likely used as temporary stopover habitats for energy replenishment and rest immediately prior to northward migration (Table 4, Figure 2-1, 2-2).

2. Home range analysis

1) Overall home range

Based on the Minimum Convex Polygon (MCP) and Kernel Density Estimation (KDE) analyses, the mean home range size for all individuals was estimated at 113.38 ± 251.36 ㎢ using 100% MCP, 39.55 ± 71.30 ㎢ using 95% KDE, and 7.21 ± 12.19 ㎢ for the core area (50% KDE). A considerable variation was observed among individuals. For instance, ke2404 (936.96 ㎢) and ke2410 (157.00 ㎢) exhibited extensive spatial use, whereas ke2409 (4.91 ㎢) and ke2406 (6.77 ㎢) remained within more restricted ranges. This disparity is primarily attributed to pre-migratory inland movements by ke2404 and ke2410 toward Junam Reservoir and the Joman River.

When considering only the GNSS records obtained prior to their inland dispersal, the average home range of these individuals within the Nakdong River Estuary was substantially smaller: 25.99 ± 15.68 ㎢ (100% MCP), 12.72 ± 8.64 ㎢ (95% KDE), and 2.81 ± 2.32 ㎢ (50% KDE; Table 5).

2) Individual home range analysis

Following its tagging on 17 December 2024, ke2401 was tracked for 65 days and primarily used areas including Baekhapdeung, Doyodeung, Eulsukdo and Maekdo Ecological Park. It initiated northward migration from Eulsukdo on 19 February 2025. Its estimated home range size was 27.84 ㎢ (MCP), 24.15 ㎢ (95% KDE), and 2.32 ㎢ (50% KDE).

Ke2402 was tracked for 86 days and exhibited consistent habitat use around Eulsukdo and Maekdo Ecological Park before initiating its northward migration on 12 March 2025. Its estimated home range size was 19.08 ㎢ (MCP), 10.93 ㎢ (95% KDE), and 2.71 ㎢ (50% KDE).

During its 79 day tracking period, ke2403 frequently moved among Baekhapdeung, Daemadeung, Eulsukdo and Maekdo Ecological Parks. Its estimated home range size was 27.87 ㎢ (MCP), 8.07 ㎢ (95% KDE), and 1.38 ㎢ (50% KDE).

Ke2404 was tracked for 91 days and actively used Eulsukdo, Maekdo Ecological Park, and the western Nakdong River between December 2024 and February 2025. It briefly visited Junam Reservoir between 24 and 28 February, returned to the estuary, and then relocated to Junam Reservior again on 1 March. This behavior is consistent with a pre-migratory inland shift, likely representing exploratory movement prior to northward migration. Its total home range was estimated at 936.96 ㎢ (MCP), 263.24 ㎢ (95% KDE), and 45.40 ㎢ (50% KDE). When limited to movements within the estuary, the estimated range was reduced to 30.48 ㎢, 5.33 ㎢, and 0.70 ㎢, respectively.

Ke2405 was tracked for 78 days and primarily used Baekhapdeung, Eulsukdo, Maekdo and Samnak Ecological Parks. It briefly passed through the western Nakdong River on 4 March before initiating northward migration. Its estimated home range size was 30.72 ㎢ (MCP), 21.27 ㎢ (95% KDE), and 4.25 ㎢ (50% KDE), reflecting stable use of estuarine habitats throughout the wintering period.

Ke2406 remained within Baekhapdeung, Daemadeung, and Eulsukdo Ecological Park for 73 days and departed the estuary via the Myeongji-dong tidal flats on 27 February. Its space use was highly localized, with an estimated home range of 6.77 ㎢ (MCP), 1.70 ㎢ (95% KDE), and 0.32 ㎢ (50% KDE).

Ke2407 was tracked for 91 days and utilized a broad range of habitats, including Baekhapdeung, Daemadeung, Jinudo, Eulsukdo, Maekdo, and Samnak Ecological Parks. Its movement pattern indicates a sequential space-use strategy, with an estimated home range of 54.42 ㎢ (MCP), 15.72 ㎢ (95% KDE), and 2.38 ㎢ (50% KDE).

Ke2408 was tracked for 56 days and primarily used Baekhapdeung, Eulsukdo, and Maekdo Ecological Parks. It initiated its northward migration on 11 February via the upper Joman River in Gimhae. The individual shoeed strong site fidelity, with a compact home range estimated at 14.03 ㎢ (MCP), 7.30 ㎢ (95% KDE), and 1.12 ㎢ (50% KDE).

Ke2409 was tracked for 72 days and remained within Daemadeung, Jangjado, and Eulsukdo Ecological Park before departing from Eulsukdo on 28 February. It exhibited the smallest home range among all individuals in the study: 4.91 ㎢ (MCP), 0.98 ㎢ (95% KDE), and 0.20 ㎢ (50% KDE).

Ke2410 was tracked for 88 days and moved extensively among Eulsukdo, Maekdo and Samnak Ecological Parks, as well as Dunchido, Chideung, Yulha Stream, and the Joman River in Gimhae. In January, it displayed exploratory movements among these sites and permanently relocated to the Joman River on 28 January. This behavior is consistent with a pre-migratory inland shift and likely represents scouting behavior prior to migration. Its total home range was estimated at 157.00 ㎢ (MCP), 91.82 ㎢ (95% KDE), and 15.70 ㎢ (50% KDE). When limited to movements within the estuary, the estimated range decreased to 14.76 ㎢ (MCP), 27.83 ㎢ (95% KDE), and 7.60 ㎢ (50% KDE), respectively.

Ke2411 was tracked for 72 days and used a range of habitats, including Maekdo, Eulsukdo, Baekhapdeung, and Doyodeung, as well as downstream aquaculture zones. It also made occasional excursions to low-salinity coastal waters. The individual initiated northward migration on 26 February. Its estimated home range was 56.98 ㎢ (MCP), 8.36 ㎢ (95% KDE), and 2.05 ㎢ (50% KDE).

Ke2412 was tracked for 65 days and moved among Maekdo, Daejeo, Eulsukdo, Baekhapdeung and Doyodeung. It initiated northward migration from Eulsukdo on 19 February. Its estimated home range was 24.05 ㎢ (MCP), 21.01 ㎢ (95% KDE), and 4.68 ㎢ (50% KDE).

3) Monthly variation in home range

Monthly variation in home range size showed relatively restricted movement during December 2024 and January 2025, followed by an expansion in February and March 2025. A focused analysis of the 10 individuals that remained within the Nakdong River Estuary revealed average home range sizes of 53.84 ㎢ (100% MCP), 14.21 ㎢ (95% KDE), and 1.93 ㎢ (50% KDE) (Table 6, Figure 3). This pattern likely reflects renewed spatial expansion in March by individuals engaging in pre-migratory inland dispersal.

4) Spatial trends in core area use by month

An analysis of monthly spatial distribution patterns indicated that from December 2024 to February 2025, most individuals consistently used the Eulsukdo area as their core habitat. However, by March 2025, there was a clear shift in core-use areas toward Maekdo Ecological Park. Notably, four individuals used the Maekdo region as a core area within their 50% KDE range during this period. This shift may have been driven by changes in food availability, spatial saturation, or pre-migratory behavior during the late wintering season (Figure 3).

3. Seasonal trends in wintering population size (October 2024 – April 2025)

Bird monitoring conducted in the Nakdong River Estuary from October 2024 to April 2025 recorded no Whooper Swans (Cygnus cygnus) in October. In November, 1,220 individuals were observed, followed by a sharp increase to 2,250 in December and a peak of 2,300 in January 2025. The number then slightly declined to 2,100 in February, before dropping sharply to fewer than 500 in March and under 100 in April (Figure 4).

This monthly pattern suggests that the Nakdong River Estuary functions as a key early-season wintering site, supporting a large influx of swans by late autumn and providing stable habitat conditions through January (Tables 5 and 6). The rapid decline in numbers after March corresponds with satellite tracking data, indicating that most individuals departed the estuary around mid-March and dispersed to inland wetlands such as Junam Reservoir.

These results highlight strong consistency between field-based population monitoring and satellite-based home range analyses.

Discussion

This study confirms that the Nakdong River Estuary in Busan functions as a stable and ecologically important wintering site for Whooper Swans (Cygnus cygnus). GNSSbased real-time tracking of 12 individuals demonstrated consistent use of a limited number of core areas including Eulsukdo, Maekdo and Samnak Ecological Park, Baekhapdeung, and Daemadeung for foraging, resting, and energy storage. These patterns suggest that the estuary has a high ecological carrying capacity sufficient to meet the energetic demands of overwintering swans.

Most individuals exhibited a sedentary wintering pattern, remaining within the estuary and showing no dispersal to adjacent inland areas. This behavior is likely influenced by both the ecological stability of the estuarine environment and supplementary feeding programs aimed at migratory waterbirds (Hong, 2016).

The estuary’s aquatic habitats including mudflats, sandbars, and diverse plant communities offer consistent access to water and foraging resources, including lotus (Nelumbo nucifera Gaertn.), Schoenoplectus spp., reeds, benthic invertebrates, and other aquatic vegetation. Maekdo Ecological Park, in particular, has been identified as a highly suitable habitat for long-term swan use, owing to waterfront restoration efforts and seasonal vehicle restrictions that reduce human disturbance (Hong, 2016;Hong, 2020). These habitat features are consistent with prior observations of predominant foraging and sedentary behavior in the region (Lee, 2022).

Home range analysis indicated restricted spatial movements during December and January, followed by gradual expansion in February and a pronounced increase in March. With the exception of ke2404 and ke2410 individuals that dispersed to Junam Reservoir and the Joman River most swans remained within the Nakdong River Estuary throughout the wintering period. This pattern suggests a strategy of energy accumulation and pre-migratory exploration, consistent with the findings of Lee et al. (2023) and Jia et al. (2024).

The increased use of Maekdo Ecological Park in March suggests a preference for habitats offering abundant food resources and minimal human disturbance. In contrast, the frequent use of Eulsukdo, along with adjacent Daemadeung and Jangjado, during early winter highlights the ecological importance of exposed mudflats as foraging grounds for aquatic vegetation and benthic invertebrates (Shin, 2016). These spatial patterns were identified through monthly KDE-based core area analyses and supported by direct field observations conducted across the estuary.

Some individuals were observed visiting low-salinity waters near aquaculture areas downstream of Doyodeung, likely foraging on macroalgae such as Gamtae and Porphyra, as previously reported in Hadong County (Kim et al., 2017a). These observations highlight the species foraging plasticity and regional variation in diet.

Following mid-March, several individuals undertook inland movements to Junam Reservoir and Haepyeong Stream, where they made brief stopovers lasting 2–5 days. These stepwise migratory movements align with the East Asian–Australasian Flyway (EAAF) strategy (Lee et al., 2023;Ostapenko, 2013) and underscore the conservation value of Junam as a key staging habitat.

These movements also reflect emerging trends of wintering site fragmentation and population divergence, as reported by Choi et al. (2018) and Gu (2022). Maintaining ecological connectivity among the Nakdong River Estuary, Junam Reservoir, and Haepyeong Stream should therefore be considered a conservation priority.

Individuals such as ke2404 and ke2410 exhibited exploratory movements, indicating intraspecific variation in migratory strategies and site fidelity similar to patterns observed in swan populations in Northern Europe and Russia (Chisholm and Spray, 2002;Ostapenko, 2013). Such variability may be associated with physiological condition, age, sex, or breeding history, highlighting the need for integrated studies combining movement ecology and physiological metrics.

The seasonal concordance between field-based monitoring and GNSS-based tracking particularly the observed population decline after mid-March reinforces the reliability of spatial tracking methods. These findings underscore the importance of developing biodiversity monitoring systems based on quantitative movement data.

Historically, the Nakdong River Estuary has supported approximately 20–26% of the national wintering population of Whooper Swans in Korea. In recent years, however, population density in the estuary has declined, largely attributed to urban expansion, river modification, and the reduction of sandbar habitats (Choi et al., 2018;Hong, 2020).

Ongoing management efforts should prioritize high-use areas such as Maekdo and Eulsukdo by implementing measures including vehicle access restrictions, water quality control, and minimization of anthropogenic structures. Furthermore, integrating real-time tracking data with avian disease surveillance may enhance responses to HPAI outbreaks (Newman et al., 2009), thereby extending the implications of this research to both biodiversity conservation and public health.

This study utilized GNSS-based tracking to investigate individual home range dynamics and pre-migratory inland shifts in 12 Whooper Swans wintering in the Nakdong River Estuary. Most individuals exhibited high site fidelity, remaining within clearly defined core areas throughout the winter, followed by inland dispersal beginning in mid-March. These findings underscore the estuary’s dual function as both a primary wintering site and a strategic staging area along the East Asian–Australasian Flyway (EAAF).

Home range analysis revealed compact space use during the early wintering period, with expanded movements beginning in February, likely reflecting a strategy for optimized foraging and energy conservation. The further expansion observed in March suggests exploratory behavior preceding migration. Notably, individuals moving inland to Junam Reservoir did not return to the Nakdong River Estuary before initiating northward migration, consistent with pre-migratory exploratory shifts described by Lee et al. (2023) and Ostapenko (2013).

This pattern differs from the long-term habitat fragmentation-driven population divergence highlighted by Choi (2018) and Lee et al. (2022), as the observed movements reflect short-term, individual-level variation rather than persistent separation among populations.

Most individuals remained within the Nakdong River Estuary and did not undertake long-distance movements, suggesting that the site sufficiently met their ecological requirements. This high site fidelity may have been reinforced by supplemental feeding efforts, warranting further investigation into the quantitative relationship between food provisioning and habitat use.

The movement data generated in this study provide valuable insights for policy applications beyond enhancing ecological understanding, including HPAI transmission pathway monitoring, emergency response planning, and the development of climate-adaptive conservation strategies. However, our tracking began in mid-December after the initial arrival of Whooper Swans at the Nakdong River Estuary, which limited our ability to analyze first arrival dates, arrival sites, and early stopover locations. Future research should aim to capture and track individuals from the early autumn migration period to comprehensively analyze arrival dynamics and intermediate staging areas, thereby contributing more effectively to HPAI transmission monitoring and the development of systematic conservation plans.

Outbreaks of highly pathogenic avian influenza (HPAI) in East Asia have been linked to migratory waterbirds, including swans, making real-time movement data crucial for understanding potential transmission pathways (Newman et al., 2009). Integrated, annual-cycle conservation strategies that connect breeding, staging, and wintering habitats are essential for effective species management. Long-term tracking, combined with assessments of habitat quality and reproductive success, will further strengthen conservation efforts for Whooper Swans and other migratory waterbirds.

This study employed GNSS-based tracking to analyze the winter home range dynamics and pre-migratory inland shifts of 12 Whooper Swans (Cygnus cygnus) in the Nakdong River Estuary, South Korea. Most individuals exhibited high site fidelity, consistently using a limited number of core areas throughout the wintering period. Movement patterns revealed restricted space use during early winter, followed by gradual expansion in February and exploratory behavior in March, reflecting adaptive foraging and migration strategies.

The estuary’s stable habitat conditions, including low human disturbance and abundant aquatic vegetation— particularly in Maekdo Ecological Park—supported longterm occupancy. Short-distance inland dispersals to Junam Reservoir and Haepyeong Stream around mid-March emphasize the estuary’s dual role as both a primary wintering site and a strategic staging area along the East Asian– Australasian Flyway (EAAF).

These findings highlight the importance of sustained habitat protection, ecological connectivity among key sites, and integration of real-time tracking with avian disease surveillance. The results also underscore the potential policy applications of movement data for managing HPAI risks and developing climate-adaptive conservation strategies. Long-term, annual-cycle conservation efforts linking breeding, staging, and wintering habitats will be essential to ensuring the stability of migratory waterbird populations across flyways.