Shared initial migration axis and divergent annual-cycle strategies of whooper swans in the Nakdong Estuary, South Korea

Using GNSS tracking data from 12 Whooper Swans tagged in the Nakdong Estuary, South Korea, we examined whether individuals within a single wintering population shared an initial northward migration axis and later diverged in annual-cycle strategies. Eleven birds followed a common west-central axis within Korea during early spring migration, but northern destinations later diverged into four types: inland-route (n = 7), high-latitude (n = 1), Far East (n = 2), and short-distance/interrupted (n = 2). Among the six individuals for which winter return could be assessed, three remained within the 20-km return zone, one re-entered the zone and later redistributed, and two did not return. For the four individuals with interpretable round-trip records, spring migration tended to be longer and to involve greater stopover use than autumn migration, although differences were not statistically significant. Exploratory weather and NDVI comparisons suggested that stopover duration and timing may vary with pre-arrival freezing conditions and vegetation phenology. This pattern was evident across routes, return outcomes, and seasons. These results indicate strategic diversity within a shared wintering population and support the Nakdong Estuary as an important wintering area and early departure hub for northward migration.