More stopovers facilitate migratory waterfowl tracking shifting vegetation green-up

Climate change is altering the phenology of food resources, and migratory birds may respond by adjusting migration timing to track shifting food availability (i.e. their phenological sensitivity). The ability to adjust migration timing can vary among species according to traits such as ecological guild, body size and migration distance. Stopover sites are critical components of migration, allowing birds to refuel and acquire updated environmental information. However, empirical evidence on how stopover use enhances phenological sensitivity to shifting spring green-up remains lacking. In this study, we investigated how stopover frequency influences the phenological sensitivity of migratory waterfowl at breeding areas. We hypothesized that individuals using more stopover sites would better align their migration timing with shifts in vegetation green-up. We reconstructed 220 spring migration paths from five East Asian migratory waterfowl species using high-resolution tracking data collected from 2015 to 2023. Spring vegetation green-up was derived from remote sensing data. We then fitted linear mixed models to test how stopover frequency influences the change of arrival timing at breeding sites. Individuals using a greater number of stopover sites showed closer alignment between their migration timing and local spring vegetation green-up at breeding areas. More frequent stopover use was associated with stronger phenological sensitivity, suggesting greater behavioural plasticity in tracking shifting resources. Loss of stopover habitat may therefore limit their capacity to respond to changing resource phenology, underscoring the importance of stopover conservation for promoting resilience to climate change.