Departing With the Wind: Spring Migration Timing in Brent Geese From Their Most Important Staging and Wintering Site,...

The timing of migratory departure represents a critical behavioural decision for long-distance migrants, influencing arrival condition at breeding grounds and reproductive success. Despite growing recognition of wind as a proximate departure cue, the relative importance of multiple meteorological variables, and whether wind selectivity changes over the departure window, remain poorly quantified at the individual level for most species. We examined meteorological drivers of individual spring migration departure in Dark-bellied Brent Geese (Branta bernicla bernicla) from the German Wadden Sea, the species' principal staging area along the East Atlantic Flyway. GPS tracking data from 43 individuals contributing 47 departure events across seven seasons (2015-2024) were combined with hourly meteorological data extracted at each bird's staging location and analysed using Cox proportional hazards models across 18,036 hourly risk intervals. The best-supported model combined wind assistance at 100 m altitude-consistent with the median sustained flight altitude of 69 m-and atmospheric pressure, with departure probability increasing by 20% per additional m/s of tailwind (HR = 1.20, 95% CI 1.12-1.29) and 10% per additional hPa (HR = 1.10, 95% CI 1.06-1.15). Relative humidity was a competitive secondary predictor, with higher humidity reducing departure probability (HR = 0.959 per %, 95% CI 0.932-0.986). Time-varying analyses revealed that wind selectivity was strongest early in the season (HR ≈ 1.69) and declined to near unity by early June (HR ≈ 0.98), reflecting increasing migratory urgency. Atmospheric pressure showed no time-varying effect, acting as a stable synoptic quality filter rather than an urgency-modulated cue. These findings demonstrate that Brent Geese fine-tune spring migration onset by integrating local wind conditions and synoptic pressure to exploit transient atmospheric opportunities, highlighting the potential vulnerability of this migration system to projected shifts in North Atlantic wind regimes under ongoing climate change.