Atmospheric conditions facilitate mass migration events across the North Sea

Crossing large bodies of water can be extremely risky for terrestrial migrants and yet such migrants have been observed to cross hundreds and even thousands of kilometres of water. However, the mechanisms that enable nocturnal migrants to cross large bodies of water successfully remain unclear. In the north of the Netherlands, autumn migration of birds occurs mainly along a southwesterly axis and can be predicted very successfully. However, on rare occasions, intense and unexpected migration events occur with a significantly different track direction. We hypothesized that these events represent birds arriving from Norway and other Scandinavian countries after crossing the North Sea barrier. We implemented a back-trajectory model calibrated with radar data from autumn 2006–2008 to test our hypothesis, assuming that birds maintain a constant heading and airspeed during flight. A total of 14 events were identified. In some cases measured mean ground speeds were twice as high as mean airspeeds. Model results demonstrated that birds took advantage of atmospheric conditions to cross the North Sea quickly. In the majority of cases, birds could be tracked to Norway, Denmark or Sweden when maintaining a constant airspeed and heading en route, but not in all cases. Thus migrants must be flexible in their reaction to wind to take advantage of dynamic and heterogeneous atmospheric conditions experienced en route. The integration of measurements with simulation modelling provides a powerful framework to improve our understanding of how animals move through a dynamic environment and the consequences of their behaviour.