Assessing vector navigation in long-distance migrating birds

Birds migrating between distant locations regularly perform long continuous flights lasting several days. What compass mechanism they use is still a mystery. Here, we use a novel approach, applying an individual-based model, taking compass mechanisms based on celestial and geomagnetic information and wind into account simultaneously, to investigate what compass mechanism likely is used during long continuous flights and how wind drift or compensation affects the resulting tracks. We found that for the 6 cases of long continuous migration flights, the magnetoclinic route could best explain the route selection in all except one case compared with the alternative compass mechanisms. A flight strategy correcting for wind drift resulted most often in routes ending up closest to the predicted destinations. In only half of the cases could a time-compensated sun compass explain the migration routes observed with sufficient precision. Migration from Europe to the Siberian tundra was especially challenging to explain by one compass mechanism alone, suggesting a more complex navigation strategy. Our results speak in favor of a magnetic compass based on the angle of inclination used by birds during continuous long-distance migration flights, but also a capacity to detect and correct for drift caused by winds along the route.