Geomagnetic field affects spring migratory direction in a long distance migrant

Night-migrating song birds travel to and from their wintering and breeding areas often separated thousands of kilometers apart and are clearly capable of finding intended goal areas from a distant location. Displacement experiments provide a useful way to highlight orientation and navigational skills in migrants. To investigate which cues birds actually use to compensate for displacement and the exact mechanism of each cue, experiments with manipulation of single cues are required. We conducted a simulated displacement of lesser whitethroats Sylvia curruca on spring migration. Birds were displaced not geographically but in geomagnetic space only, north and south of their breeding area to test whether they incorporate information from the geomagnetic field to find their breeding area. Lesser whitethroats held in southeast Sweden but experiencing a simulated displacement north of their breeding area (Norway) failed to show a consistent direction of orientation, whereas birds displaced south of their breeding area (Czech Republic) exhibited consistent northerly orientation, close to the expected seasonally appropriate direction, after displacement toward the trapping location. The absence of a clear compensatory direction in birds displaced north might be due to unfamiliar magnetic information or lack of sufficient information such as a magnetic gradient when moving around. By isolating one orientation cue, the geomagnetic field, we have been able to show that lesser whitethroats might incorporate geomagnetic field information to determine latitude during spring migration.