Navigation by extrapolation of geomagnetic cues in a migratory songbird

Displacement experiments have demonstrated that experienced migratory birds translocated thousands of kilometers away from their migratory corridor to unfamiliar areas can orient towards and ultimately reach their intended destinations. This implies that they are capable of “true navigation”, commonly defined as the ability to return to a known goal after displacement to a completely unknown location without relying on familiar surroundings, cues that emanate from the destination, or information collected during the outward journey. In birds, true navigation appears to require previous migratory experience, and it is generally assumed that, to correct for displacements outside the familiar area, birds initially have to gather information within their year-round distribution range, learn predictable spatial gradients of some environmental cues within it and extrapolate from those to cues of unfamiliar magnitude ̶ the gradient hypothesis. However, the nature of the cues used, and evidence for actual extrapolation remains elusive. Geomagnetic cues (inclination, declination and total intensity) provide predictable spatial gradients across large parts of the globe and could serve for navigation. We tested the orientation of long-distance migrants, Eurasian reed warblers (Acrocephalus scirpaceus), exposing them to geomagnetic cues of unfamiliar magnitude only encountered beyond their natural distribution range. The birds demonstrated re-orientation towards their natural migratory corridor as if they were translocated to the corresponding geographic location but only when all naturally occurring magnetic cues were presented, not when declination was changed alone. This result represents direct evidence for migratory birds’ ability to navigate using geomagnetic cues extrapolated beyond the range of magnitude they previously experienced