Geomagnetic navigation is a well-known phenomenon in birds. Relocation experiments have demonstrated that migratory birds that are translocated thousands of miles away from their normal migratory route have the capability to orient and correct themselves towards their expected migratory route. This ability to correct one’s migratory route after displacement is called ‘true navigation.’
It has long been assumed that true navigation required previous migratory experience. The ‘gradient hypothesis’ states that birds initially gather information within their year-round distribution range, learn predictable spatial gradients of environmental cues within it, and extrapolate unfamiliar magnitudes from those. This hypothesis was first postulated by Wallraff (1974) and later developed by others. However, it is still not clear how birds perceive environmental cues for orientation and the evidence for extrapolation remains elusive. It is assumed that geomagnetic fields are one of the navigation cues because they can provide predictable spatial gradients across large parts of the planet.
In a recent study, however, Dmitry Kishkinev et al. (2021) showed that when birds were exposed to magnetic cues of unfamiliar magnitude (those encountered beyond their natural distribution range) they were still able to navigate back to their migratory route. In this laboratory experiment, Eurasian Reed Warblers (Acrocephalus scirpaceus) were exposed to ‘virtual magnetic fields’ and their behavior was monitored. First, birds responded to novel magnetic fields as if they were displaced to the equivalent location. However, changing one magnetic cue only (declination) did not result in re-orientation. The ‘‘virtual displacement’’ only worked when all magnetic cues matched a real place. The birds demonstrated re-orientation toward their migratory route as if they were translocated to the corresponding location but only when all naturally occurring magnetic cues were presented, not when declination was changed alone. Their study shows that Eurasian Reed Warblers can use a combination of cues derived from the Earth’s magnetic field to detect a displacement, even if all of these cues are of unfamiliar magnitude, and adjust their migratory direction accordingly, i.e., they are able to perform magnetic true navigation without previous experience.
This might explain observations so-called vagrants (birds that are detected outside of their expected migratory route): normally, most birds can correct their migration route after weather involving strong winds or storms produce a displacement. This occurs when birds are blown from their normal route by strong winds and the distances involved are so great that any initial disorientation by adverse weather would be compensated before arriving to their final destination. Reverse migration, however, is an understudied phenomenon that could also explain displacements. When reverse migration occurs, some birds behave as if their perception of north and south is opposite. It’s as if their brain has stored their migration map in reverse.
Even as we learn more about migration, the mysteries of how birds manage to make transcontinental flights and re-orient after weather disruptions continue to thrill and scientific inquiries deepen. Research involving satellite transmitters and other devices, for example, are revealing remarkable capacities for flight and orientation on the migration journeys of some species, like the 12,000 kilometers non-stop of Bar-tailed Godwits (Limosa lapponica) from Alaska to New Zealand (Hedenström 2010).
Hedenström, A. (2010). Extreme endurance migration: what is the limit to non-stop flight? PLoS Biology 8(5) e1000362
Kishkinev, D., Packmor, F., Zechmeister, T., Winkler, H.-C., Chernetsov, N., Mouritsen, H. & Holland, R.A. (2021). Navigation by extrapolation of geomagnetic cues in a migratory songbird. Current Biology in press. https://doi.org/10.1016/j.cub.2021.01.051
Wallraff, H.G. (1974). Das Navigationssystem der Vögel. Oldembourg.