Even if blown off course by storms, migratory birds are able to realign their course if they’ve already performed one migration in their life. This remarkable navigational ability has been one of the enduring mysteries of behavioural biology.
Now, research published in the journal Current Biology by an international team shows for the first time, how birds are able to navigate by using both Earth’s global magnetic field and localized magnetic anomalies.
Different parts of the Earth have a distinct ‘geomagnetic signature’ according to their location. This is a combination of the strength of the magnetosphere, Earth’s global geomagnetic field, the magnetic inclination or the dip angle between magnetic field lines and the real horizon, and the magnetic declination, or the angle between directions to the geographic and magnetic North poles.
For the new study, adult reed warblers, a small passerine bird that breeds across Europe in summer and migrates to sub-Saharan Africa in winter, already familiar with their migration route, and its general magnetic signatures, were held in captivity for a short period before being released back into the wild, and exposed in special cages, equipped with strong magnets, to a simulation of the Earth’s magnetic signature at a location thousands of miles beyond the birds’ natural migratory corridor.
Despite remaining physically located at their capture site and experiencing all other sensory clues about their location, including starlight and the sights, smell and sounds of their actual location, the birds still showed the urge to begin their journey as though they were in the location suggested by the magnetic signal they were experiencing.
They oriented themselves to fly in a direction which would lead them ‘back’ to their migratory path from the location suggested to them by the magnetic information they were receiving.
This, according to the researchers, shows that the earth’s magnetic field is the key factor in guiding reed warblers when they are blown off course.
“The overriding impulse was to respond to the magnetic information they were receiving.” explained Richard Holland of Bangor University’s School of Natural Sciences and co-author of the research article .”What our current work shows is that birds are able to sense that they are beyond the bounds of the magnetic fields that are familiar to them from their year-round movements, and are able to extrapolate their position sufficiently from the signals. This fascinating ability enables bird to navigate towards their normal migration route.”
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