Bird-brained? Not at all: Reed warblers reveal a magnetic map

Scientists are still trying to unravel all the mechanisms involved. Now, one group of scientists believe that they have revealed one system being used by some migrating birds, and it reveals a fascinating 'world-map' that many of us would marvel at.

One of the problems thrown up by covering long distances is how to estimate your current position. While latitude was a relatively simple problem to resolve, it took human geographers, map-makers, scientists, clock-makers and mariners centuries to resolve the problem of how to locate themselves longitudinally, or on the west- east axis, as they crossed vast oceans. The system which eventually resolved the problem for human explorers involved accurate time-keeping.

One aspect of the system used by some birds has been revealed for the first time and involves detecting the declination or variation that occurs because the geographic 'true' north pole and magnetic north are not in the same place.

In Europe, this difference between the true and magnetic north, increases as you move from east to west. When using a map and compass, this changing declination needs to be accounted for, especially if you are to reach a long distance destination. It would appear that the birds detect the declination or variation between the true north and the magnetic north, and from that, their longitudinal axis.

Writing in the current issue of Current Biology (August 2017) an international team of biologists, including Richard Holland and Dmitri Kishkiniev of Bangor University in the UK, explain how they identified for the first time, that mature reed warblers are able to detect the declination from magnetic north, and use the scale of the declination or change from true north to geo-locate themselves to a longitude, from which they orient themselves towards their autumnal migration from Russia to Africa.

"How birds have resolved the longitudinal problem has been a scientific mystery," says Dr Richard Holland of Bangor University. "It seems that a bird as unassuming as the reed warbler, may have a geographic map or memory that enables it to identify its longitudinal position on the globe, only by detecting the magnetic north pole and its variance from true north. This, combined with other external cues, which may include the strength of the magnetic field, star positions or smells enables it to locate its current position and orient itself during a long migration."

In a bid to prove their theory, the group temporarily caught both mature and juvenile Eurasian Reed Warblers. Holding the birds in small funnel shaped cages that record their orientation tendencies, the team recorded the birds' tendency to orient towards their migratory route from Russia to North Africa before changing the magnetic field around the aviaries to represent an 8? variance from true north (reflecting Aberdeen's actual declination). Still contained within orientation cages, mature birds were found to temporarily orient themselves as though they were embarking on a migration from Aberdeen, around 900 miles away from their location in Russia!

The immature birds appeared confounded by the new magnetic signal and were not able to orient themselves, suggesting that this magnetic map is learnt by experience.

The group have opened a window to reveal one system which these birds, and possibly other species use. The revelation however raises as many questions as it answers- how do the birds detect the magnetism? How do they learn the map?