Migrating birds posses a sixth sense that allows them to detect the difference between magnetic and true north, new research suggests.
By measuring the variation, which changes depending on location, the birds can navigate in a way that defeated human sailors for centuries, scientists believe.
Every orienteering enthusiast knows that while a compass needle points north under the influence of the Earth's magnetic field, the north pole as viewed on a map can be one or more degrees further east or west.
When navigating using a map and compass, this "declination" has to be accounted for, especially when travelling long distances.
Scientists have now shown that at least one species of migratory bird, the reed warbler, can sense declination and use the information to plot its longitudinal point on Earth.
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The problem of longitude - the co-ordinate that specifies an east-west axis position on the Earth's surface - proved unresolvable until the invention of accurate clocks.
Reed warblers do not need clocks, at least not mechanical ones.
The way they do it remains a mystery, but the birds can use declination to work out their equivalent of longitude, the researchers found. In this way, they keep themselves pointing in the right direction during their autumnal migration from Russia to Africa.
Dr Richard Holland, from the University of Bangor in Wales, said: "How birds have resolved the longitudinal problem has been a scientific mystery.
"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."
The findings are reported in the journal Current Biology.
