The calculation of longitude was one of the great scientific challenges of the 18th century. Very valuable prizes were offered for its solution, while countless sailors died from the delay. Yet birds solved the problem millions of years ago. Their solution, at least for reed warblers, has now been revealed as measuring magnetic declination – the angle between magnetic north and true north – as well as the more widely used intensity and inclination.

Calculating latitude is a relatively easy task, since (much to the annoyance of flat earthers) different stars can be seen depending on the distance from the equator. Longitude is much harder, and prior to GPS required very accurate clocks that could compare local time with that at a known location.

As a new paper in Current Biology notes: “Birds do not appear to possess a time-difference clock sense.” Despite this, some manage to go on east-west migrations, as well as the more common north-south ones, homing in on specific sites. Moreover, the paper says “Night-migratory songbirds can correct for east-west displacements to unknown locations.”

The authors caught 15 reed warblers (Acrocephalus scirpaceus) in western Russia on their way to Africa. These were placed inside a magnetic field with the same strength as that of the Earth locally, but a declination differing by 8.5 degrees. The artificial field mimicked that near Edinburgh. The birds responded as if they were indeed now in Scotland, turning 151º to fly east-south-east, rather than west-south-west.

 "We've shown for the first time that magnetic declination may be a component of the magnetic navigational map, at least in some long-distance migratory birds," said Dr Nikita Chernetsov of the Zoological Institute of the Russian Academy of Sciences, in a statement.

The research also revealed a learned component. Twenty-five birds too young to have undergone a migration through Western Europe became confused on exposure to the altered field, trying to fly in random directions.

"Reed warblers seem to learn the large-scale spatial pattern of the declination gradient during their annual movements, just like they learn other gradients, inclination, and total intensity," Chernetsov said. "As magnetic declination mainly varies along the east-west axis, it provides the possibility to measure longitude."

Unresolved questions include how reed warblers understand the implications of a gradient from a location they have never experienced, and whether all migratory birds use the same technique. The discovery could have been of great use to navigators in previous centuries, who might have found compasses that could measure magnetic declination easier to build than clocks that would keep time on a rolling ship. Nevertheless, it is not clear if there are any technological applications for which it could be used today.