In the 1700s, scientists, sailors, engineers and explorers were trying to work out how to navigate east to west at sea. The stakes were high; many lives were lost because sailors could not reliably measure longitude. Eventually, the invention of accurate timepieces allowed sailors to use the time in their current location, and the time at a fixed reference, to calculate longitude. But how do long-distance migrants navigate longitude at sea without a time-difference clock sense? Nikita Chernetsov of the Zoological Institute of Russian Academy of Sciences and an international team of collaborators suspected that for Eurasian reed warblers, magnetic fields might provide an answer.
Some species navigate east to west using magnetic inclination, which is the angle of magnetic field lines relative to the Earth's surface, and magnetic field intensity. Because these cues vary independently, they can create a bi-coordinate map, which migrants use to get their bearings. The problem is that in many parts of the world, these cues run in parallel and so cannot be used to measure longitude. The research team suspected that if warblers could locate true north using celestial cues, and detect magnetic north, then they could measure magnetic declination – the angle between true north and magnetic north. Together, magnetic inclination and magnetic declination create a grid that migrants might use to navigate.
To test this idea, the team caught adult migrants flying over Russia. To check whether catching these birds disrupted their flight path, they released them into small, funnel-shaped enclosures where they could hop around; at night, the captive birds became restless and tended to move in the same direction as migrants. The birds were put in the funnels on a clear, starry night, so that they could use the stars to navigate; they moved in a WSW direction. The team then manipulated the magnetic field around the birds’ enclosures, holding magnetic inclination and intensity constant but altering magnetic declination. This meant that while the birds were geographically in Russia, they were surrounded by the geomagnetic field found in Dundee, Scotland. This time, when the birds were released into the enclosure on a clear, starry night, they changed their direction of movement by 151 deg and moved in an ESE direction. This enormous change suggests that migratory reed warblers do use magnetic declination to navigate.
The team then repeated this experiment with juvenile birds, reasoning that because these inexperienced migrants had not yet established their functional map, they should be disorientated by a change in magnetic declination if they use magnetic declination to navigate. Juveniles that were caught and released into the same enclosures moved in a WSW direction, just like adult migrants, but when magnetic declination was manipulated around these birds, they moved around randomly. This further supports the idea that magnetic declination is a cue that helps birds chart their course east to west.
This work reveals an elegant solution used by long-distance migrants to the longstanding challenge of navigating at sea.