Attractive theory of navigation

Birds sense the disparity between true and magnetic north to help them navigate, new research has shown...
17 August 2017


Reed Warbler


Some of the best navigators on the planet are birds, and they also have all the right attributes to be great pilots too - excellent vision, stamina, and an incredible sense of direction. But in this latter case, how do birds find their way around?

Scientists think that they have a form of in-built compass that enables them to tune-in to the Earth's magnetic field. But there's still a problem with that explanation: it would enable them to navigate north to south by following the strength of magnetic field lines, but it wouldn't tell them where they were around the planet in an East-West direction. Or could it?

By magnetising migrating reed warblers in Russia, Bangor University researcher Richard Holland reckons they're exploiting the difference between magnetic and true North to work out where they are. Writing in Current Biology, Holland and his colleagues used a coil apparatus to expose groups of birds to an 8-degree counter-clockwise shift in the local magnetic field.

If the birds were merely using the local magnetic field as a basic compass, the team reasoned, then the animals should show a tendency to orientate themselves on a heading no more than 8 degrees from normal. But they didn't. The magnetised birds re-oriented their potential flight paths by nearly 90 degrees, showing a tendency to orient in an East-South-East direction rather than the West-South-West heading they should have taken.

"It was as though they were flying to West Africa from Aberdeen, in Scotland, rather than much farther east in Russia," says Holland.

The team think that this happened because the birds are sensitive not only to the intensity of the local magnetic field but also to a component known as the "declination". This is the disparity between true and magnetic north, the extent of which varies across the Earth's surface from east to west. By sensing this disparity, the birds can use it, together with the intensity of the local field, like a grid pattern across the planet to pinpoint their location, even in unfamiliar areas.

This is something they need to learn though, because when Holland's team exposed newly-hatched reed warblers that had yet to make a migratory journey to their magnetic field apparatus, the animals were completely confused and reoriented themselves in all sorts of different directions afterwards. 

"So this is something they have to learn, and then remember," Holland suggests. 

What the new paper cannot speculate on is how the birds are able to perform this feat of magneto-detection.

"There are two theories," says Holland. "One is that the birds have photopigment molecules in their eyes that are sensitive to magnetic fields, meaning the birds can see magnetic field intensity. The other idea is that birds and other animals might have cells that contain iron particles which pick up changes in magnetic fields, although where those are we don't know..."


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