Magnetic fields guide migratory birds

There’s a growing body of evidence that birds can detect magnetic fields like the one around the Earth, possibly by “seeing” them. And this, researchers think, accounts for how migrating birds, like reed warblers, manage to find their way, seemingly unerringly, half way around the planet. But it’s more subtle than just using the magnetic field like a compass. What Oxford University’s Joe Wynn thinks is happening is that the birds are pre-programmed by their upbringing to fly in a certain direction, but they use the angle - or inclination - of the Earth’s field, to work out how far north or south they’ve travelled, so they know where to stop. Anoushka Handa reports…

Anoushka - 1% left on my phone. No, no, no, no, no, no. Phone's dead, Maps is dead. How on earth do I get home? I wouldn't have to worry about this as a songbird. The earth's magnetic field is key to guiding birds on their journey, but it changes year by year. So, how do reed warblers, a type of songbird, know exactly where to fly home to? If the magnetic field changes year on year, surely they'd also get lost like me. A study involving 80 years worth of data on travel of reed warblers has given us an inclination of how these birds find their way home. Joe Wynne tells us more.

Joe - We found that birds pay attention to something called inclination. Magnetic inclination is the angle between the Earth's magnetic field and the Earth's circuits, and this is one of a number of cues that can be extracted from the Earth's magnetic field.

Anoushka - The magnetic inclination acts essentially as a stop sign for these birds? Is that right?

Joe - Yes. I mean the big problem with using a single cue is that there are lots of places on earth with the same magnetic inclination. In the case of inclination, specifically, because it varies with latitude, all the points on the same latitude have more or less the same inclination. The question we had when we found that birds seemed to be following an inclination was -  "They have to use something else, surely." Birds would be travelling along an already determined direction, maybe they inherit it genetically? They would be traveling in this direction and they would wait until they hit the magnetic inclination value that they thought represented their home, and then they stop.

Anoushka - The inclination is the same in different areas of the earth. How do they know that this is the place that's home and not the place on the other side of the world that has exactly the same inclination.

Joe - I like to think about this like those little cars that you have as a kid that you wind back and then release. The reed warbler is like a small car in the sense that, when it's released, it travels in a straight line. And I kind of imagine the inclination as like a brick wall -  the little car hits the wall and then it stops. In the same way, the reed warbler is travelling in a straight line until they encounter the inclination that they think represents their reading, and then they stop. We understand very well that songbirds probably inherit the direction that they go in when they leave the nest for the first time when they're going South, so it's not unreasonable to suggest that such a mechanism might also help them on return, but it's obviously very hard to say with the data that we've got.

Anoushka - Are there any other animals that have these magnetic campuses and can find their way home using the Earth's magnetic field?

Joe - There's some good evidence for the use of magnetic sensitivity in sea turtles, sea turtles use the magnetic field to remember where to return to. And there's similarly some pretty good evidence in salmon for the use of mag reception in order to find where they spawned.