Alan writes in, acknowledging that many animals use the Earth's magnetic field for navigation purposes. Given the shifts in poles and field intensity we experience due to convection currents in the Earth's core, how do they stay on course? James Tytko asked Miriam Liedvogel, professor of ornithology, to help find the answer...
James - The best supported ideas include the presence of a tiny compass needle of magnetic iron oxide in the beaks of some birds. Or there's the radical pair hypothesis, which explains magnetoreception with quantum mechanics. Mirjam Liedvogel is director of the Institute of Avian Research in Germany.
Miriam - You can think of the Earth's magnetic field as a giant bar magnet. So there is a north pole and a south pole, and the intensity is highest at the poles. And then it gradually decreases towards the equator. And another feature that the Earth's magnetic field, because its spheric orientation, also provides is a difference in the inclination angle. So the angle of this vector, how it enters the Earth's surface, changes. So it's perpendicular at the poles, and it is almost horizontal to the Earth's surface at the magnetic equator. So for birds, we know the inclination is the feature they are using. So they don't care about polarity, actually.
James - Due to turbulence in the flow of molten metal in the Earth's core, which generates the magnetic field, it can be susceptible to changes in intensity. Magnetic poles can wander, for example. The north pole has moved from Canada to near Siberia over the past century. So how do migratory animals react to this turbulence?
Miriam - Yeah, so it's a good question. And we don't know exactly how they do it, but we know that they can cope with fluctuation. I mean, there's daily fluctuation in intensity, also probably inclination. It's not a static entity. It's also not so that they exclusively rely on magnetic cues. So for their migratory journeys, for example, if it's a night migratory bird, the magnetic field is an extremely reliable cue. It's ubiquitously present. But night migrants also use star patterns or the rotational center of the sky or the starry sky. I mean, some other animals use the Milky Way or any other sort of celestial cues to orient as well. And they integrate this information into a very robust navigation program. So if one cue is absent or gives funny signals that doesn't really make sense for what the animal knows it should do, then it can also just say, OK, well, I rely or I prioritise some other cue.
James - Animals really are master navigators, orienting themselves using the sun, the stars, other cues besides the multiple modalities of magnetoreception, potentially also working in tandem, as we've described today.
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