How do birds navigate?

Birds are some of the planet’s best navigators - many migrating thousands of miles each year. But how do they 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 would it? By magnetising migrating reed warblers in Russia, Richard Holland from the University of Bangor reckons they're exploiting the difference between magnetic and true North to work out where they are - as Chris Smith found out…

Richard - Every year, obviously, there are billions of birds migrating between Europe and Africa and then back again the next year and these are reed warblers which are quite a long distance migrant. These ones that we’ve been looking at, they are migrating from a place called the Courish spit in Russia and they migrate down to Western Africa.

What is still something of a mystery is how are these birds that are able to make these huge journeys are able to actually navigate during that because we know from previous experiments we’ve done that if you displace them from their normal path to a place they’ve never been to before, they can actually correct for this. It’s not that they’re following familiar routes, they’re able to extract some information from the environment that allows them to calculate their position, even when they’ve been displaced to a place they’ve never been to before.

Chris - What do you think that might be?

Richard - There’s a number of theories, but one theory is the Earth’s magnetic field is providing at least some of the cues. We have some evidence that birds are able to use the strength of the magnetic field to indicate where they are on a north/south axis - what map readers call latitude. So the Earth’s magnetic field is stronger as you approach the magnetic poles and weaker at the equator.

But what has actually remained a mystery to us is what they can then use to decide where they are on an east/west axis - what a map reader would call the longitudinal axis. It was also a problem for human navigators which famously was solved through the longitude prize using a very accurate clock and referencing Greenwich Mean Time to the time of sunrise or sunset or the stars.

Birds don’t appear to do that. We’ve tested that already to see where the birds are similarly using some sort of clock to try and calculate their position on this east/west axis, and that doesn’t seem to be the case.

Chris - So if you jet lag the birds, in other words, they still find their way?

Richard - They still find their way. It doesn’t seem to affect them.

Chris - So they must be doing something else?

Richard - Yeah. There’s been a proposal around for a long time based around something that we call declination. Now declination, again, is something that map readers will be aware of. Declination is the difference between the actual physical north pole, so the top of the Earth and the location of the Earth’s magnetic pole that a magnetic compass will point to.

Chris - In other words there is a difference between what we call true north of the planet and magnetic north? On the ordnance maps that was always an arrow that was slightly off from grid north and you had to reset your compass a few degrees to account for that.

Richard - Exactly.

Chris - And you’re saying what that birds can register that?

Richard - Our experiments seem to suggest that the birds are actually using this. So as you go from east to west across Europe, the degree difference between magnetic and true north changes and it’s therefore, potentially, a cue to this position of the east/west axis on the longitude axis. So what we did in our experiments was to shift the magnetic field by 8 degrees and keep all other cues the same so the birds actually were in Russia. And the birds were sat inside a device called a helmholtz coil, which allows us to change the strength and direction of the magnetic field.

So all we did was make it point 8 degrees to the left of where it would normally point and the birds response to that was quite remarkable. If they’d just simply been using the magnetic field direction as a compass cue they should have only shifted their orientation by 8 degrees. What happened was, instead of heading southwest as they normally do, the shifted dramatically round to southeast and actually responded as if they were in Aberdeen.

Chris - That’s almost like a 90 degree change in direction?

Richard - A 90 degree change in direction.

Chris - From an 8 degree shift in the magnetic field?

Richard - From an 8 degree shift, exactly. So they’re actually this magnetic compass cue as a GPS device, effectively, to locate their position on the Earth.

Chris - Is this a learned thing, Richard, or is is something they are born with? In other words, if you take animals that have done their migratory journey once before, are they the same in this behaviours as animals that have just popped out of the egg and they are doing it for the first time?

Richard - No. And that’s another interesting finding from our experiments. Birds that responded with this dramatic shift were birds that had previously already made a migratory journey. When we showed birds that were first ever making their first migratory journey, they became completely confused by the change in declination.

Chris - So, in other words, they encounter this declination, the distortion of the Earth’s magnetic field away from true north When they’re making their journey they integrate that and learn to use that as a guidance cue, so when you then deform or distort that cue when they’re older by putting them in this coil apparatus that’s why they then go off course?

Richard - Exactly yes, so it’s something that they’ve learned.

Chris - Are we any closer yet to nailing how they are “seeing” this magnetic field, and not the magnetic field but the change in intensity of the magnetic field?

Richard - That’s a whole field within the field itself in fact. There are two hypotheses for how birds can detect the magnetic field: one is that they actually effectively detect it through the eye. We think there are some photoreceptive chemicals that also react to the Earth magnetic field. There’s another competing hypothesis that suggests that  animal and birds in particular have sensory cells with small magnetic particles in them, and that these magnetic particles move in response to the magnetic field and that signals the strength of the magnetic field.

These two competing hypotheses people continue to test to see just how much evidence they can find for these. But those are the most likely ways in which we think birds will detect the magnetic field at the moment.