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From NPR News, this is ALL THINGS CONSIDERED. I'm Melissa Block.

ROBERT SIEGEL, host:

And I'm Robert Siegel.

The ability of birds to migrate to find their way year-in year-out to the same locations is one of nature's great wonders. Scientists know that the birds possess an internal magnetic compass. And now, a study from Germany finds that the garden warbler, a common migratory bird in Europe, passes information about the Earth's magnetic field to the brain through the eye. And that suggests that in some way, birds can see the magnetic field.

Henrik Mouritsen is professor of neurosensory science at the University of Oldenberg in Germany and joins us from there. I tried to say "they can see" with quotation marks around that. Is it fair to say they can see or it looks they can see?

Professor HENRIK MOURITSEN (Neurosensory Science, University of Oldenberg): Well, the indications we have gathered over the last four, five, six years suggests that, indeed, the birds may perceive the compass direction of the Earth's magnetic field as a virtual image. That is correct.

SIEGEL: How did you arrive at that? Or what have you been doing for the past few years to lead you to that - near you to that conclusion anyway?

Prof. MOURITSEN: Well, there were some very interesting behavioral experiments done by mainly a group in Frankfurt, Germany. And what they had shown was that when you ask a bird to orient according to the magnetic field, you can turn the field and a bird will start jumping at different direction in a cage. If you do this experiment on the white light, it works perfectly. But if you do it on the monochromatic light, the bird seems to have trouble if you have yellow or red light, whereas if you have green or blue light, they do it just fine. And that was the first indication that the visual system must play some kind of role in the magnetic compass of birds.

SIEGEL: Are neuroscientists able to ascertain that another species is seeing something that? I mean, is that something that you feel you can verify?

Prof. MOURITSEN: Well, what we have done is basically to act on a paper in 2000, where some physicists suggested how a light-sensitive molecule could theoretically sense the Earth's magnetic field. And we basically found that that type of molecule exists in these birds. And we tried to look into the brain of a bird while it was doing magnetic compass orientation in the lab. And if you look into the brain of a bird doing this kind of behavior, you will see that the only parts of the brain being highly active processing sensory information during such a magnetic orientation task is parts of the visual system; in night migratory birds, using their magnetic compass at night, but not in non-migratory birds.

SIEGEL: Now, this obviously might explain something fascinating about birds. But also, that molecule that actually could translate magnetic fields into some visual impulse, some form of vision, that must be quite an important molecule.

Prof. MOURITSEN: Well, it certainly seems to be for the birds.

SIEGEL: Yes.

Prof. MOURITSEN: A similar molecule is also found in humans, but it doesn't seem to have this - give rise to this effect in humans. And the birds actually have many different kinds of these molecules. And it seems like one of these kinds have evolved into having a different role to play inside the eye of the bird.

SIEGEL: Well, what logically would be, say, the next experiment down this line of research to further explore the relationship between vision and magnetic field?

Prof. MOURITSEN: Well, we would, of course, love to listen in to the neurons and see if we can understand the language they are speaking to the brain, so to say. So we would like to record from neurons - and we are in the process of doing this - so that we can see if the nervous signals that originates in the eye, if they are modified by changing the direction of the field. That is not an easy thing to do, but we are trying.

SIEGEL: These are rather small birds you're talking about. To say it's not easy seems a great understatement.

Prof. MOURITSEN: Well, but there are people recording from neurons in a fruit fly and that's even smaller, so I guess everything is possible. We, of course, have ethical consideration that I will not kill more than a very, very few of these birds. Therefore, we have to do it very carefully.

SIEGEL: Well, Professor Mouritsen, thank you very much for talking with us.

Prof. MOURITSEN: Thank you.

SIEGEL: That's Henrik Mouritsen, professor of neurosensory science at the University of Oldenberg in Germany.

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