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Migrating birds don't use the Global Positioning System, so how do they know where they're going? Biologists had two theories, both based on birds' ability to sense Earth's magnetic field and use it as a compass. One theory held that clusters of iron-mineral crystals in the birds' upper beaks detect magnetic information and relay it through the trigeminal nerve to the brain. The other theory proposed that the magnetic field is detected by light-sensitive molecules in birds' eyes that communicate the information to a specialized brain region by means of a visual signal. Working with European robins (Erithacus rubecula), Henrik Mouritsen at the University of Oldenburg in Germany and his colleagues have found evidence that favours the second theory, and simultaneously disproved the first (see page 1274). Mouritsen tells Nature more.

How did you prove the second theory?

We proposed that if we inactivated cluster N — the specialized forebrain region that we believed processes signals from light-sensitive magnetosensory molecules in birds' eyes — then the magnetic compass would fail. We inactivated the region by injecting a neurotoxin, then tested the birds in small wooden huts on campus that were fitted out with magnetic coils. We observed the birds' behaviour in Earth's normal magnetic field as well as in two experimental conditions, one with the geomagnetic field turned 120 degrees anticlockwise and the other with a vertical component of the magnetic field inverted. The birds didn't orient at all.

And how did you disprove the first theory?

We cut the nerve connecting the crystal clusters in the birds' upper beak to the brain. We then put them through the same tests. They oriented perfectly in all cases.

How did you know that the neurotoxin you used to inactivate cluster N didn't cause more extensive brain damage?

Birds can use the Sun and the stars as a compass, so we tested them outside at sunset and in a planetarium set to simulate local stars. They oriented wonderfully in both situations.

Why are your findings significant?

If a migratory bird species is facing extinction, conservationists might move it to a new breeding or wintering ground. The birds are shipped to a new area, but once they're released, they go straight back 'home'. For such endeavours to succeed, we need to understand the birds' navigational system to manipulate it during transportation or know how their sensory system develops and move them before they can imprint on where they were born.