Follow the light

Researchers have long been intrigued with the way some birds, mammals, reptiles, insects, and other groups of creatures use Earth's magnetic fields to navigate. For birds, the compass mechanism remains somewhat mysterious.

Now, researchers have shown that, in principle, light-driven chemical reactions in a bird's eye could play a key role. In essence, similar reactions could allow migratory birds to "see" the Earth's magnetic field as a visual pattern that they can use to orient themselves as they travel. The team, led by Kiminori Maeda at Oxford University, has demonstrated in the lab for the first time that the kind of chemical reactions needed to drive this visual compass can take place in a magnetic field as weak as Earth's.

Until now, no one had demonstrated the effect.The use of a photochemical compass stands in contrast to another mechanism scientists have explored, based on the presence of tiny amounts of the mineral magnetite in the birds' bodies. Still, several scientists have inferred that a migratory songbird's eyes could play a key role in magnetic navigation.

This has come in part through experiments that show that birds are unable to align themselves with Earth's magnetic field if one eye is blocked. And their ability to orient themselves seems to depend on the wavelength of ambient light. Both theory and experiments had started to narrow the mechanism down to light's interactions with chemicals in the birds' retinas. The results appear in this week's issue of the journal Nature.

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