A small European robin perched on a branch can lose its bearings simply because the light around it changes colour. How can a migrating bird steer by a field it cannot see?
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The magnetic compass was first detected in the European robin in the 1960s, and since then in more than twenty other species, mostly songbirds, but also pigeons, a sandpiper and the domestic chicken. It does not read the north-south polarity of the Earth's field, but its orientation, which is enough for the bird to find its way.
The most striking feature is its dependence on light. In robins and in Australian silvereyes, the compass works under blue, turquoise or green light, up to about 565 nanometres. Under yellow light, from about 590 nanometres, the birds lose their direction.
The most studied explanation involves a protein, cryptochrome, which reacts to blue light and forms pairs of radicals. This mechanism remains a well-supported hypothesis, not directly proven in a living bird, and another possibility, magnetite crystals, is still debated.
