MIGRATING birds probably use a chemical compass to navigate with, rather than a physical one.
Pigeons have particles of magnetite in their beaks, which some researchers have suggested could align with the Earth鈥檚 magnetic field and give birds a directional cue. But a study of European robins (Erithacus rubecula) indicates that birds are more likely to rely on chemistry.
Thorsten Ritz of the University of California at Irvine and his colleagues placed captive robins in cages and manipulated the length of their days and nights to simulate the onset of spring. By altering the lighting, the researchers could cause the birds to attempt to migrate north.
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If they exposed the birds to an artificial oscillating magnetic field exactly aligned with Earth鈥檚 natural magnetic field, the birds set off in the right direction. If the fields were not aligned, the birds were confused. 鈥淭hey became disorientated,鈥� says Ritz.
The oscillating field was too weak to affect any magnetite particles in the robins. Instead, it may be affecting pigments in the robin鈥檚 eye. In photosynthesis, electrons hop between pairs of pigment molecules, altering their energy state. The researchers speculate that the magnetic fields trigger a similar process in the robin鈥檚 eye, producing a visual cue that the birds use to see which way is north (Nature, vol 429, p 177).