Each fall, eastern North American monarch butterflies migrate 4000 kilometers south to central Mexico. In daylight, the butterflies navigate by the Sun's position and their antennal circadian clocks. But under overcast skies, they rely instead on a magnetic compass, Guerra et al. found. The team put monarchs in a flight simulator surrounded by a magnetic coil, measuring responses to horizontal, vertical, and intensity changes in the magnetic field. Monarchs, they found, navigate north or south using the change in dip of Earth's magnetic field lines with latitude. And as in birds, the compass is light-sensitive; monarchs need ultraviolet-to-blue wavelengths to find their way.
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