The birds recalibrate their internal clock at every dusk and dawn. This means that for every degree of longitude a bird covers between sunrise and sunset, its clock falls four minutes out of sync with local time. In consequence, a Manx shearwater misjudges where the Sun should be and veers off course clockwise when it flies east, or anticlockwise when it flies west.
Scientists had already shown something similar in other birds by catching penguins and quickly moving them across lines of longitude, but before Lewin’s study no one had clearly demonstrated it in wild animals moving on their own, mostly because the effect is small, and so requires
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