All northern migrants wintering north of the equator set off on their spring journey in conditions of increasing daylengths, whereas those wintering south of the equator set off in conditions of decreasing daylengths. Such flexibility in wintering area within species again implies the importance of an internal clock in influencing the timing of spring departure, for outside the breeding season individuals may be exposed to markedly different daylength regimes from year to year. If these birds use daylength change to calibrate their internal clocks, they must do so in their breeding areas, at a latitude which they occupy consistently from year to year. Because birds can measure daylength, the longest day (at the summer solstice) could provide a useful baseline against which any internal clock could be re-set, but so far as I am aware, this possibility has not been tested. Despite the use of an internal clock as the primary timer, birds must presumably also respond to prevailing daylengths in order to adjust the timing and speed of migration to their particular wintering latitude, as explained above.
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