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Many animals perform their migratory movements within discrete time periods of the day/night cycle. Among migratory songbirds the majority of species generally restrict their migratory flights to the night. This makes their nocturnal departure timing a key factor determining the potential duration of migratory flights, which in turn affects their travel speed. Previous correlative studies revealed that part of the variation in nocturnal departure timing of migratory songbirds is explained by the respective length of the night, with birds departing earlier when nights are short. As the length of the night is inextricably linked to the time within the season, it has been impossible to ascertain which of the two factors drives variation in nocturnal departure timing. To virtually decouple both factors, that is, length of the night and time within the season, we conducted a series of short-term experiments using migratory songbirds caught at stopover. These birds were exposed to nights of either natural or shortened length. We found that birds exposed to shortened nights showed slightly earlier nocturnal departures than those exposed to the natural length of the night. This coincided with a seasonal pattern in the start of nocturnal migratory behaviour (migratory restlessness during the experimental period and actual departures following release) among birds, irrespective of the experimental treatment. Based on these results we suggest that birds' nocturnal departure timing pursues an innate seasonal schedule of migratory activity, which is adjusted to the current length of the night. Such a mechanistic framework provides the potential for cross-calibrating birds' innate seasonal schedule of migratory activity with the current spatiotemporal progress during migration. |
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