Flexibility in the diel migratory schedule of an Arctic songbird despite robust 24‐h activity rhythms across the annual cycle

Abstract

Many birds schedule their activity to a specific phase of the diel (24‐h) light–dark cycle. Two notable exceptions are the nocturnal migration of many otherwise diurnal songbirds and the diverse activity patterns of birds during the continuous light above the Polar Circles during summer. Assumptions about the phase relationship between migratory activity and the diel cycle can be incorrect, and the diel activity of Arctic migratory birds during the year is largely unknown. We used the snow bunting Plectrophenax nivalis to investigate whether breeding songbirds above the northern Polar (Arctic) Circle have 24‐h activity rhythms with distinct active and inactive phases across the annual cycle, and whether their migration aligns with a specific phase of their activity rhythm. We found that male snow buntings maintain a robust 24‐h activity rhythm with distinct active and inactive phases across most of the annual cycle, including during polar summer, but the robust 24‐h periodicity attenuated during vernal migration. Birds scheduled long and short flights across their diel activity rhythm. However, shorter flights most often began during the active phase, and the longest flights most often began during the transition between the active and inactive phase of the diel activity rhythm. This indicates that snow buntings can flexibly schedule their flights across their prominent diel activity rhythm, although their longest flights during migration typically overlapped with their normal inactive phase, probably corresponding to nighttime. Several open questions, however, remain about the generality of our results for other songbirds, such as: what is the phase relationship between the diel activity rhythm and flight to the diel light–dark cycle, and how do historical accounts of diel migration phase align with empirical activity data for other species?