Data from: When the sun never sets: diverse activity rhythms under continuous daylight in free-living arctic-breeding birds

Circadian clocks are centrally involved in the regulation of daily
behavioural and physiological processes. These clocks are synchronized to
the 24 h day by external cues (Zeitgeber), the most important of which is
the light–dark cycle. In polar environments, however, the strength of the
Zeitgeber is greatly reduced around the summer and winter solstices
(continuous daylight or continuous darkness). How animals time their
behaviour under such conditions has rarely been studied in the wild. Using
a radio-telemetry-based system, we investigated daily activity rhythms
under continuous daylight in Barrow, Alaska, throughout the breeding
season in four bird species that differ in mating system and parental
behaviour. We found substantial diversity in daily activity rhythms
depending on species, sex and breeding stage. Individuals exhibited either
robust, entrained 24 h activity cycles, were continuously active
(arrhythmic) or showed ‘free-running’ activity cycles. In semipalmated
sandpipers, a shorebird with biparental incubation, we show that the
free-running rhythm is synchronized between pair mates. The diversity of
diel time-keeping under continuous daylight emphasizes the plasticity of
the circadian system, and the importance of the social and life-history
context. Our results support the idea that circadian behaviour can be
adaptively modified to enable species-specific time-keeping under polar
conditions.