Supplementary material from "Evidence for latitude-driven changes in diel rhythms in a wide-ranging seabird"

Diel rhythms synchronised to Earth’s photic cycle are near-ubiquitous amongst animals living in regions with distinct day-night cues. Where such cues are reduced or absent, however, activity patterns may weaken, reorganise, or become decoupled from the light–dark cycle, which may allow for more flexible behavioural expression. Using a dataset of >900 free-ranging black-legged kittiwakes from colonies spanning a broad latitudinal gradient (40-81°N), we show considerable population-level variation in diel activity patterns, from clear 24-hour rhythms with distinct peaks at dawn and dusk at lower latitudes, to variable and non-diel rhythms with flattened activity patterns across the 24-hour day at high latitudes. These patterns were consistent with the wide spectrum of light-dark cycles experienced across their breeding range, supported by our finding that activity was strongly predicted by sun elevation, suggesting alignment with the light-dark cycle. Together, our findings reveal substantial within-species variation in rhythmicity across latitudes and demonstrate that changes in the photic environment can reshape the temporal organisation of activity even within a single, widely distributed species.