Urban effects on timing and variability of diel activity differ across passerine species and seasons

Life on Earth is adapted to rhythmic cycles in environmental conditions throughout the day and year via diel patterns of behavioural activity. Urban conditions can disrupt such behavioural rhythms of activity. However, most studies so far have investigated urban effects on patterns of activity of single species in a single season. Additionally, we know little about the level of between- and within-individual variation in urban and non-urban populations, and whether they differ. Here, we use automated radio telemetry to record patterns of daily activity in six passerine species (blackbird, robin, great tit, blue tit, dunnock and chaffinch) across two urban and two forest populations during the pre-breeding and post-breeding seasons. We investigate urban effects on five activity-related traits: time of activity onset, time of activity end, duration of diurnal activity, level of diurnal activity and level of nocturnal activity. We employ statistical tools that allow us to estimate urban effects on mean phenotypic values but also quantify urban versus forest differences in between-individual and within-individual phenotypic variation. We found the strongest urban effects on time of activity onset in blackbirds and robins during both the pre- and post-breeding seasons: urban populations of blackbird and robin started their daily activity earlier than their forest counterparts. We did not find this effect in the other species. Urban populations of all species showed higher levels of nocturnal activity than forest populations, but this effect was not offset by lower diurnal activity, suggesting that urban birds may incur higher daily energetic demands. Lastly, our analysis revealed large and consistent differences in variation in the investigated timing traits. Onset and end of daily activity were more variable in urban birds between individuals, implying lower population synchronisation, and more variable within individuals, implying less consistent behaviour, than in their forest counterparts. Conversely, activity levels were more variable in forest birds. We conclude that, for birds, urban life is associated with less rest, less consistency and lower synchronicity, but that effect sizes depend on species and time of the year. Our results warn against generalising the effects of urbanisation on daily rhythms of birds and call for future studies to understand the mechanisms behind species and seasonal differences.