Variation in diel timing in great tits is affected by the timing of their social mate

Diel rhythms are driven by genetic and environmental components. These rhythms are mediated by the circadian clock, and entail rhythmicity of various physiological and behavioural traits. Although individuals show to some extent repeatable timing (i.e., chronotype), there is ample variation of diel timing observed within and between individuals of the same species. Here, we investigated various environmental factors, including timing of the social partner, that could explain day-to-day variation within individuals. Synchronisation with the social partner during provisioning timing could increase breeding success, and decrease extrapair paternity opportunities for females during the fertile period in case of consistent timing across the breeding stages. Therefore, we also investigated fitness consequences of between-individual variation. We first assessed the magnitude of between- and within-individual variation in the timing of nest visits by great tits (Parus major). We monitored nest visits of males and females in 37 broods during chick provisioning in 2020 and 2021. Next, we explored the responsiveness of the diel timing to environmental variables, specifically comparing abiotic and social factors. The onset of nest visits varied significantly with day within the breeding season, rainfall and the diel timing of the breeding partner but not with night temperature. In response to the partners onset, females responded stronger compared to males. By contrast, offset was generally more variable within individuals and less variation was explained by the environmental variables. Both males and females delayed their activity offset with the progressing season and females also had a later onset with more daytime rainfall. Further, the reproductive output and extrapair paternity were independent of parental chronotype and their synchronisation within pairs. It is possible that consistency of chronotypes is less important for reproductive success than the ability to plastically respond to changing environmental conditions. Thus, the next step could be to investigate potential individual differences in plasticity which could be even linked to specific chronotypes. This information might be crucial to predict how species can cope with unpredictable environmental conditions.