Reproductive fitness is associated with female chronotype in a songbird

Research on biological rhythms has revealed widespread variation in diel timing within populations. Repeatable individual chronotypes have been linked to performance in humans but, in free-living species, benefits of chronotype are poorly understood. To address this gap, we investigated fitness correlates of incubation patterns in female songbirds (great tit, Parus major) at urban and forest sites. We confirm repeatable chronotypes (r [≥] 0.31) and show novel links between chronotype and reproductive success. In both habitats, females that started activity earlier in the day raised more fledglings. We also observed that forest females started their day at a similar time throughout the breeding season, whereas urban females tied their onset of activity closely to sunrise. Our study points to possible mechanisms that underlie chronotype variation and provides sought-after evidence for its relevance to fitness.