Female chronotype is not related to annual and lifetime reproductive success in a free-living songbird.

Circadian clocks play a crucial role in regulating the sleep-wake rhythm of organisms, aligning their activity with fluctuating environmental factors, such as light intensity. Still, significant and consistent interindividual differences in the timing of activity, known as chronotypes, have been observed across various species, but whether this affects fitness is still unknown. While previous studies have primarily focused on annual reproductive success, few studies have examined associations between chronotype and lifetime reproductive success. Here, we, therefore, study the association between chronotype, i.e. the emergence time from the nest box in the morning at the onset of reproduction, and annual reproductive success, lifetime reproductive success and longevity in free-living female great tits. We used a long-term dataset of individually marked birds, with the number of eggs, fledglings and recruits produced by a female serving as proxies for reproductive success and the age at death as a measure of longevity. Intriguingly, we did not find significant relationships between chronotype and reproductive success or longevity, and hence, no evidence for directional selection on chronotypes. As we found neither evidence of stabilizing nor disruptive selection, we could not show evolutionary implications of individual variation in chronotypes. Further experimental investigations and complementary studies in other populations will be necessary to understand whether and to what extent chronotypes are adaptive and whether our results are generalizable.