Variation in behavioural and physiological chronotypes and their fitness consequences in great tits

Birds cope with diel changes in their environments across the 24 h day by regulating behavioural and physiological processes, such as rest and activity, metabolism and body temperature. Individuals differ in timing of these processes, for example due to genes and environmental conditions. Individuals with consistent differences in diel timing have been categorised into chronotypes, along a spectrum from early to late. However, diel timing can be temporarily modified within individuals to cope with specific situations and thus, to gain higher fitness. This plasticity is thought to be higher for behaviour than for underlying physiology, raising the risk of internal desynchronisation within the individual.This study investigates individual consistency and plasticity of diel rhythms of activity and body temperature and associated fitness benefits and costs. Chronotype might affect fitness by overall timing, but also by differences in plasticity. For example, individuals could benefit from adjusting their timing in challenging situations. However, desynchronisation of behaviour and physiology could come with fitness cost and counteract this benefit.Here, we used remotely collected data on diel activity and skin temperature to determine between and within individual variation in wild great tits during the breeding season. For this, individuals were equipped with temperature-sensitive radio transmitters, while additional information on female incubation patterns was collected using small temperature loggers inside the nest. Additionally, fitness proxies were extracted from brood monitoring. Our study will give new insights into the relation between physiological and behavioural rhythmicity within and between individuals, and its consequences for fitness.