Clocks in the wild: biological rhythms of great tits and the environment

Biological clocks play a fundamental role in the physiological and behavioural processes of organisms. Internal timekeepers evolved to anticipate environmental changes, the most important of these being the geophysical light-dark cycle, to coordinate external changes with the timing of internal processes. Research into functions of the biological clock during captive studies has provided valuable insight into mechanisms by which clocks function, and how small environmental changes can affect the clock and its outputs. However, biological clocks have so far been understudied in ecology.
\nIn this thesis, this gap in knowledge was addressed by placing studies of chronobiology into the context of the natural environment. A model species in avian ecology, the great tit (Parus major) was used to investigate biological rhythms in the wild at three levels; behaviour, transcripts and life histories. This thesis investigated how features of the natural environment shapes rhythms of behaviour and physiology in a wild animal, using experimental and observational approaches.
\nDifferences in timing of individual rhythms, or chronotype, may provide wild animals with different consequences for fitness. In this thesis, individual behavioural rhythms of incubating great tits were quantified for birds in city and forest environments. There were strong effects of both the number of days to hatching and site on timing of incubation activities, where city birds rose earlier, and stayed out later, than forest birds. Maternal chronotype was then linked to fitness traits.
\nCity birds face a number of new challenges in the urban habitat. The impacts of one feature of the urban habitat, artificial light at night, was tested using a forest nest box system. Nestling great tits were experimentally exposed to low-level artificial light at night, and aspects of condition and clock and immune gene transcripts were compared for nestlings under light at night and dark-night control. Nestlings under light at night treatment weighed less than control birds, and suppressive effects of light at night treatment were found for genes involved in the core pathways of the circadian clock and immune system. Time of day differences were also observed in transcript levels of genes. 
\nParasitic infections can cause consequences for fitness and reproductive success of wild birds. In this study, effects of infection with avian malaria parasites on nestling condition and immune system were investigated, at city and forest sites. The prevalence of Leucocytozoon parasites was higher at forest sites than city sites and increased with the season. Infection had no suppressive effects on immune genes of nestlings, and no negative effects on condition were found.
\nIn mammals, malaria is otherwise known as the “circadian disease” due to rhythmic development of parasites during their life cycle. In this study, host-parasite interactions with avian malaria parasites were investigated in the context of biological rhythms in wild great tits. Transcript levels of nestlings were determined by field sampling across a temporal profile and linked to infections with Leucocytozoon parasites. Leucocytozoon infection reduced overall transcript levels for circadian clock and immune gene targets, but did not alter the timing of expression. 
\nThis study ultimately demonstrated the importance of biological clocks for the ecology of great tits and provided important advances for studies of clocks in the wild.

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