Fitness and social interactions in the great tit (Parus major)
Keywords: great tit
social behaviour
Parus major
kin selection
familiarity

Social behaviour is ubiquitous in vertebrate systems, but for many taxa the consequences of sociality remain unexplored, especially for non group-living species that are sometimes dismissed as nonsocial. Although studies of group-living and highly gregarious species make a major contribution to our understanding of this phenomenon, they do not allow the detection of more subtle consequences of social behaviour that may be operating in species with more cryptic social patterns.
This thesis aimed to investigate the fitness consequences of sociality in the great tit (Parus major ). Although great tits do not live in groups, their territoriality during breeding and aggregation in large foraging flocks during winter allows the potential for social interactions in a variety of fitness-relevant contexts. Using 41 years of breeding data from the Wytham Woods great tit population I performed an analysis of between-year territorial familiarity between breeders older than one year and its effect on reproductive success. I found that familiarity with the nearest neighbour and the number of familiar neighbours had positive effects on the female’s clutch size and the breeders’ probability of nesting success, respectively. Both effects were robust to the inclusion of fine-scale spatial factors, and were identified to be underpinned mostly by intrinsic differences between individual breeders. The benefit of familiarity for nesting success led me to hypothesise that familiar neighbours may engage in cooperative activities such as predator nest defence – a hypothesis that I subsequently tested experimentally. To investigate the effects of between-year familiarity on nest defence I performed a field experiment using the natural variation in familiarity in territorial neighbours present in the population. The experimental manipulation revealed that familiar neighbours were more likely to join each other’s nest defence than neighbours that were unfamiliar or neighbours that were first-time breeders. In the light of the importance of nest defence for predator deterrence and nesting success, it is likely that nest defence coalitions are one of the mechanisms through which individuals benefit from familiarity with neighbours.
To test the effects of sociality on a finer timescale, between winter and the subsequent breeding season, I performed a detailed analysis of the stability of relationships in winter foraging flocks and individuals fitness. Because the largest determinant of fitness is whether an individual breeds or not, I analysed the effect of social stability on the probability of breeding, as well as the effect on measures of reproductive success for individuals that were recorded breeding. I found that individuals were more likely to breed when their social stability was high, and that individuals that had more stable social relationships fledged heavier young. These effects seem to be underpinned by phenotypic plasticity of individuals in the former case, and intrinsic differences between individuals in the latter.
An analysis of within-season associations in overwintering juveniles revealed a strong spatial effect on the rate of association between juveniles in the vagrancy stage of natal dispersal. Independent of the spatial factors, the study revealed kin structure in the rate of associations driven by a high rate of association between brothers. The presence of kin structure reveals the potential for kin selection in the winter population.
Overall, this thesis reveals complex, existing and potential fitness consequences of sociality operating at different times in the life cycle and at different temporal scales. It also demonstrates that even in a species where social interactions occur much more cryptically than in some gregarious organisms, their consequences for individual fitness are still apparent, suggesting that such impacts of sociality may be more widespread than previously anticipated.