| Summary: | The fundamental puzzle of personality is why personality – between-individual differences and within-individual consistency of behaviour – exists. Personality has been measured in many taxa and understanding the origins of personality means understanding a common basis to all behaviours. This study uses the isolated Lundy Island house sparrow Passer domesticus system to give a whole-population overview of how personality is formed, maintained, and may influence fitness. House sparrows are globally successful, and might help us to understand the role of personality – inflexible behaviour – in a highly adaptable species. This research uses cross-fostering, where offspring are exchanged between broods, to separate genetic and environmental effects on personality, and first examines whether cross-fostering has inherent biases. We establish that cross-fostering is linked to spatio-temporal breeding heterogeneity, changes to the rearing environment, and, possibly, observer-based sampling bias, influencing individual survival and potentially study outcomes. Thus, we provide practical guidelines for reducing such bias. Second, this thesis investigates how personality traits develop and might link to fitness by measuring three main personality traits. Our results indicate that heritability varied widely among personality traits. No traits were correlated across contexts, implying no cross-context constraint between these traits. Physiological state, in this case nestling mass, was an important factor shaping personality in nestlings. Social broods also shaped nestling personality, though personality was more similar within social broods, which is contrary to theoretical predictions. Lastly, personality was weakly correlated with fitness in females but not in males, and male behaviour might influence female behaviour and reproductive investment. Therefore, the partner can probably modulate the link between fitness and behaviour. This thesis shows that personality can have stable genetic and physiological bases, but social interactions are associated with more similar personalities between interacting individuals. The fitness consequences of personality might be sex specific and moderated by partners.
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