Animal personality, consistent between-individual variation in suites of behaviours, has a genetic component, associates with fitness and is subjected to selection. Recent studies have shown that early developmental effects and environmental conditions experienced by parents also affect personality traits, even over multiple generations. Yet, the mechanisms underlying such transgenerational regulation remain unknown, while determining them is crucial to understand how development affects heritable traits in evolutionary processes. A likely mechanism involved in such epigenetic regulation is DNA methylation, since this can stably alter gene expression in response to environmental factors without the need for structural modifications of the DNA sequence. However, ecological research connecting natural variation in DNA methylation to behavioural trait variation is almost absent. Here we propose to bridge that gap by associating DNA methylation to variation in exploratory behaviour in the great tit (Parus major), a well-developed model for studying the genetics of animal personality. We will combine state-of-the-art genome-wide techniques with targeted measurements on major candidate genes using a unique long-term wild study population and an existing experimental F2-cross population. We will conduct targeted experiments, manipulating DNA methylation during early development in order to determine environmental causes of variation in DNA methylation and we will explore its ecological and evolutionary consequences. This unique approach for investigating the causes and consequences of DNA methylation in relation to animal personality will answer key questions regarding the consistent-but-plastic nature of behavioural traits and will significantly enhance our understanding of how behavioural variation is maintained in wild populations.