This study investigates how personality traits in birds, specifically the exploratory behavior in novel environments, influence fitness in a fluctuating environment. The research focuses on great tits (Parus major), a common bird species in Europe and Asia. Individual great tits differ in their exploratory behavior, with "fast" explorers being more bold and aggressive, and "slow" explorers being more cautious and sensitive to external stimuli. These behavioral differences are highly repeatable and have a substantial genetic component, indicating that personality traits are heritable. The study found that selection pressures on personality traits fluctuate annually, with opposite effects on males and females each year. Annual adult survival was related to exploratory behavior, but the direction of selection differed between sexes and years. Similarly, the number of offspring surviving to breeding was related to the parents' personalities, with selection pressures changing between years. These changes in selection pressures coincided with changes in environmental conditions, such as beech mast, which affects competition for food and space. The results suggest that fluctuating environmental conditions lead to changes in competition, which in turn influence selection pressures on personality traits. This fluctuating selection helps maintain genetic variation in personality traits, as it prevents a single personality type from becoming dominant. The study also found that disassortative mating (pairing individuals with different personalities) may be an adaptive strategy, as it allows parents to produce offspring with intermediate traits that may be more viable under fluctuating environmental conditions. The findings highlight the importance of environmental variability in shaping the maintenance of genetic diversity in personality traits. The study underscores the complex interplay between individual behavior, environmental conditions, and evolutionary processes in natural populations.This study investigates how personality traits in birds, specifically the exploratory behavior in novel environments, influence fitness in a fluctuating environment. The research focuses on great tits (Parus major), a common bird species in Europe and Asia. Individual great tits differ in their exploratory behavior, with "fast" explorers being more bold and aggressive, and "slow" explorers being more cautious and sensitive to external stimuli. These behavioral differences are highly repeatable and have a substantial genetic component, indicating that personality traits are heritable. The study found that selection pressures on personality traits fluctuate annually, with opposite effects on males and females each year. Annual adult survival was related to exploratory behavior, but the direction of selection differed between sexes and years. Similarly, the number of offspring surviving to breeding was related to the parents' personalities, with selection pressures changing between years. These changes in selection pressures coincided with changes in environmental conditions, such as beech mast, which affects competition for food and space. The results suggest that fluctuating environmental conditions lead to changes in competition, which in turn influence selection pressures on personality traits. This fluctuating selection helps maintain genetic variation in personality traits, as it prevents a single personality type from becoming dominant. The study also found that disassortative mating (pairing individuals with different personalities) may be an adaptive strategy, as it allows parents to produce offspring with intermediate traits that may be more viable under fluctuating environmental conditions. The findings highlight the importance of environmental variability in shaping the maintenance of genetic diversity in personality traits. The study underscores the complex interplay between individual behavior, environmental conditions, and evolutionary processes in natural populations.