Sex differences in social dominance emerge late in life in a gregarious bird.

Social dominance shapes group structure and the collective behaviour of gregarious animals. Senescence, the progressive physiological deterioration associated with ageing, may influence behaviour, including social dominance, while reproductive roles may lead to different onset times or rates of senescence between the sexes. However, how ageing shapes sex differences in sociality remains poorly understood, largely due to the scarcity of long-term studies comparing male and female social behaviour. To address this gap of knowledge, we monitored dominance hierarchies during foraging in an ageing, semi-natural population of adult common waxbills (Estrilda astrild). Our study lasted 5 years, which is more than most adult waxbills would live in the wild, to facilitate detecting effects of senescence. At the onset of the study, sex differences in dominance were negligible but became pronounced with ageing during breeding seasons. In contrast, sex differences remained weak during non-breeding seasons. Females showed stronger longitudinal and seasonal changes in dominance than males, whose dominance patterns remained more consistent across breeding and non-breeding periods. Nonetheless, social dominance also increased, on average, as the number of years until death decreased. Although sex differences in animal sociality are often thought of as largely fixed, our findings reveal a dynamic pattern in which sex differences in dominance emerge with ageing and are context-dependent, varying across breeding seasons. Overall, our results suggest that senescence and seasonal context shape sociality later in life.