Sex-biased dispersal in the Arabian babbler (Argya squamiceps)

Sex-biased dispersal plays a central role in shaping social structure and reproductive strategies in many bird species. In the Arabian Babbler Argya squamiceps, a cooperatively breeding species with pronounced male philopatry, dispersal occurs individually or in kin-based same-sex coalitions. We investigated sex differences in dispersal using 17 years of longitudinal field observations encompassing 64 dispersal events involving 156 individuals. We tested seven predictions, comparing females and males in (1) age at dispersal, (2) coalition size, (3) likelihood of dispersing alone, (4) dispersal distance, (5) duration of the transience period, (6) likelihood of encountering aggression upon joining a new group and (7) natal group size. Females dispersed at a younger age, in smaller coalitions and from smaller natal groups than males. Females also tended to disperse more often alone, and to further-away groups, though not significantly. No significant sex differences were found in transience periods, or likelihood of encountering aggression upon joining a new group. These patterns suggest that females are in more urgent need to disperse, probably because of limited reproductive opportunities within their natal groups and their subordinate position in the social hierarchy. Although we did not directly measure dispersal costs, our findings suggest that females may incur greater cumulative costs. A feedback dynamic may occur, where female-biased dispersal reinforces male philopatry, amplifying sex differences. Our results highlight the complex interplay between reproductive constraints, social organization and sex-specific dispersal strategies in cooperative breeders, and underscore the value of long-term behavioural data in understanding the evolutionary dynamics of sex-biased dispersal.