Social niches and sex assortment: uncovering the developmental ecology of brown-headed cowbirds, Molothrus ater

Variation in individual interaction patterns creates different social niches. Social niches afford differential exposure to social information that individuals utilize to develop species-typical behaviours. Brown-headed cowbirds depend on social interaction to develop competent reproductive behaviour. We used a fission–fusion paradigm to perturb two flocks of birds: one adult flock (N = 38) and one juvenile flock (N = 24). We split the two flocks, then recombined them while recording the frequency of individual approaches. Both adult and juvenile females maintained stable social niches, but males did not. Females maintained equivalent levels of gregariousness and attractiveness across perturbations. Juvenile females maintained the highest levels of sex assortment. The consistency of female niche construction may be explained by females’ reliance on interaction with other females to assess males. Early sex assortment may also foster the development of social preferences and influence reproductive choices.

► Juvenile and adult flocks of brown-headed cowbirds were perturbed using a fission–fusion procedure. ► Females showed high repeatability in gregariousness and attractiveness. ► Adult female gregariousness was correlated with their attractiveness. ► Juvenile females maintained higher levels of sex assortment than adult females.