Sex differences in familiarity preferences within fission-fusion brown-headed cowbird, Molothrus ater, flocks

Many vertebrates inhabit dynamic and loosely structured groups where group size and social composition continually fluctuates. The ability to sustain nonrandom interaction preferences across group changes is an important aspect in maintaining social organization. In two experiments, we explored the strength and persistence of social preferences for familiar conspecifics in brown-headed cowbirds. In the first experiment, we demonstrated that females preferentially associate with familiar females when introduced into a flock containing novel individuals. In the second experiment we investigated the consistency of familiarity preferences across a series of social introductions. Females maintained preferences to approach familiar conspecifics, and the individual strength of those preferences remained consistent across introductions. Male preferences changed across the introductions. In the first introduction, males showed a significant preference to approach familiar conspecifics, but increased their approaches towards novel conspecifics in subsequent introductions. Our findings suggest that female cowbirds are an important factor in maintaining social organization through their enduring associations with familiar individuals, whereas males facilitate social integration by extending connections towards novel individuals during periods of change.