Effects of Social Relationships on the Collective Motion of Bird Flock*

Collective animal motion has long been modeled using self-propelled particles that are assumed to be identical and to follow same interaction rules. In nature, however, group members can be quite different, and such differences may shape the group behavior. Here, we study how social relationships in bird flocks affect the local interactions and group dynamics. We used 3D optical tracking to study flocks of jackdaws (Corvus monedula), a highly social corvid species that forms lifelong, monogamous pair bonds. We show that jackdaw flocks contain discrete pairs that are likely to reflect the pair bonds. We find that paired birds interact with fewer neighbors than unpaired birds and use less energy when flying. Pairing thus appears to grant energetic benefits. However, we also find that flocks with more pairs have shorter velocity correlation lengths, in agreement with a generic self-propelled particle model, indicating that social relationships may inhibit efficient information transfer through the group. Our results reveal a critical tension between individual- and group-level benefits during collective behavior in species with differentiated social relationships, and have many evolutionary and cognitive implications.