Heterogeneous structure in mixed-species corvid flocks in flight

► Most models of collective movement assume homogeneous, independent group members. ► In mixed jackdaw-rook flocks, heterogeneity clearly affects aerial flock structure. ► Birds aggregate by species, with the larger, dominant rooks at the front of flocks. ► Conspecifics align closely and neighbouring jackdaws fly in very close proximity. ► Both species often fly in discrete pairs, probably reflecting monogamous pair bonds.