The relationship between leadership and gregariousness in mixed-species bird flocks

Among the various species that form mixed-species bird flocks, " nuclear species" are thought to be important in flock formation and maintaining flock cohesion. Such nuclear species have been noted to occur in large groups on their own and to lead flocks, but the relationship between leadership and intraspecific group size has not been quantitatively tested at a large scale. Using a dataset of descriptive studies of terrestrial flock systems collected over 75 y, we found that intraspecific group size was significantly larger in flock leaders than in species that attend the same flocks but do not lead. The relationship held in a reduced dataset with phylogenetically-independent flock systems. We discuss a framework for explaining the connection between leadership and intraspecific group size, contrasting between those hypotheses that emphasize that gregariousness serves to attract the attention of other species, and those hypotheses that suggest that gregariousness leads to kin-selected behavior from which other species can also benefit.