Within-flock heterogeneity in exploration affects flock behaviour in a highly gregarious bird

How individual differences translate into group outcomes is a controversial issue and depends on the phenotypic composition of the group. Research on the effect of within-group homogeneity/heterogeneity in phenotypic composition provided contrasting results, with some studies supporting benefits of homogeneity and others the opposite. We characterized a personality trait in a gregarious bird, the European starling (Sturnus vulgaris), and investigated how different degrees of within-flock variation in exploration affect flock behavioural responses. Based on consistent individual differences in exploration, we formed flocks of 4 birds differing in mean and variation of exploration scores. These flocks underwent an exploration test and a perturbation test to assess escape response. We found that, compared to less exploratory individuals, more exploratory individuals enter the test room earlier and start and stop flying sooner upon frightening (escape response). Flocks with a more homogeneous distribution of the individual exploration scores are significantly faster to enter the exploration room, but there is no clear effect on the escape response. The mean exploration score of the flock or the maximum exploration score of purported ‘key’ individuals within groups do not explain differences in group performance. The results indicate that it may not be the individual that drives the flock behaviour, but rather the phenotypic variation within the group, and suggest that a homogenous distribution of personality types could speed up collective decision-making in gregarious species.