Within-flock differences in exploratory tendency and flock performance in a highly gregarious bird

How individual differences translate into group outcomes is a timely and debated issue. Recent studies, especially in social arthropods and fish, focus on diversity of personality traits. These studies suggest that the phenotypic group assortment by personality type of an animal group, including the presence of "keystone" individuals, leads to group-level personality differences and can strongly impact both group and individual outcomes. However, little attention has been given to the variation of a given trait within a group. Theory predicts that phenotypic homogeneity rather than heterogeneity yields the optimal group performance, especially in an anti-predatory context, but the experimental support includes mainly morphological traits, e.g. body size or colouration. Here, we focus on personality and group level differences in a highly gregarious bird, the European starling (Sturnus vulgaris). We investigated how different degrees of within-flock variation in exploration affect flock outcomes in exploratory behaviour and in escape response after a frightening stimulus. First, we established consistent individual differences in exploration. Then, flocks of 4 birds were formed to obtain gradual differences in mean and in variation of exploration scores among flocks. Flocks underwent an exploration test and a perturbation test. More exploratory individuals entered the test arena earlier, tended to start flying more rapidly and also stopped flying sooner upon frightening. Flocks with a more homogeneous distribution with respect to this personality trait were significantly faster to enter the test room, but no effect on the escape response emerged. The mean exploration tendency of the flock or the maximum exploration tendency of purported key individuals within groups did not play a notable role in explaining such differences in group performance. Our results indicate that it is not the individual that predicts/drives the flock outcome, but rather a group feature, namely phenotypic variation within the group.