Predator Presence Influences The Geometry of Rosy Starling Flocks

Synopsis Aerial flocks of birds provide striking examples of collective behavior, yet how flock structure responds dynamically to environmental context remains poorly understood. Such flocks are particularly vulnerable to predation, and interactions between individuals and their external environment can generate distinct group-level shapes. The rosy starling (Pastor roseus), a wintering species in India, forms large flocks at foraging, resting, roosting, and breeding sites. At dusk, these birds frequently form flocks while moving from foraging areas to roosting locations; however, the nature of their interactions with surrounding ecological factors during this phase has not been quantified. Here, we identify 12 canonical shapes exhibited by aerial flocks of rosy starlings during their movement to roosting sites. We analyze how flock shapes change over time, how these dynamics are influenced by the presence of predators, and how transitions between shapes occur. Our results reveal pronounced shape dynamics associated with predator presence, with shape transitions driven by external influences. These findings support the idea that flocking and its geometries function as adaptive mechanisms for predator avoidance. Overall, this study advances our understanding of aerial flocking behavior in birds and provides a framework for future investigations into the ecological and evolutionary drivers of collective motion in starling flocks.