The study investigates the scale-free correlations in bird flocks, specifically starling flocks, to understand how collective response to environmental perturbations is achieved. By analyzing the three-dimensional positions and velocities of individual birds, the researchers found that the spatial correlation between bird velocities does not have a constant range but scales linearly with the linear size of the flock. This indicates that behavioral correlations are scale-free, meaning that the change in the behavioral state of one bird affects all other birds in the group, regardless of the group's size. Scale-free correlations extend the effective perception range of individuals, enhancing the group's ability to respond collectively to environmental changes. The results suggest that bird flocks behave as critical systems, poised to respond maximally to environmental perturbations. The study also discusses the conditions under which such correlations may arise and the implications for understanding collective animal behavior.The study investigates the scale-free correlations in bird flocks, specifically starling flocks, to understand how collective response to environmental perturbations is achieved. By analyzing the three-dimensional positions and velocities of individual birds, the researchers found that the spatial correlation between bird velocities does not have a constant range but scales linearly with the linear size of the flock. This indicates that behavioral correlations are scale-free, meaning that the change in the behavioral state of one bird affects all other birds in the group, regardless of the group's size. Scale-free correlations extend the effective perception range of individuals, enhancing the group's ability to respond collectively to environmental changes. The results suggest that bird flocks behave as critical systems, poised to respond maximally to environmental perturbations. The study also discusses the conditions under which such correlations may arise and the implications for understanding collective animal behavior.