Before revision:  Yet the spatial correlations of such fluctuations  have a range as wide as the entire group : the speed change of a bird influences and is influenced by that of birds on the other side of the flock. Long-range correlations and limited speed fluctuations set conflicting constraints on the mechanism controlling the individual speed of each bird, as the factors boosting correlations also tend to amplify speed fluctuations and vice versa.
After revision:  Yet the spatial correlations of such fluctuations have a range as wide as the entire group . Scale-free correlations and limited speed fluctuations set conflicting constraints on the mechanism controlling the individual speed of each bird, as the factors boosting correlations also tend to amplify speed fluctuations and vice versa.