Fluctuation Vector.

The two-dimensional projection of the velocity vectors (Figure 4A) and the fluctuation vectors (Figure 4B) of individuals within the flock at one step. The number of individuals is 150. Figure 4A appears that the individuals that are represented by the velocity vector align in nearly the same direction. However, if we take the fluctuation vector from Figure 4A, then we can observe two large correlated domains in the flock (the upper and lower domains). The shape from these two large correlated domains is very similar to empirical flocks.