  Recent investigations have provided important insights into the complex
structure and dynamics of collectively moving flocks of living organisms. Two
intriguing observations are, scale-free correlations in the velocity
fluctuations, in the presence of a high degree of order, and topological
distance mediated interactions. Understanding these features, especially, the
origin of fluctuations, appears to be challenging in the current scheme of
models. It has been argued that flocks are poised at criticality. We present a
self-propelled particle model where neighbourhoods and forces are defined
through topology based rules. The force fluctuations occur spontaneously, and
gives rise to scale-free correlations in the absence of noise and in the
presence of alignment of velocities. We characterize the behaviour of the model
through power spectral densities and the Lyapunov spectrum. Our investigations
suggest self-organized criticality as a probable route to the existence of
criticality in flocks.
