  The most conspicuous trait of collective animal behaviour is the emergence of
highly ordered structures. Less obvious to the eye, but perhaps more profound a
signature of self-organization, is the presence of long-range spatial
correlations. Experimental data on starling flocks in 3d show that the exponent
ruling the decay of the velocity correlation function, C(r) ~ 1/r^\gamma, is
extremely small, \gamma << 1. This result can neither be explained by
equilibrium field theory, nor by off-equilibrium theories and simulations of
active systems. Here, by means of numerical simulations and theoretical
calculations, we show that a dynamical field applied to the boundary of a set
of Heisemberg spins on a 3d lattice, gives rise to a vanishing exponent \gamma,
as in starling flocks. The effect of the dynamical field is to create an
information inflow from border to bulk that triggers long range spin wave
modes, thus giving rise to an anomalously long-ranged correlation. The
biological origin of this phenomenon can be either exogenous - information
produced by environmental perturbations is transferred from boundary to bulk of
the flock - or endogenous - the flock keeps itself in a constant state of
dynamical excitation that is beneficial to correlation and collective response.
