Before revision:  Here, building on experimental field data on starling flocks with group sizes spanning an unprecedented interval of over two orders of magnitude, we develop a novel type of speed control mechanism that generates scale-free correlations yet allowing for biologically plausible values of the flocks' speed. The bare-bones hallmark of this new control is a strongly nonlinear speed-restoring force that ignores small deviations of the individual speed from its natural reference value , while ferociously suppressing larger speed fluctuations.
After revision:  Here, using a field-theoretical approach, we demonstrate that a marginal speed confinement that ignores small deviations of the individual speed from its natural reference value , while ferociously suppressing larger speed fluctuations.