  Experiments find coherent information transfer through biological groups on
length and time scales distinctly below those on which asymptotically correct
hydrodynamic theories apply. We present here a new continuum theory of
collective motion coupling the velocity and density fields of Toner and Tu to
the inertial spin field recently introduced to describe information propagation
in natural flocks of birds. The long-wavelength limit of the new equations
reproduces Toner-Tu theory, while at shorter wavelengths (or, equivalently,
smaller damping), spin fluctuations dominate over density fluctuations and
second sound propagation of the kind observed in real flocks emerges. We study
the dispersion relation of the new theory and find that when the speed of
second sound is large, a gap sharply separates first from second sound modes.
This gap implies the existence of `silent' flocks, namely medium-sized systems
across which neither first nor second sound can propagate.
