  We propose a hydrodynamic description of the homogeneous ordered phase of
polar flocks. Starting from symmetry principles, we construct the appropriate
equation for the dynamics of the Goldstone mode associated with the broken
rotational symmetry. We then focus on the two-dimensional case considering both
"Malthusian flocks" for which the density field is a fast variable that does
not enter the hydrodynamic description and "Vicsek flocks" for which it does.
In both cases, we argue in favor of scaling relations that allow to compute
exactly the scaling exponents, which are found in excellent agreement with
previous simulations of the Vicsek model and with the numerical integration of
our hydrodynamic equations.
