  We study the large-time behavior of continuum alignment dynamics based on
Cucker-Smale (CS)-type interactions which involve short-range kernels, that is,
communication kernels with support much smaller than the diameter of the crowd.
We show that if the amplitude of the interactions is larger than a finite
threshold, then unconditional hydrodynamic flocking follows. Since we do not
impose any regularity nor do we require the kernels to be bounded, the result
covers both regular and singular interaction kernels. Moreover, we treat
initial densities in the general class of compactly supported measures which
are required to have positive mass on average (over balls at small enough
scale), but otherwise vacuum is allowed at smaller scales. Consequently, our
arguments of hydrodynamic flocking apply, mutatis mutandis, to the agent-based
CS model with finitely many Dirac masses. In particular, discrete flocking
threshold is shown to depend on the number of dense clusters of communication
but otherwise does not grow with the number of agents.
