  The modeling and investigation of the dynamics and configurations of animal
groups is a subject of growing attention. In this paper, we present a continuum
model of flocking and use it to investigate the reaction of a flock to an
obstacle or an attacking predator. We show that the flock response is in the
form of density disturbances that resemble Mach cones whose configuration is
determined by the anisotropic propagation of waves through the flock. We
analytically and numerically test relations that predict the Mach wedge angles,
disturbance heights, and wake widths. We find that these expressions are
insensitive to many of the parameters of the model.
