  We consider the effect of introducing a small number of non-aligning agents
in a well-formed flock. To this end, we modify a minimal model of active
Brownian particles with purely repulsive (excluded volume) forces to introduce
an alignment interaction that will be experienced by all the particles except
for a small minority of "dissenters". We find that even a very small fraction
of dissenters disrupts the flocking state. Strikingly, these motile dissenters
are much more effective than an equal number of static obstacles in breaking up
the flock. For the studied system sizes we obtain clear evidence of scale
invariance at the flocking-disorder transition point and the system can be
effectively described with a finite-size scaling formalism. We develop a
continuum model for the system which reveals that dissenters act like annealed
noise on aligners, with a noise strength that grows with the persistence of the
dissenters' dynamics.
