  We use a well known model (T. Vicsek et al. Phys Rev Lett 15, 1226 (1995))
for flocking to test mutual information as a tool for detecting order-disorder
transitions, in particular when observations of the system are limited. We show
that mutual information is a sensitive indicator of the phase transition
location, in terms of the natural dimensionless parameters of the system which
we have identified. When only a few particles are tracked, and when only a
subset of the positional and velocity components are available, mutual
information provides a better measure of the phase transition location than the
susceptibility of the data.
