  The self-organised motion of vast numbers of creatures in a single direction
is a spectacular example of emergent order. We recreate this phenomenon using
actuated non-living components. We report here that millimetre-sized tapered
rods, rendered motile by contact with an underlying vibrated surface and
interacting through a medium of spherical beads, undergo a phase transition to
a state of spontaneous alignment of velocities and orientations above a
threshold bead area fraction. Guided by a detailed simulation model, we
construct an analytical theory of this flocking transition, with two
ingredients: a moving rod drags beads; neighbouring rods reorient in the
resulting flow like a weathercock in the wind. Theory and experiment agree on
the structure of our phase diagram in the plane of rod and bead concentrations
and power-law spatial correlations near the phase boundary. Our discovery
suggests possible new mechanisms for the collective transport of particulate or
cellular matter.
