  As the constituent particles of a flock are polar and in a driven state,
their interactions must, in general, be fore-aft asymmetric and non-reciprocal.
Within a model that explicitly retains the classical spin angular momentum
field of the particles we show that the resulting asymmetric contribution to
interparticle torques, if large enough, leads to a buckling instability of the
flock. Precisely this asymmetry also yields a natural mechanism for a
difference between the speed of advection of information along the flock and
the speed of the flock itself, concretely establishing that the absence of
detailed balance, and not merely the breaking of Galilean invariance, is
crucial for this distinction. To highlight this we construct a model of
asymmetrically interacting spins fixed to lattice points and demonstrate that
the speed of advection of polarisation remains non-zero. We delineate the
conditions on parameters and wavenumber for the existence of the buckling
instability. Our theory should be consequential for interpreting the behaviour
of real animal groups as well as experimental studies of artificial flocks
composed of polar motile rods on substrates.
