  We modify the standard Vicsek model to clearly distinguish between intrinsic
noise due to imperfect alignment between organisms, and extrinsic noise due to
fluid motion. We then consider the effect of a steady vortical flow, the Taylor
Green vortex, on the dynamics of the flock, for various flow speeds, with a
fixed intrinsic particle speed. We pay particular attention to the morphology
of the flow, and quantify its filamentary. Strikingly, above a critical flow
speed there is a pronounced increase in the filamentarity of the flock, when
compared to the zero flow case. This is due to the fact that particles appear
confined to areas of low vorticity; a familiar phenomena, commonly seen in the
clustering of inertial particles in vortical flows. Hence the cooperative
motion of the particles gives them an effective inertia, which is seen to have
a profound effect on the morphology of the flock, in the presence of external
fluid motion. Finally we investigate the angle between the flow and the
particles direction of movement and find it follows a power law distribution.
