  Sheep are gregarious animals, and they often aggregate into dense, cohesive
flocks, especially under stress. In this paper, we use image processing tools
to analyze a publicly available aerial video showing a dense sheep flock moving
under the stimulus of a shepherding dog. Inspired by the fluidity of the
motion, we implement a hydrodynamics approach, extracting velocity fields, and
measuring their propagation and correlations in space and time. We find that
while the flock overall is stationary, significant dynamics happens at the
edges, notably in the form of fluctuations propagating like waves, and
large-scale correlations spanning the entire flock. These observations
highlight the importance of incorporating interfacial dynamics, for instance in
the form of line tension, when using a hydrodynamics framework to model the
dynamics of dense, non-polarized swarms.
