  When birds come together to form a flock, the distribution of their
individual velocities narrows around the mean velocity of the flock. We argue
that, in a broad class of models for the joint distribution of positions and
velocities, this narrowing generates an entropic force that opposes the
cohesion of the flock. The strength of this force depends strongly on the
nature of the interactions among birds: if birds are coupled to a fixed number
of neighbors, the entropic forces are weak, while if they couple to all other
birds within a fixed distance, the entropic forces are sufficient to tear a
flock apart. Similar entropic forces should occur in other non-equilibrium
systems. For the joint distribution of protein structures and amino-acid
sequences, these forces favor the occurrence of "highly designable" structures.
