  We numerically explore the behavior of repelling and aligning self-propelled
polar particles (boids) in 2D enclosed by a damped flexible and elastic
loop-shaped boundary. We observe disordered, polar ordered (or jammed) and
circulating states. The latter produce a rich variety of boundary shapes
including; circles, ovals, irregulars, ruffles, or sprockets, depending upon
the bending moment of the boundary and the boundary to particle mass ratio.
With the exception of the circulating states with non-round boundaries, states
resemble those exhibited by attracting self-propelled particles, but here the
confining boundary acts in place of a cohesive force. We attribute the
formation of ruffles to instability mediated by pressure on the boundary when
the speed of waves on the boundary approximately matches the self-propelled
particle's swim speed.
