  We study fidelity decay in classically chaotic microwave billiards for a
local, piston-like boundary perturbation. We experimentally verify a predicted
non-monotonic cross-over from the Fermi Golden Rule to the escape-rate regime
of the Loschmidt echo decay with increasing local boundary perturbation. In
particular, we observe pronounced oscillations of the decay rate as a function
of the piston position which quantitatively agree with corresponding
theoretical results based on a refined semiclassical approach for local
boundary perturbations.
