The evolution of unstable barotropic vortices is studied numerically. Exact solutions to the equation of potential vorticity conservation under the "rigid lid" condition, as well as nonsteady-state configurations, are set as initial states in the evolutionary experiments. The examined "shielded modon" structures usually collapse within one to several synoptic periods and radiate vortex pairs propagating westward and eastward. The latter are shown to be modons of Larichev and Reznik. The westward dipoles are identified as "nonlocal modons," that is, vortical cores of stationary nonlinear Rossby waves. In the case of standing Stern modons, some small initial perturbations induce slow westward drift and subsequent collapse of the vortex structure due to the Rossby wave radiation, others lead to their transformation into Larichev and Reznik's modons. This conclusion is supported by the results of a numerical integration of the linear stability problem.