Red admiral caterpillars will feed off the leaf shelter until they reach a point where they are no longer concealed.  At that point, caterpillars will wander to another leaf, construct a new shelter and repeat the process.  This makes caterpillar hunting somewhat easlier when you can visibly note several shelters on one plant as caused my one or more mobile larvae.  Please note in the Western U.S., that stinging nettle plants that have been stripped by dozens, if not hundreds of larvae are NOT red admiral caterpillars; but are those of the milbert's tortoiseshell whose females lay their eggs in large clusters as opposed to singletons (red admirals.)