Spread the bread with butter, cut into 12 (2-inch) squares, and press the buttered side of the bread gently into the parsley to coat it. In a small bowl combine well the radish and the onion and divide the mixture among the bread squares. Roll up each herring strip into a coil and arrange the herring and the capers decoratively on the bread.
The canapes may be made 30 minutes in advance and kept covered and chilled. Arrange the canapes on a platter.