With a small sharp knife cut a 1/8 inch (1 mm) slice off the bottom of the eggs so they will stand up. Then slice off the top 1/3 of each egg, and chop the scraps of egg white finely and set them aside. Gently squeeze the yolks out of the eggs, being careful not to break the white cases. Mash the yolks and the sardines together with a fork, then beat the mayonnaise into the egg-sardine mixture. Add the lemon juice, mustard, cayenne and salt, mixing thoroughly. Taste and adjust the seasoning. Spoon the filling into the egg white cases, dividing it evenly among them and mounding it on top. Mix the reserved chopped whites together with the parsley and chives in a shallow bowl, and dip the filled eggs into the mixture, rolling to coat the tops thoroughly. Place a caper or two on the top of each egg. Stand the eggs on a chilled platter and serve at once, or cover the platter loosely with waxed paper and refrigerate until ready to serve.