Ground chicken is perhaps the trickiest ground meat to form into burgers because it is so wet and mushy. Chill the meat well - or even partially freeze it - and work with moistened hands to prevent sticking. Because chicken can be so sticky, it helps to cook the burgers on a lightly oiled mesh grill screen that you set directly on the grilling grid.
Because the chicken is so moist, it's important that the other ingredients be dry. Drain the capers and pat them dry between sheets of paper towels. Herbes de Provence are available in supermarkets, but if you cannot find them, use dried oregano or thyme, or a mixture of the two. If the chicken seems too moist and you are afraid the patties will break when they are transferred to the grill, put them in the freezer for no longer than 30 minutes to firm but not freeze them solid.
Serve with Mustard Sauce