Step 1Spray a grill rack set 4 inches from the heat source with nonstick cooking spray. Heat the grill to medium.
Step 2Place the hens in a large plastic food bag. Combine the orange juice, vinegar, oil, garlic, rosemary, shallot, zest, soy sauce, 1/2 teaspoon of salt, red pepper flakes and pepper in a small bowl. Pour the marinade over the hens. Turn the bag around several times until the hens are evenly coated with the marinade. Marinate 4 to 6 hours, refrigerated, turning the bag occasionally. Bring the hens to room temperature 30 minutes before grilling.
Step 3Remove the hens from the marinade; discard the marinade. Lightly sprinkle the hens with salt and pepper.
Step 4Place the hens skin side down on the rack. Grill, turning with tongs 3 to 4 times, until the meat is fully opaque (insert a knife in the joint between the breast and thigh to check; there should be no redness), and the skin is brown and crispy. This should take about 40 minutes. Brush each side of the hens with the glaze on the last turn.
Step 5Transfer them to a platter. Brush the tops with the glaze again. Arrange the Warm Spinach Salad on a platter. Place the hens attractively on top. Garnish the platter with the orange slices and rosemary sprigs. Serve immediately. Alternately, serve the hens and salad on separate platters; this way, the hens can be served warm or at room temperature.