Preheat the oven to 375 degrees F. With kitchen shears or a sharp knife, cut along either side of one of the hens' backbone and remove it. Press the hen flat, skin-side down. Cut in half down the middle of the breastbone. Place the hen halves, skin-side up, in a large roasting pan. Repeat with remaining hens. Season the hens with salt and pepper. Roast 30 minutes. Combine the syrup, mustard and sage in a bowl. When the hens have roasted 30 minutes, brush with half the maple/mustard mixture. Roast 10 minutes more, and then brush with the remaining maple/mustard mixture. Roast 10 to 15 minutes more, or until cooked through.