Remove giblet packages from ducks, and reserve for another use. Rinse ducks, and pat dry. Remove excess fat and skin. Rub ducks evenly with salt and pepper. Squeeze juice from 1 orange quarter into each duck cavity. Fill duck cavities with remaining orange quarters, onions, carrots, celery, and parsley sprigs. Prick legs and thighs with a fork, avoiding breast area. Place ducks, breast sides up, on a wire rack in an aluminum foil-lined shallow roasting pan. Tie legs together with kitchen string.
Stir together orange marmalade and next 5 ingredients in a small saucepan. Cook mixture over medium heat, stirring constantly, 2 to 3 minutes or until orange marmalade is melted. Remove and reserve 3/4 cup mixture.
Bake ducks at 350° for 1 1/2 hours; brush evenly with orange sauce, and bake 25 to 30 more minutes or until a meat thermometer inserted into thickest portion of thigh registers 180°. Garnish, if desired. Serve with reserved 3/4 cup orange mixture.
Orange-Glazed Hens: Substitute 3 (5- to 7-lb.) whole chickens for ducks. Do not prick chickens. Proceed as directed. Bake at 350° for 1 1/2 hours; brush evenly with orange glaze, and bake 30 more minutes. Brush with orange glaze; shield with aluminum foil. Bake 15 minutes or until a meat thermometer inserted into thickest portion of thigh registers 170°. Garnish, if desired. Serve with reserved 3/4 cup orange mixture.