Preheat oven to 350 degrees F. Line a large roasting pan with foil.

Remove both wingtips at the top joint with kitchen scissors or a heavy knife; reserve wing tips. Reserve neck (discard the rest of the giblets). Trim the excess skin and fat around the neck and cavity opening. Grate 1/4 teaspoon orange zest and set aside. Cut oranges into quarters. Set aside 3 sections and place the remaining orange sections inside the duck; fold the wings under and tie the legs together with kitchen string. With a skewer or fork, prick holes in the skin without piercing the flesh. (This allows fat to drain during roasting.)

Whisk marmalade and 2 tablespoons soy sauce in a small bowl. Loosen the skin over the breast and thigh meat and rub about 3 tablespoons of the marmalade mixture under the skin. Place breast-side down on a rack in the prepared roasting pan.

Roast the duck for 1 hour. Remove the duck from the pan and carefully pour off the fat. Return the duck to the rack, breast-side up. Continue roasting, basting every 15 minutes with another 2 tablespoons of the marmalade mixture, until an instant-read thermometer inserted into the thigh without touching bone registers 165 degrees F, 1 to 1 1/4 hours more.

Meanwhile, place the reserved wing tips and neck in a medium saucepan, add broth and bring to a boil. Reduce heat and simmer, stirring occasionally, until the broth is reduced by half, 5 to 7 minutes. Strain and discard solids; return the broth to the pan. Squeeze the juice from the reserved orange sections into a small bowl; add cornstarch and whisk until smooth. Stir the juice mixture, reserved zest and brown sugar into the broth; cook over medium heat, stirring, until thickened, 1 to 2 minutes. Stir in the remaining 1 tablespoon soy sauce.

Transfer the duck to a cutting board; let rest for 15 minutes before removing the string and carving. Serve with the sauce on the side.

Equipment: Kitchen string