Melt suet in large saucepan over low heat, stirring occasionally. Stir in bird seed and sunflower seeds. Cool 15 to 20 minutes. Refrigerate 30 to 45 minutes or until mixture starts to set but is not solid.
Meanwhile, with point of small knife, punch 2 holes opposite each other in each orange half, 1/2 inch down from cut edge. Cut 6 pieces of string to desired length for hanging. Push ends of 1 string through holes, from outside to inside, in 1 orange half; tie each end in a knot.
Place each orange half in small bowl or place orange halves in baking pan. Spoon suet mixture into each orange half, pressing in gently and mounding in center. Refrigerate 1 hour or until firm. Hang outdoors. DO NOT EAT.