Use a sharp knife to cut the oranges in half. Spoon the fruit into a bowl, working carefully to avoid cutting the skin. Don't worry about getting every piece of fruit out; It's okay if some stick to the shell. Set the scooped fruit aside for a snack or fruit salad.
In each of the four shells, use a small wooden skewer or nail to poke a hole in the shell, then make another directly opposite the first hole. Thread the end of a piece of string or twine through each hole (use the skewer or a nail to help push it through the holes, if necessary) and tie off.
Place the peanut butter in an empty container. Add 1 cup cornmeal or rolled oats and 1/4 cup raisins or sunflower seeds (if desired) and stir or knead into peanut butter. The resulting mixture should be slightly sticky. If it's more crumbly than sticky, chop up a tablespoon or two of the fruit you removed in the previous step and work it into the mixture to add moisture.
While holding the string or string handle out of the way, spoon some of the peanut butter mixture into each of the four shells.
Take the filled baskets outside and hang them from a bush or tree for the birds to enjoy. Deciduous shrubs and trees with short, sturdy twigs are ideal; Evergreen shoots may not be strong enough to support the weight of the baskets.
