The song of the yellow warbler signals to other yellow warblers that a particular area has been appropriated by the singer as its own feeding territory. Although the singing deters other yellow warblers from taking over the feeding territory of the singer, other yellow warblers may range for food within a portion of the singer' s territory. However, a warbler sings a special song when it molts (sheds its feathers). Other yellow warblers will not enter the smaller core territory of a yellow warbler singing its molting song. Ttherefore yellow warblers, which can only fly short distances during molting, have no competition for the food supply within the range of their restricted flying. The argument makes which one of the following assumptions? ['The core areas contain just enough food to sustain one yellow warbler while it molts.', 'The core areas of each feeding territory are the same size for each molting warbler.', 'Warblers often share their feeding areas with other kinds of birds, which often do not eat the same insects or seeds as warblers do.', 'There are no birds other than yellow warblers that compete with yellow warblers for food.']