Numerous investigations have indicated that the phenomenon of song overlapping, where one bird starts singing while another of its kind is already singing, may indicate aggression. These investigations rely exclusively on receiver-response experiments, which aim to draw conclusions about the intent of a signal based on the reactions of others. However, any reaction—even a lack of reaction—can be interpreted as a response to perceived aggression. Therefore, __________________. Which one of the following most logically completes the argument?