Various studies have concluded that song overlapping, the phenomenon where one bird begins a song while another of its species is singing, is a signal of aggression. These studies are based solely on receiver-response tests, which seek to derive conclusions about the intent of a signal based on how others respond to it. However, any response -- even no response -- can be interpreted as a reaction to perceived aggression. Ttherefore, __. Which one of the following most logically completes the argument? ['receiver-response tests can provide no insight into bird behavior', 'the conclusion of these studies is unconvincing', 'song overlapping is likely not a signal of aggression', 'birds do not respond in a predictable manner to signals of aggression']