Various studies have concluded that song overlapping, the phenomenon

Here's the passage:

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. Therefore, __________________.

We see that the passage says that the 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."

Then, the passage brings up an issue with that approach: "However, any response-even no response-can be interpreted as a reaction to perceived aggression."

The conclusion, "Therefore, __________________," follows from the fact that that issue exists.

So, what would follow from that fact?

Here's (C).

(C) Song overlapping is likely not a signal of aggression

Notice that the issue brought up in the passage is that "any response ... can be interpreted as a reaction to perceived aggression."

The passage does not say that the responses are not likely truly reactions to perceive aggression. Rather, the issue is simply that the responses may not be reactions to perceived aggression.

So, we don't have support for the conclusion that "Song overlapping is likely not a signal of aggression." It may be a signal of aggression. The studies simply don't show conclusively that it is. In other words, the studies don't show that it likely is or that it likely is not a signal of aggression.

Now, lets consider choice (E).

(E) The conclusion of these studies is unconvincing

We see that this choice is supported by the passage. After all, if "any response ... can be interpreted as a reaction to perceived aggression," then it's not clear whether the responses observed are indeed reactions to perceive aggression. So, the studies' conclusion that "song overlapping ... is a signal of aggression" is not well supported, and therefore "is unconvincing."

So, (E) the best choice to place after "therefore."