This is unsupported because we don’t know how birds respond to the signal of aggression based on what the author tells us.

This is unsupported because even if receiver-response tests could perceive any response as aggression, there may be other ways they could provide insight into bird behavior outside of evaluating the purpose of song overlapping.

This is unsupported because even though we have reasons to doubt the existing studies conclusion, we cannot go so far as to conclude the opposite. We merely have to acknowledge that the prevailing hypothesis is unsupported.

This is unsupported because song overlapping may be used to communicate even if we have reasons to doubt existing studies showing it is a signal of aggression.

This is strongly supported because the studies rely on a method that could show any response to be a signal of aggression. This means we have little reason to believe that song overlapping is uniquely a sign of aggression.