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,
birds do not respond in a predictable manner to signals of aggression