The social interaction role of song in song sparrows: implications for signal design

Many territorial songbirds use singing as an interactive social signal to reduce inter-neighbor aggression. Communication theory predicts that territorial songbirds may use repertoires of signals to indicate graded levels of aggressive motivation. This theory is supported in song sparrows, a species that uses several different song-based signals such as song-type matching to escalate or de-escalate aggression during counter-singing interactions. However, birds cannot type match if they do not share the song type their rival is singing, raising the question of how they might signal aggression instead. We present evidence for two alternative signaling strategies that non-sharing neighbors could use to communicate aggressive motivation. In the first case, a bird may ‘similarity match’ a rival’s song by singing the most similar song in his repertoire, even if he cannot type match. Another solution would be for neighbors to agree to treat specific pairs of non-similar types as matches by convention. The conventional match is potentially a new class of signal that territorial neighbors may use along with type and similarity matching to maintain a repertoire of aggressive motivation signals.