Good neighbour, bad neighbour: song sparrows retaliate against aggressive rivals

Many territorial animals, despite being in direct competition for resources such as space, food and mates, show reduced aggression towards their neighbours. This situation is called the Dear Enemy effect. One explanation of the Dear Enemy effect is that it is due to a conditional strategy like Tit for Tat where territory holders cooperate by reducing aggression towards neighbours that also show reduced aggression, but retaliate against aggressive neighbours. Previous research found evidence for such a conditional strategy in migratory species but not in species with long-term association between neighbours, suggesting that long-term neighbours might be engaged in more ‘forgiving’ strategies. We tested this hypothesis in male song sparrows, Melodia melospiza, which are resident year-round in our population (leading to long-term associations between neighbouring birds) and display the Dear Enemy effect. We found that following a simulated intrusion by a neighbour, song sparrow males responded more strongly to playback of this neighbour than to playback of a neutral neighbour from their respective boundaries, consistent with a conditional retaliation strategy. We suggest that the primary effect of an intrusion by a neighbour might be to increase the perceived risk of cuckoldry by the intruding male, and increased aggression and vigilance towards this neighbour might be a strategy to prevent cuckoldry.