Territory owners discriminate between aggressive and nonaggressive neighbours

In a territorial system where males vary in fighting ability or attractiveness to females, not all neighbours should be equally threatening to a territory owner. Selection should act on territory owners to recognize consistent individual differences between neighbours, assess which neighbours represent greater threats, and respond appropriately when challenged. In this study, we examined whether male song sparrows, Melospiza melodia, assess the level of threat posed by individual neighbours and respond differently to neighbours that differ in their level of aggressiveness. Aggressive males should represent a greater threat because they are more likely to engage in fights over territorial boundaries, and may also be preferred by females as extrapair mates. We used playback experiments to assay male territorial aggression, and then to test whether males responded differently to their aggressive and nonaggressive neighbours. We found that male song sparrows respond more strongly to their aggressive neighbours than to their nonaggressive neighbours. Thus, male song sparrows differentiate between neighbours that differ in aggressive response and appear to respond adaptively towards the individual that poses the greatest threat.