• Moser-Purdy, Christopher


The dear enemy effect arises when territorial animals respond more intensely to unfamiliar strangers than to familiar neighbours. This widespread behavioural phenomenon occurs because strangers represent a threat to both an animal's territory and parentage, whereas neighbours represent a threat only to parentage. Recent research in birds demonstrates some flexibility in the dear enemy effect across the breeding season. Given that neighbours often sire extrapair young, male animals may benefit by responding more aggressively to neighbours during periods of female fertility. Here we investigate the hypothesis that the dear enemy effect varies with female fertility by testing the prediction that male birds will respond more strongly to neighbours when their own mates are fertile than when they are not fertile. We conducted a playback experiment with wild song sparrowsMelospiza melodia, repeating playback sessions to paired territorial males over the course of a breeding season, including periods when females were fertile and periods when they were not. Male song sparrows displayed a dear enemy effect only when their social mate was not fertile. We conclude that male song sparrows adjust behaviour towards neighbours based on their own mate's fertility status, presumably because neighbours threaten a territorial male's parentage during his breeding partner's fertile period. When paternity is not at stake, reduced aggression towards neighbours may enhance fitness, but when paternity is at stake, normal levels of aggression towards neighbours may be favoured as a mate-guarding tactic


Our playback experiments took place between 18 April - 22 May 2015 and 8 April - 15 May 2016; these periods correspond roughly to pair formation through nest building, egg laying and incubation in our study population. 25 subjects were banded with unique combinations of coloured leg bands and a Canadian Wildlife Services numbered band to facilitate individual identifacation. From the original 29 playback subjects, we excluded two individuals that did not respond to any playback subjects, we excluded two individuals that did not respond to any playback trials, three individuals that never paired with a female, and two individuals that moved their breeding territory part-way through the study period. AFter these exclusions we were left with 22 males for analysis