Seasonal variation in response to neighbors and strangers by a territorial songbird

Reduced aggression toward territorial neighbors, termed the 'dear-enemy' effect, is thought to arise because territorial animals benefit by avoiding contests with neighbors with whom they have already established relationships. The dear-enemy effect has been described in many taxa, but few studies have considered whether or not neighbors' relationships are affected by changes in the social environment. In this study, I tested whether Carolina wrens, Thryothorus ludovicianus, behaviorally discriminate between neighbors and strangers in two different social environments: in spring when territories have been established for several months, and in fall when an influx of new birds claiming territories might de-stabilize wren neighborhoods. Comparisons of responses of territorial males to playbacks of songs from neighbors and strangers showed that Carolina wrens show the dear-enemy effect in spring, but not in fall in this design. The apparent lack of a differential response to neighbors and strangers in fall might be due to a reduction in aggression toward strangers. This study provides evidence that seasonal changes in the dear-enemy effect coincide with seasonal changes in the social environment.