Territorial Interactions in Sympatric Song Sparrow and Bewick's Wren Populations

The similarity in song, habits, and habitat in the Song Sparrow and Bewick's Wren stimulated a study of their interactions in western Washington. Both species respond to models and song of the other species more intensely than to the Black-capped Chickadee, but less intensely than to their own species. Both birds showed a gradual decrease in aggressive behavior to conspecifics over the breeding season, the Bewick's Wren response rising somewhat early in the season before gradually declining over the rest of the study period. While the Bewick's Wren showed a strong decrease in response levels to the Song Sparrow over time, the Song Sparrow showed a significant drop in aggressive response toward the Bewick's Wren at the time the first brood was fledged, the response rising again at the time of the appearance of the second clutch. The Song Sparrow and the Bewick's Wren represent a sustained bidirectional system of interspecific aggression. This system may be more sensitive to changes in breeding behavior over the season than is the intraspecific system. Where they coexist they forage at slightly different heights and on slightly different food, but overlap sufficiently so that aggression and similarity of song have evolved as still other mechanisms to reduce competition.