The sparrow and the hawk: aggressive signaling under risk of predation

Eavesdropping by predators imposes a major cost on signalers, which in turn have evolved a number of strategies to deal with this cost. These strategies however, have not been well studied in the context of aggressive signaling. Here, we report an experiment on male song sparrows (Melospiza melodia) in which we experimentally increased the perceived predation risk by playing Cooper's hawk (Accipiter cooperi) calls or control Northern flicker (Colaptes auratus) calls in the midst of a simulated conspecific territorial intrusion and assessed the change in signaling strategies. We found that song sparrows clearly discriminated between the hawk call and the flicker call. Specifically, subjects decreased number of songs and wing wave displays (a visual signal of aggressive intent) and increased alarm calling during the hawk playback. However, the change in signaling behaviors did not persist when the simulated intruder resumed his challenge, despite the fact that the subjects were still alarmed as indicated by high rates of alarm calling. Additionally, we found no evidence for the eavesdropping avoidance hypothesis as an explanation for the low amplitude of soft song, the most reliable signal of aggression in this species. These results suggest that male song sparrows flexibly adjust their signaling effort in response to both the predation risk and the need to defend their territory against an intruder.