Our new paper on aggressive signaling under predation risk has just been published in Behavioral Ecology. In an experiment where we played Cooper’s hawk calls (vs. northern flicker calls in controls) in the middle of a simulated intrusion, we found that that song sparrows cease aggressive signaling , and increase alarm calling during hawk playbacks consistent with an increased perceived risk of predation as a result of the hawk playback. Once the hawk playback was over and the simulated intruder returned however, they rapidly resumed aggressive signaling at the same levels as before, although they kept alarm calling a higher rate after hawk playbacks compared to flicker playbacks.
These results suggest aggressive signaling may indeed carry costs in terms of detection by predators (as suggested by the fact that signaling mostly ceases during hawk playback). This finding might help explain why under-signalers (those who signal at low levels but behave aggressively) have a survival advantage as we reported in our recent paper in Evolution.
The experiment also was designed to test the eavesdropping avoidance hypothesis as an explanation for the low amplitude of soft song: the most reliable signal of aggression in this and several other songbirds. Given that birds did not increase soft song use after hawk playback relative to flicker playback, we found no support for this hypothesis (as in the only other study, by Searcy and Nowicki, 2006, that explicitly tested this hypothesis). It may therefore be time to look at the other hypotheses and test those in future research.