Meeting Abstract
Male Bachman’s sparrows (Peucaea aestivalis) have large vocal repertoires containing 35-45 song types and multiple categories of songs and calls. We examined the territorial defense function of the vocal repertoire of this species by quantifying singing behaviors and aggressiveness in response to a simulated territorial intrusion. We compared vocal and other behaviors between the intrusion and post-intrusion periods, compared more aggressive birds (attackers) with less aggressive birds (non-attackers), and tested for signals that predict attack. During intrusion, subjects switched among their song types at higher rates, and gave more “excited songs” and calls, however song switching and excited songs did not differ between more and less aggressive birds. These behaviors appear to serve an agonistic function but do not predict aggression. More aggressive birds gave more calls and sang more low amplitude “whisper songs.” In line with previous studies of aggressive signaling in songbirds, Bachman’s sparrows appear to use low amplitude song to threaten rivals. Based on our results, the Bachman’s sparrow provides another species in which to test hypotheses about the evolution of quiet threat signals. The eavesdropping avoidance hypothesis predicts that other acoustic traits besides amplitude should be shaped by selection to minimize signal propagation and transmission range. In contrast to most species that use low-amplitude songs to threaten rivals, the whisper songs of Bachman’s sparrows do not differ in acoustic structure from their broadcast primary songs, except for amplitude. This provides an opportunity to explore whether reduced amplitude is sufficient to reduce the costs of eavesdroppers, and to test alternative hypotheses to explain the use of low amplitude song as a threat.
