Are signals of aggressive intent less honest in urban habitats?

Abstract

The effect of urban noise on animal communication systems is one of the best examples of how anthropogenic change affects animal social behaviour. Urban noise often drives shifts in acoustic properties of signals but the consequences of noise for the honesty of signals - that is, how well they predict signaler behaviour, is unclear. Here we examine whether honesty of aggressive signaling changes in urban living song sparrows (Melospiza melodia). Aggressive signaling in song sparrows consists of close-range signals in two modalities that predict a subsequent attack: the low amplitude soft songs (an acoustic signal) and wing waves (a visual signal). Male song sparrows living in urban habitats display more territorial aggression than males living in rural habitats, but whether the honesty of close-range signals is affected by urbanization has not been examined. If soft songs are less effective in urban noise, we predict that they would be less reliably associated with attack in these habitats compared to rural habitats. We found that while acoustic noise was higher in urban habitats, the urban birds still sang more soft songs than rural birds during a simulated territorial intrusion. Furthermore, high rates of soft songs and low rates of loud songs predicted attacks in both habitats. Finally, we found evidence for a potential multimodal shift: urban birds tended to give proportionally more wing waves than soft songs than rural birds. These results indicate that urbanization might have a limited effect on the overall honesty of aggressive signals in song sparrows.