Are signals of aggressive intent less honest in urban habitats?

Abstract

18The effect of urban noise on animal communication systems is one of the best examples of how 19 anthropogenic change affects animal social behaviour. It is well documented that urban noise 20 drives shifts in the acoustic properties of signals but the consequences of noise for the reliability 21 of signalsthat is, how well they predict signaler behaviour, is unclear. Here we examine 22 whether reliability of aggressive signaling in song sparrows (Melospiza melodia) differs between 23 urban and rural habitats. Previous research in this species showed that urban males are more 24 aggressive than rural males. We predicted that the main signal of aggressive intent, low-25 amplitude soft songs, would be less effective in urban noise and that urban birds should signal 26 their aggressive intent via loud songs. We found that while acoustic noise was higher in urban 27 habitats, the urban birds still sang more soft songs and rural birds. Furthermore, high rates of soft 28 songs and lack of loud songs predicted attacks in both habitats. Finally, we found evidence for a 29 potential but incomplete multimodal shift due to urban noise. These results indicate that 30 urbanization, despite the well documented effects on signal structure has a limited effect on the 31 reliability of the signals in song sparrows. 32 33