Immediate signaling flexibility in response to experimental noise in urban, but not rural, white-crowned sparrows

Background noise can interfere with acoustic communication. Signal modifications have the potential to increase signal-to-noise ratios and reduce the masking effect of noise. Immediate signaling flexibility, a type of vocal plasticity, allows animals to modify their signal to optimize transmission depending on ambient noise conditions. Results from previous studies provide conflicting evidence about whether expression of immediate signaling flexibility is dependent upon the signaler having prior experience with noisy environments. To improve our understanding of vocal plasticity, we examined immediate signaling flexibility in white-crowned sparrows Zonotrichia leucophrys nuttalli in urban and rural locations. We experimentally broadcast noise with three different spectral profiles-city (low frequency), inverse of city (high frequency), and white (equal intensity at all frequencies)-successfully to 107 males and measured multiple features of songs produced prior to and during the experiment. We predicted that all males would adjust their song structure during the noise broadcast, but that the extent of plasticity would be greater in noisier territories and the manner of adjustment would depend on the type of experimental noise broadcast. Instead, we found that only urban males exhibit immediate signaling flexibility, through which bandwidth decreases in response to experimental noise. Signal modification did not change with type of noise broadcast or territory background noise. Although songs were less flexible than predicted, the relative reduction in signal bandwidth in urban birds could improve their signal-to-noise ratio during the noise broadcast.