Patterns of Song across Natural and Anthropogenic Soundscapes Suggest That White-Crowned Sparrows Minimize Acoustic Masking and Maximize Signal Content

Abstract

Soundscapes pose both evolutionarily recent and long-standing sources of selection on acoustic communication. We currently know more about the impact of evolutionarily recent human-generated noise on communication than we do about how natural sounds such as pounding surf have shaped communication signals over evolutionary time. Based on signal detection theory, we hypothesized that acoustic phenotypes will vary with both anthropogenic and natural background noise levels and that similar mechanisms of cultural evolution and/or behavioral flexibility may underlie this variation. We studied song characteristics of white-crowned sparrows (Zonotrichia leucophrys nuttalli) across a noise gradient that includes both anthropogenic and natural sources of noise in San Francisco and Marin counties, California, USA. Both anthropogenic and natural soundscapes contain high amplitude low frequency noise (traffic or surf, respectively), so we predicted that birds would produce songs with higher minimum frequencies in areas with higher amplitude background noise to avoid auditory masking. We also anticipated that song minimum frequencies would be higher than the projected lower frequency limit of hearing based on site-specific masking profiles. Background noise was a strong predictor of song minimum frequency, both within a local noise gradient of three urban sites with the same song dialect and cultural evolutionary history, and across the regional noise gradient, which encompasses 11 urban and rural sites, several dialects, and several anthropogenic and natural sources of noise. Among rural sites alone, background noise tended to predict song minimum frequency, indicating that urban sites were not solely responsible for driving the regional pattern. These findings support the hypothesis that songs vary with local and regional soundscapes regardless of the source of noise. Song minimum frequency from five core study sites was also higher than the lower frequency limit of hearing at each site, further supporting the hypothesis that songs vary to transmit through noise in local soundscapes. Minimum frequencies leveled off at noisier sites, suggesting that minimum frequencies are constrained to an upper limit, possibly to retain the information content of wider bandwidths. We found evidence that site noise was a better predictor of song minimum frequency than territory noise in both anthropogenic and natural soundscapes, suggesting that cultural evolution rather than immediate behavioral flexibility is responsible for local song variation. Taken together, these results indicate that soundscapes shape song phenotype across both evolutionarily recent and long-standing soundscapes.

White‐crowned sparrow males show immediate flexibility in song amplitude but not in song minimum frequency in response to changes in noise levels in the field

“…Song minimum frequency in this species has changed across generations (approximately 30 years) in association with changes in the transmission properties of its environment (Derryberry, ) and with changes in background noise levels in its soundscape (Luther & Baptista, ; Luther & Derryberry, ). Further, we found evidence of spatial variation in song minimum frequency across soundscapes consistent with the acoustic adaptation hypothesis (Derryberry et al., ). Findings in this study suggest that these temporal and spatial patterns of variation are likely not the result of immediate flexibility in song minimum frequency.…”

“…Our results suggest that (1) males do not show flexibility in song minimum frequency in response to fluctuations in noise levels on an immediate time scale, (2) males do show immediate flexibility in song amplitude, and (3) that white‐crowned sparrows do not face a physiological constraint on singing louder at these frequencies. Together, these results show support for immediate flexibility in song amplitude as a behavioral response to the soundscape and indicate additional work is needed to examine other mechanisms, such as ontogenetic adjustments, to explain previously described correlations between song minimum frequency and environmental noise levels across territories and over generations in this species (Derryberry et al., ; Luther & Baptista, ).…”

“…Our previous work in this system indicated quadratic relationships between song features and noise levels (Derryberry et al., ). Therefore, we included quadratic terms of noise levels (e.g., instantaneous 2 ) in our models.…”

“…Here, we observed the singing behavior of free‐living, male Nuttall's white‐crowned sparrows ( Zonotrichia leucophrys nuttalli ) in the San Francisco, CA area (Figure ). Song minimum frequency varies spatially with environmental noise levels in this species (Derryberry et al., ; Luther, Phillips, & Derryberry, ). One mechanism to explain this correlation is that the song phenotype has changed over generations.…”

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

“…Background noise is a strong predictor of song minimum frequency at a local scale as well, with minimum frequency higher at noisier sites within urban and rural populations (Derryberry et al. ). In this study, we conducted noise broadcast experiments on randomly selected free‐living adult male NWCS from urban and rural populations.…”