Increased attenuation and reverberation are associated with lower maximum frequencies and narrow bandwidth of bird songs in cities

As urbanization expands globally, the communication systems of an increasing number of species are affected. Because bird song is a long-distance signal used to attract mates and defend territories, the evolution of bird song is often shaped by habitat structure and background noise. These potential drivers of song evolution are more often studied in natural areas than in urbanized areas, leaving open the question of how anthropogenic changes to the landscape are affecting the evolution of bird song. One songbird that persists in both urbanized and rural areas in North America is the White-crowned Sparrow (Zonotrichia leucophrys). Our previous work demonstrates that increased background noise in cities and in natural habitats affects acoustic adaptation in this species. However, we lack information about how sound transmits in urban and rural habitats. Because cities tend to have different physical properties than rural areas, it is pertinent to understand the influence of urban habitat structure on song evolutionary processes. Here, we test the acoustic adaptation theory and hypothesize that differences in the sound transmission properties of urban and rural habitats affect song divergence between urban and rural populations. We conducted sound transmission trials of tones from 1 to 8 kHz on 59 White-crowned Sparrow territories in three urban and three rural locations around San Francisco and Point Reyes, California, at varying heights and rates of note production. We also recorded and analyzed songs of males from each location to see if differences in signal–noise ratio, attenuation, reverberation, and distortion predicted song divergence. We found that urban locations have higher attenuation and reverberation than rural locations and urban songs tend to have short whistles, faster trills, and narrower bandwidth. These findings partially support acoustic adaptation theory, though faster trills in the city may instead be driven by cultural drift or sexual selection. Overall, our results add to a rapidly growing area of research and allow us to better understand the complexity of influences on song divergence.

Duke Scholars

Author Sara E Lipshutz Biology