City living: Smaller territories and shorter communication distance for urban songbirds

Background/Question/Methods

: Many studies have demonstrated that urban noise can interfere with animal communication by masking acoustic signals such as birdsong, but the functional consequences of impaired communication are still not well understood. Although many bird species sing at higher amplitude in noise pollution, communication distance is still reduced in noisy urban soundscapes. Song is a long-distance signal that functions to attract a mate and defend a territory, so it stands to reason that a reduction in communication distance could negatively influence a male’s reproductive or competitive success. Reduced territorial success could manifest as more frequent territorial intrusions, reduced territory quality, or reduced territory size. Here we examine the relationship between communication distance and territory size in Nuttall’s white-crowned sparrows (Zonotrichia leucophrys nuttali) across an urban-rural gradient in the San Francisco Bay Area in Spring 2021 and 2022. We mapped the territories of male white-crowned sparrows and calculated territory size as the 95% utilization distribution using a kernel density estimator. We measured the amplitude of each male’s songs as well as background noise levels on his territory, which we used to calculate the communication distance of each song. We used linear regression to assess the relationship between communication distance and territory size.

Results/Conclusions

: We found that the communication distance of a bird’s songs and the environmental noise level on his territory significantly predict his territory size, such that birds with shorter communication distance tend to have smaller territories. This result suggests that communication distance may be one factor influencing size of songbird territories. This finding strengthens our understanding of the link between communication and its fitness-related functions – an important frontier in the study of birdsong. It also underlines the potential for anthropogenic impacts on animal behavior to be both subtle and important to the ecology of the species.