Communication distance predicts territory size for an urban songbird

Many studies demonstrate that urban noise interferes 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 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. We examined the relationship between communication distance and territory size in white-crowned sparrows (Zonotrichia leucophrys) across an urban-rural gradient in the San Francisco Bay Area in Spring 2021. We mapped the territories of male White-crowned sparrows and calculated territory size as the 75% utilization distribution using a kernel density estimator. We measured the amplitude of each male’s songs as well as background and ambient noise levels on his territory, which we then used to calculate the communication distance of each song. We found the mean communication distance of each individual and assessed its relationship to territory size. the communication distance of a bird’s songs significantly predict his territory size, such that birds with shorter communication distance tend to have smaller territories. This suggests that communication distance may influence the size of songbird territories. In keeping with this trend, urban birds had significantly smaller territories than rural birds. 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 complexity of the impacts of the anthropogenic soundscape upon animal behavior.