Communication distance predicts territory size: implications 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. 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 ter-ritory quality or reduced territory size. Although many bird species sing at higher amplitude in noise pollution, communication distance is still reduced in noisy urban soundscapes. Therefore, we examined the relationship between communication distance and territory size for male white-crowned sparrows, Zonotrichia leucophrys, across an urban-rural gradient in the San Francisco Bay Area, California, U.S.A., in Spring 2021. We mapped territories and calculated territory size as the 75% utilization distribution using a kernel density estimator. We measured the song amplitude as well as territory background and ambient noise levels, which we then used to calculate the communication distance of each song. We found that the communication distance of a bird's songs significantly predicted his territory size, such that birds with shorter communication distance tended 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 behaviour.& COPY; 2023 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.