Many avian species live, breed, and communicate in urban areas. To survive and reproduce in these areas, birds must transmit their signals to intended receivers. As an arena for acoustic communication, 2 salient features of the urban environment are an abundance of reflective surfaces and a high level of low-frequency anthropogenic noise. Each presents unique communication challenges, with hard surfaces reflecting and distorting high frequencies and noise masking low-frequency song components. Based on this, we predicted that noise level would affect minimum song frequency and urban development (percentage of impervious surface) would affect maximum frequency and frequency range. We compared the effects of urban development and noise on songs of 6 bird species at 28 sites along an urban to rural gradient, across a broad range of noise levels. We found that minimum song frequency increased as noise level increased for 2 of 6 species, with 5 of 6 species showing a strong trend in the predicted direction. Species with lower frequency songs were more affected by noise. Maximum frequency and frequency range decreased for 2 of 6 species as urban development increased, and this effect was stronger for species with higher frequency songs. For some species, minimum frequency only increased with noise at less urban sites and similarly, maximum frequency and frequency range only decreased with urbanization at quiet sites, suggesting a trade-off between different vocal adjustments. Ours is the first study to investigate how noise and urban development affect song frequency characteristics of multiple bird species.
