Urban-associated drivers of song variation along a rural–urban gradient

Most studies demonstrating associations between avian song and urbanization have tested noise as the primary driver, but alternative explanations remain largely unexplored. In particular, urban-associated changes in vegetation, density of conspecifics, and morphology also may promote changes in song. In this study, we 1) identified relationships between urbanization and song characteristics and 2) evaluated the extent to which altered song was explained by variation in noise level, vegetation, social context, and morphology of individual birds. We monitored the territories and recorded songs of northern cardinals (Cardinalis cardinalis) within forest stands in central Ohio that varied with surrounding urbanization. As landscapes surrounding forests urbanized, songs were longer, sung at faster rates, and comprised of higher-frequency notes. Noise best predicted minimum song frequency, with individuals singing at higher frequencies in environments with higher levels of background noise. However, temporal attributes of song (e.g., syllable rate, length) were best explained by conspecific densities, which are substantially greater in urban than rural landscapes. Morphology and vegetation did not predict any song attributes tested. These findings show that although anthropogenic noise may shape urban-associated changes in song, other features of the urban environment may be more important contributors to patterns in song variation.