Swainson's thrushes vary song structure and singing behavior across ambient noise gradients and rapidly adjust songs in response to experimental traffic noise

Anthropogenic noise pollution has a variety of negative consequences for animals, with observed effects ranging from physiological processes to community structures. Sound waves from anthropogenic noise can mask animal signals, creating challenges for animal communication. Songbirds have been observed to combat noise pollution by changing their song rate, duration, frequency, or amplitude, with different species employing different strategies. We examined whether Swainsons thrushes (Catharus usulatus) adjust their song structure under different traffic noise regimens using both population level surveys and individual level noise playback experiments. Swainsons thrushes song frequency varied with background traffic noise levels across the surveyed population, but other song parameters did not. When confronted with realistic levels of experimental traffic noise playback, individual thrushes rapidly changed their song structure, increasing both the duration and the minimum frequency of their songs, before quickly returning towards baseline levels after the noise playback ended. Together, these results suggest that Swainsons thrushes are adjusting their singing to reduce acoustic masking in noisy areas and further indicate that some songbirds might continually assess ambient noise levels and only adjust their signals when necessary.