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

Abstract

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 Swainson's thrushes (Catharus usulatus) adjust their songs under different traffic noise regimens using both population-level surveys and individual-level noise playback experiments. Swainson's thrushes' introductory note 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 the acoustic features of their songs, increasing the duration of their songs and adjusting the frequency parameters of the introductory notes before quickly returning towards baseline levels after the noise playback ended. Together, these results suggest that Swainson's thrushes adjust 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.