No frequency shift in the “D” notes of Carolina chickadee calls in response to traffic noise

Abstract

Loud, low-frequency traffic noise can mask song- bird vocalizations, and populations of some urban songbird species have shifted the frequency of their vocalizations up- ward in response. However, the spectral structure of certain vocalization elements may make them resistant to masking, suggesting that species that use these notes could be more successful in areas with high levels of traffic noise. To test this idea, we recorded Carolina chickadees (Poecile carolinensis), whose calls feature "D" notes with an overtone spectral struc- ture, along a traffic noise gradient in Durham and Orange Counties, North Carolina, USA. Frequency parameters of "D" notes did not change with noise level suggesting the possibility that these notes can be communicated effectively in noise, but further investigation is needed to test this hy- pothesis directly. In addition, we performed a playback exper- iment demonstrating how the use of spectrograms to measure note frequencies is unreliable, especially when recordings are made in noisy areas. We used an alternative method based on the predictable frequency structure of "D" notes. Our experi- ment is one of few that address the effects of urban noise on calls produced by both sexes as opposed to song produced onlybymalesduringthebreedingseason.Understandinghow vocalizationswithdifferentspectralstructuresmaybeaffected differentiallybytrafficnoisewillincreaseourabilitytopredict how the expansion of noisy areas may impact songbird com- munity composition in the future.