Birds and Anthropogenic Noise: Are Urban Songs Adaptive?

TLDR

In cities with intense low‑frequency traffic noise, birds have been observed to sing louder and at a higher pitch, and some studies suggest this pitch shift may reduce masking and even drive reproductive isolation. This study models signal transmission to compare the benefits of raised song amplitude versus pitch in urban environments. Using great tit and blackbird, the authors calculated communication distances under varying urban noise and forest conditions. Results show that increased pitch only marginally extends communication distance, whereas amplitude adjustments greatly improve it, indicating pitch changes are likely not an adaptive masking strategy but a side effect of higher amplitude singing.

Abstract

In cities with intense low‐frequency traffic noise, birds have been observed to sing louder and at a higher pitch. Several studies argue that higher song pitch is an adaptation to reduce masking from noise, and it has even been suggested that the song divergence between urban and nonurban songs might lead to reproductive isolation. Here we present models of signal transmission to compare the benefits of raised song amplitude and song pitch in terms of sound transmission. We chose two bird species that sing with higher pitch in urban areas, the great tit (Parus major) and the blackbird (Turdus merula). For both species, we calculated communication distances in response to different levels of urban noise and in their natural forest habitats. We found that an increase in vocal pitch increased communication distance only marginally. In contrast, vocal amplitude adjustments had a strong and significantly larger effect. Our results indicate that frequency changes of urban songs are not very effective in mitigating masking from traffic noise. Increased song pitch might not be an adaptation to reduce signal masking but a physiological side effect of singing at high amplitudes or an epiphenomenon of urbanization that is not related to signal transmission.