Effect sizes of divergence in urban noise and song minimum frequency of grey-cheeked fulvettas Alcippe morrisonia morrisonia

Abstract

Noise pollution, one of the most prominent features of urbanization, is an important factor influencing the vocal behavior of urban wildlife. Studies have reported that many songbirds raise their song minimum frequencies in response to urban noise. It has been proposed that this increased minimum frequency (IMF) of songs is an adaptation that allows urban populations to cope with the masking effect of noise pollution. However, urban populations of some songbirds do not exhibit significant IMF compared with non-urban populations; thus, the notion that IMF is an adaptation to urban noise has been questioned. Furthermore, the effects of IMF might be influenced by both noise levels and the acoustic structures of songs. Here, we employed dichotomous and gradient effect size approaches to investigate IMF regarding two distinct acoustic structures (whistled and harmonic) in songs of six grey-cheeked fulvetta Alcippe morrisonia morrisonia populations in Taiwan, three with high noise pollution and three with low noise pollution. We found that when using the dichotomous approach, paired populations with significant divergence in noise levels exhibited weak or insignificant divergence in the minimum frequencies for both whistled and harmonic phrases. In contrast, we found that when using the gradient approach, the effect size of noise-level divergence was strongly correlated with the effect size of divergence in the minimum frequency of the harmonic phrase and only moderately correlated with the effect size of divergence in the minimum frequency of the whistled phrase. These findings suggest that noise pollution has a more pronounced effect on the divergence in the minimum frequency of harmonic phrases used in short-range communication, compared to the whistled phrases used in long-range communication. For population comparisons on the IMF, adopting a gradient approach could provide insights into the impact of noise pollution on the acoustic structures of songs across various communication ranges.