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

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
nonurban 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. We conclude that 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.