Effects of anthropogenic noise and urbanization on the entropy of male song in the gray catbird, Dumetella carolinensis
As urbanization increases, so does anthropogenic noise pollution, and animals that communicate acoustically are increasingly impacted by noise-masking of their signals. Oscine songbirds have been shown to change their behavior to avoid masking from low-frequency anthropogenic noise such as by raising the minimum frequency of their songs. In contrast to most bird species that have been studied which sing short, stereotyped songs, the gray catbird, Dumatella carolinensis, sings long bouts with an immediate variety of different song elements and employs mimicry of other species’ songs. This singing style makes quantifying song differences of catbird populations along an urban gradient difficult. I hypothesized that urbanization would have an effect on male catbird song entropy, which is the dispersion of energy across a frequency spectrogram. Previous findings demonstrate that gray catbirds raise the minimum frequency of their song in response to anthropogenic noise; therefore, decreasing song bandwidth (Rhodes 2020, Moseley Lab unpublished data). These findings suggested males would sing with decreased entropy in response to anthropogenic noise, due to a smaller range to expend energy. I predicted that aggregate and average entropy would significantly vary among males, among sites, and among habitat types. Specifically, I predicted that increased ambient noise levels (dB) would have a negative relationship with both measures of entropy meaning that males in noisier sites would sing with lower entropy. Using standardized song selections with durations of 2 and 5 seconds between the frequencies 1150 Hertz and 11000 Hertz, I analyzed songs from 31 males freely singing at sites along an urban gradient from Washington, D.C., to Harrisonburg, VA. After testing for normality, I used a one-factor ANOVA and the Kruskal-Wallis test to determine if there was more variation within or between groups for males, sites, and habitat types. I also performed a regression analysis to examine if entropy varied with background noise level. While there were no significant differences in entropy among sites, habitat type, or ambient noise, there was a significant difference among males for all measures of entropy. These findings suggest that males use entropy to recognize individuals, similar to male and female vermilion flycatchers calling with variation in entropy (Ríos-Chelén et al., 2020). Additionally, I observed a negative relationship between increasing ambient noise levels (dB) and all measures of entropy, which was in line with my prediction, but not statistically significant. Although there was a weak relationship between these two variables, a larger sample size may support a stronger relationship between the two variables. Morphology, genetics, nesting stage, nutritional effects, early development, and the brain should be studied to understand why individual males vary in entropy and other structural components of song in gray catbirds.