Urban birdsongs: higher minimum song frequency of an urban colonist persists in a common garden experiment

Abstract

Environmental changes caused by urbanization and noise pollution can have profound effects on acoustic communication. Many organisms use higher sound frequencies in urban environments with low-frequency noise, but the developmental and evolutionary mechanisms underlying these shifts are generally unknown. We used a common garden experiment to ask whether changes in minimum song frequency observed 30 years after a songbird colonized an urban environment are a consequence of behavioural flexibility. We captured male juvenile dark-eyed juncos, Junco hyemalis thurberi, from two populations (urban and mountain) soon after they reached independence (aged 25-40 days), raised them in identical indoor aviaries and studied their songs at an age of 3 years. We found that the large population difference in minimum frequency observed in the field persisted undiminished in the common garden despite the absence of noise. We also found some song sharing between the common garden and natal field populations, indicating that early song memorization before capture could contribute to the persistent song differences in adulthood. These results are the first to show that frequency shifts in urban birdsong are maintained in the absence of noise by genetic evolution and/or early life experiences.

Keywords: acoustic communication; anthropogenic noise; cultural evolution; dark-eyed junco; song learning; urbanization.

Figure A1.

Correlation of (a) minimum and (b) maximum frequencies measured visually from the spectrogram and using a minus 30 dB threshold from the power spectrum.

Figure A2.

Comparison of the spectrogram and power spectrum techniques for measuring minimum and maximum frequency of songs recorded in the common garden environment. Each box represents the interquartile range and median, whiskers represent range of data within 1.5 times the interquartile range, and dots represent data points exceeding that range. **P < 0.001.

Figure 1.

Example spectrograms of shared song types observed between mountain- and urban-captured males in the common garden. Songs (a), (c) and (e) were produced by mountain males, and songs (b), (d) and (f) were produced by urban males. The thin line on each spectrogram marks 3 kHz to highlight frequency shifts between mountain and urban songs.

Figure 2.

Comparison of the minimum, peak and maximum frequencies of song types produced by mountain- and urban-captured males raised in a common garden environment. Each box represents the interquartile range and median, whiskers represent range of data within 1.5 times the interquartile range, and dots represent data points exceeding that range. **P < 0.001.

Figure 3.

Comparison of the (a) minimum, (b) peak and (c) maximum frequency of songs produced by mountain and urban males in the field (Cardoso & Atwell, 2011a) and in a common garden aviary environment. Box plots as in Fig. 2. *P < 0.05; **P < 0.001.