Summary
Anthropogenic noise can mask animal signals that are crucial for communicating information about food, predators and mating opportunities. In response to noise masking, signallers can potentially improve acoustic signal transmission by adjusting the timing, frequency or amplitude of their signals. These changes can be a short-term modification in response to transient noise or a long-term modification in response to chronic noise. An animal's ability to adapt to anthropogenic noise can be crucial to its success. In this study, we evaluated the effects of anthropogenic noise on the structure of red-winged blackbird song. First, we manipulated the presence of anthropogenic noise by experimentally broadcasting either silence or low-frequency white noise to subjects inhabiting quiet marshes located away from roadsides. Subjects exhibited increased signal tonality when temporarily exposed to low-frequency white noise, suggesting that red-winged blackbirds can alter their signals rapidly in response to sudden noise. Second, we compared songs produced in quiet marshes located away from roadsides with songs produced during quiet periods at roadside marshes that are normally noisy. This allowed us to test whether birds that are exposed to chronic anthropogenic noise exhibit altered song structure during temporarily quiet periods. Subjects residing in roadside marshes that are normally polluted with anthropogenic noise sang songs with increased tonality during quiet periods. Overall, our results show that anthropogenic noise influences the structure of birdsong. These effects should be considered in conservation and wildlife management.
Methodology
We tested whether red-winged blackbirds from undisturbed marshes adjusted the structure of their trills in response to experimentally broadcasted low-frequency white noise. We presented a given subject with two sequential playback treatments over the course of approximately 6 min, while simultaneously recording its vocal response (recording details provided below). For each trial, we continued the first treatment until the subject sang between 3 and 7 songs. We commenced the second treatment immediately after the first and continued it until the subject sang another 3–7 songs. Treatment order was randomized for each subject.
The two treatments were a silent control treatment in which we broadcast continuous silence, and an experimental noise treatment in which we broadcast continuous low-frequency white noise. To create the experimental noise treatment, we generated broad-spectrum white noise in Audition software (version 2.0; Adobe, San Jose, CA, USA) and then filtered the noise (bandpass filter, 0–1830 Hz) in Raven (version 1.4 Pro; Cornell Lab of Ornithology Bioacoustics Research Program, Ithaca, NY, USA). We chose an upper bandpass filter frequency of 1830 Hz because this frequency was lower than the minimum frequency of red-winged blackbird trills (minimum frequency of trills observed in our chronic noise experiment: mean ± s.e.m., 2420±6 Hz; range, 2150–2790 Hz; see below). Thus, the experimental low-frequency white noise could be removed completely in the analysis without removing the lower frequencies of the trill, thereby enabling us to measure trill structure without those measurements being affected by the white noise. Furthermore, the average spectrum of our experimental noise approximated that of typical traffic noise, which has its dominant spectral energy below 1830 Hz (Cornillon and Keane, 1977; Halfwerk et al., 2010; Wood and Yezerinac, 2006).