Many birds rely on alarm calls during nest defence to communicate about risk, but anthropogenic disturbance, including noisy industrial infrastructure, has the potential to disrupt alarm communication. Birds may overcome these effects by altering the structure of vocalizations to improve signal transmission. While previous work has demonstrated that birds can alter the structure of songs in the presence of noise, few have considered effects of noise-producing infrastructure on alarm call structure. Here, we tested whether Savannah sparrows (Passerculus sandwichensis) altered the structure of alarm calls in the presence of natural gas compressor stations or grid-powered or generator-powered screw pump oil wells, and in relation to overall ambient noise levels. Savannah sparrows called at a lower peak frequency and first quartile frequency close to gas compressor stations, but call structure did not differ with proximity to either type of oil well, or in response to ambient noise levels, suggesting that factors beyond noise amplitude, such as the physical footprint of the infrastructure, affected call structure close to compressor stations. Our results demonstrate that even unlearned, structurally simple avian vocalizations can vary in structure, and that the impacts of anthropogenic infrastructure on acoustic communication may be driven by factors beyond just noise.
Acoustic masking, alarm calls, anthropogenic noise, natural gas, vocal plasticity, vocalization structure