Mechanisms of noise disruption: masking, not distraction or increased vigilance, compromises wild bird communication

Noise from human activity is a global concern that threatens wildlife, including by disrupting acoustic communication. This disruption is often assumed to be caused by masking, where signals are difficult to hear in the presence of noise of similar frequency to the signals. However, other mechanisms can also lead to reduced responses to signals: animals may be distracted by noise and so miss the signal, or noise may increase their vigilance and so lead them to use personal information instead of responding to signals from others. Previous experimental work on superb fairy-wrens, Malurus cyaneus, found evidence that masking disrupted responses to conspecific alarm calls but could not rule out distraction. We therefore designed experiments using responses to heterospecific alarm calls to distinguish masking from distraction. We tested responses to low- and high-frequency heterospecific alarm calls during low- and high-frequency noise. Fairy-wrens responded to both heterospecific alarm calls during nonoverlapping noise, but not during overlapping noise, regardless of noise frequency, and vigilance was similar during the two types of noise. We conclude that the alarm call response was compromised by masking, not distraction or increased vigilance. Overall, the assessment of mechanisms helps predict the consequences of noise pollution and provides suggestions on managing anthropogenic noise at the community level, such as avoiding noise whose frequency overlaps either conspecific or heterospecific calls.