Western Australian Magpies alter the rate, but not the amplitude, of their territorial song in anthropogenic noise

Anthropogenic noise is considered one of the most serious forms of pollution globally and has been shown to have negative effects on the distribution, behaviour, cognition and reproductive success of animal species worldwide. Among the most commonly reported impacts of anthropogenic noise are its effects on acoustic communication. Animals may adjust the rate, amplitude, duration and/or frequency of their acoustic signals to better maintain communication when anthropogenic noise is present. One of the most commonly reported vocal adjustments in noisy conditions, an increase in amplitude known as the Lombard effect, has been reported in almost all animal species tested to date. In this study, we combine behavioural focals and amplitude measurements to investigate whether female Western Australian Magpies Gymnorhina tibicen dorsalis alter the rate and amplitude of their territorial song (known as a carol) when unmanipulated levels of anthropogenic noise are present. Magpies reduced the rate at which they carolled when loud anthropogenic noise (>50 dB) was present but, contrary to our prediction, we found no evidence that magpies adjusted the amplitude of their carols under these conditions. Reduced carolling rates during anthropogenic noise may minimize the energetic expense associated with vocalizing when it is likely that such vocalizations will be masked by anthropogenic noise. However, a reduction in carolling rate may negatively affect the sociality and territoriality of magpies, as carols are important for these aspects of life. Our study adds to the growing body of literature documenting changes to the vocal behaviour of wildlife in the presence of anthropogenic noise.