Mountain chickadees adjust songs, calls and chorus composition with increasing ambient and experimental anthropogenic noise

Vocal plasticity may allow birds to reduce masking effects of noise pollution arising from urbanization. Mountain chickadees (Poecile gambeli) use both songs and calls during the dawn chorus, which vary in masking susceptibility. Thus, increasing song or call frequency, or switching between vocalization types are all potential mechanisms to reduce masking during fluctuating noise conditions. Further, prior experience with noise pollution may be a necessary precursor to allow birds to alter signals in response to sudden noisy conditions. To determine how mountain chickadee songs, calls, and chorus composition are affected by noise, we recorded 55 males across gradients of local ambient noise and habitat urbanization in three cities in British Columbia, Canada. Of these individuals, 31 were also exposed to 5-min experimental noise treatments. Habitat urbanization was quantified through a continuous index reflecting properties of urbanized areas. Only song frequency increased with local ambient noise, and this effect varied regionally. In response to experimental noise exposure, males increased the frequency of their calls (but not of their songs), and varied their use of songs vs. calls. Interestingly, this response was dependent on local ambient noise levels: males in noisy areas shifted to using relatively more songs, whereas males in quiet areas shifted to using relatively more calls. These findings may suggest that although mountain chickadees are capable of adjusting their vocalizations, choosing a response which can lead to masking release may require prior exposure to high levels of ambient noise.