Abstract
Low-frequency urban noise can interfere with avian communication through masking. Some species are able to shift the frequency of their vocalizations upwards in noisy conditions, which may reduce the effects of masking. However, results from playback studies investigating whether or not such vocal changes improve audibility in noisy conditions are not clear; the responses of free-ranging individuals to shifted signals are potentially confounded by functional trade-offs between masking-related audibility and frequency-dependent signal quality. Black-capped chickadees (Poecile atricapillus) naturally sing their songs at several different frequencies as they pitch-shift to match conspecifics during song-matching contests. They are also known to switch to higher song frequencies in response to experimental noise exposure. Each male produces both high- and low-frequency songs and absolute frequency is not a signal of aggression or dominance, making this an interesting species in which to test whether higher-frequency songs are more audible than lower-frequency songs in noisy conditions. We conducted playback studies across southern and central British Columbia, Canada, using paired song stimuli (high- vs low-frequency songs, n = 24 pairs) embedded in synthetic background noise created to match typical urban sound profiles. Over the course of each playback, the signal-to-noise ratio of the song stimuli was gradually increased by raising the amplitude of the song stimuli while maintaining background noise at a constant amplitude. We evaluated variation in how quickly and aggressively territorial males reacted to each of the paired stimuli. We found that males responded more quickly to playbacks of high- than low-frequency songs when high-frequency songs were presented first, but not when low-frequency songs were first. This difference may be explained by high-frequency songs being more audible combined with a carry-over effect resulting in slower responses to the second stimulus due to habituation. We observed no difference in overall aggression between stimuli. These results suggest that high-frequency songs may be more audible under noisy conditions.
Highlights
Urban noise pollution is generally low in frequency and can interfere with avian communication through masking of overlapping frequencies (Rabin & Greene, 2002; Brumm & Slabbekoorn, 2005; Barber, Crooks & Fristrup, 2010)
We have shown that use of higher frequencies in male black-capped chickadees correlates with local ambient noise levels, and that territorial males adjust their frequency use to sudden, experimental increases in noise
Playback order had an interactive effect on how quickly territorial males responded such that males initially exposed to high-frequency stimuli responded more quickly to high- than to low-frequency stimuli, but males initially exposed to low-frequency stimuli responded quickly to the two treatments
Summary
Urban noise pollution is generally low in frequency and can interfere with avian communication through masking of overlapping frequencies (Rabin & Greene, 2002; Brumm & Slabbekoorn, 2005; Barber, Crooks & Fristrup, 2010). There are many examples of birds singing higher frequencies in noisy conditions (e.g., Slabbekoorn & Peet, 2003; Wood & Yezerinac, 2006; Verzijden et al, 2010; Bermúdez-Cuamatzin et al, 2011; LaZerte, Slabbekoorn & Otter, 2016; LaZerte, Otter & Slabbekoorn, 2017) These lines of evidence suggest that spectral plasticity in response to elevated noise levels may be adaptive, but from field studies there is little direct evidence that spectrally-adjusted songs improve detection and discrimination over unadjusted songs (e.g., Luther & Magnotti, 2014; but see Halfwerk et al, 2011)
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