Abstract
BackgroundUrban environments generate constant loud noise, which creates a formidable challenge for many animals relying on acoustic communication. Some birds make vocal adjustments that reduce auditory masking by altering, for example, the frequency (kHz) or timing of vocalizations. Another adjustment, well documented for birds under laboratory and natural field conditions, is a noise level-dependent change in sound signal amplitude (the ‘Lombard effect’). To date, however, field research on amplitude adjustments in urban environments has focused exclusively on bird song.MethodsWe investigated amplitude regulation of alarm calls using, as our model, a successful urban ‘adapter’ species, the Noisy miner, Manorina melanocephala. We compared several different alarm calls under contrasting noise conditions.ResultsIndividuals at noisier locations (arterial roads) alarm called significantly more loudly than those at quieter locations (residential streets). Other mechanisms known to improve sound signal transmission in ‘noise’, namely use of higher perches and in-flight calling, did not differ between site types. Intriguingly, the observed preferential use of different alarm calls by Noisy miners inhabiting arterial roads and residential streets was unlikely to have constituted a vocal modification made in response to sound-masking in the urban environment because the calls involved fell within the main frequency range of background anthropogenic noise.ConclusionsThe results of our study suggest that a species, which has the ability to adjust the amplitude of its signals, might have a ‘natural’ advantage in noisy urban environments.
Highlights
Animals that rely on acoustic communication must ensure that their vocalizations are not masked by background noise
Whilst background noise is a ubiquitous feature of natural environments, the level of noise often associated with urban settings represents a formidable challenge for individuals of many species that communicate acoustically [1,2,3]
Alarm call amplitude of Noisy miners was greater at arterial than residential roads (mean maximum amplitude: arterial = 88.6060.59dB, residential = 79.5360.90dB, t = 8.713, df = 95, p,0.001) Overall, a significant relationship was found between the background noise level at a site and the amplitude of Noisy miner alarm calls; individuals at noisier locations called more loudly than those at quieter locations, indicating that Noisy miners were exhibiting the Lombard effect in urban Melbourne
Summary
Animals that rely on acoustic communication must ensure that their vocalizations are not masked by background noise. Some birds in urban environments have the capacity to make vocal adjustments This is often achieved by altering, for example, the frequency (kHz) [8,9,10,11,12,13] and/or duration [9,10] of their signals in such a way as to avoid auditory masking by background noise. Another important form of vocal adjustment, which has been well documented for birds in laboratory studies [14,15,16,17] but has received less attention under more natural, field conditions [18–. Field research on amplitude adjustments in urban environments has focused exclusively on bird song
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