Abstract
As urbanization has expanded dramatically, the impacts of urban noise on wildlife have drawn increasing attention. However, previous studies have focused primarily on diurnal songbirds and much less on nocturnal nonpasserines such as nightjars. The savanna nightjar has recently successfully colonized urban areas in Taiwan. Using 1925 calls recorded from 67 individuals, we first investigated the individual differences of the acoustic structures; and, for those acoustic variables with significant individual differences, we examined the correlation between the acoustic structures and the ambient noise levels. We then compared the transmission efficacy of vocal individuality among three sets of acoustic variables: all acoustic variables, noise-related variables, and noise-unrelated variables. Using seven artificial frequency-shifted calls to represent seven different individuals in playback-recording experiments, we also investigated the transmission efficacy of vocal individuality and variable accuracy in three different urban noise levels (high, medium, low). We found that all 30 acoustic variables derived from the acoustic structures demonstrated significant individual differences, and 14 frequency-based variables were negatively correlated with ambient noise levels. Although transmission efficacy was significantly affected by urban noise, individuality information was still transmitted with high accuracy. Furthermore, the noise-unrelated structures (which included the maximum frequency, the maximum amplitude frequency, and the mean frequency of the call) had a significantly higher transmission efficacy of vocal individuality than the noise-related variables (which included the minimum frequency, the frequency at the start and the end of the call) in both field observation and playback-recording experiments. We conclude that these noise-unrelated acoustic features may be one of the key preadaptations for this nocturnal nonpasserine to thrive so successfully in its newly adopted urban environment.
Highlights
As urbanization has expanded dramatically, the impacts of urban noise on wildlife have drawn increasing attention
Our objectives were to investigate (1) which acoustic structures of the territorial calls were correlated with urban noise levels, (2) whether the transmission efficacy of vocal individuality was affected by varying urban noise levels, and (3) how transmission efficacy of acoustic structures and individuality, as represented by artificial frequency-shifted calls, was changed among three urban noise levels in playbackrecording experiments
In Taiwan, the savanna nightjar has become one of the most successful urban residents[38]. To better understand their acoustic adaptation to noisy urban environments, we investigated the acoustic structures of individual males’ territorial calls using two proposed hypotheses: no frequency shifting and transmission efficacy of vocal individuality
Summary
As urbanization has expanded dramatically, the impacts of urban noise on wildlife have drawn increasing attention. The noise-unrelated structures (which included the maximum frequency, the maximum amplitude frequency, and the mean frequency of the call) had a significantly higher transmission efficacy of vocal individuality than the noise-related variables (which included the minimum frequency, the frequency at the start and the end of the call) in both field observation and playback-recording experiments. We conclude that these noise-unrelated acoustic features may be one of the key preadaptations for this nocturnal nonpasserine to thrive so successfully in its newly adopted urban environment. Moseley et al.[26] suggested that the stronger positive association between urban noise and song pitch in oscines may have resulted from song development in nestlings which tend to learn less masked (higher frequency) songs over masked (lower frequency) songs in noisy situations
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