Birds inhabiting urban areas have to deal with high levels of ambient noise. Some species show a certain song flexibility that enables them to reduce noise interferences in their communications. This vocal adjustment usually implies an increase in the minimum frequencies of songs. Since urban noise is mainly made up of low frequencies (about 2.5-3.5 kHz), song of species that sing at higher frequencies could be less susceptible to being masked by anthropogenic noise. This study explores whether such species also show any kind of adjustment to noisy environments. For this purpose, the spectral and temporal parameters (note duration, maximum and minimum frequency and diversity) of the song of the European Wren Troglodytes troglodytes were analysed in three different environments (urban, periurban and rural). To evaluate the impact of noise on the vocalizations, a specific acoustic descriptor of song variability was developed. Song variability increased along the urban noise gradient from rural to urban areas and the duration of notes decreased from rural to urban zones. Urban wrens developed more complex songs with higher frequencies and longer notes than their rural counterparts, whereas peri-urban birds occupied an intermediate position, although closer to urban ones in the length of notes. These changes could be associated with higher background noise levels, although other possible causes, such as the population density, could also explain them. Maximum frequencies were mostly outside the background noise range and differed among habitats, whereas lower frequencies unexpectedly did not differ among habitats. Our results suggest that differences in song parameters among species may lead to different mechanisms of vocal adjustment. Even in wrens, with high frequency vocalisation, interference with urban anthropogenic noise could show certain changes in their vocalizations.
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