The effect on anthropogenic noise on acoustic signals is widely studied although the extent to which this is due to short-term flexibility or long-term adaptation is unclear. Anthropogenic noise may disrupt signals used to mediate aggressive interactions, leading to more physical aggression between opponents. One solution to this problem is to switch signaling effort to a less noisy modality (e.g. the visual modality). In the present study we ask whether urban and rural European robins (Erithacus rubecula) use multi-modal signals flexibly in response to anthropogenic noise during territorial defense. We predicted that during simulated intrusions with experimental noise, European robins would increase their physical aggression, respond with more visual threat displays and sing fewer songs, i.e. show a multi-modal shift. We also predicted that urban birds, living in noisier habitats, would be generally more aggressive than rural birds. The results showed that urban birds were more aggressive than rural robins, but an increase in aggression with experimental noise was seen only in the rural birds. Urban birds also used visual signals more often than rural birds. However, birds did not use visual signals more in experimental noise. Instead, both urban and rural robins sang at higher rates under noise conditions, thus increasing signaling effort in the noisy modality. These results point to a complex role of immediate plasticity and longer-term processes in affecting communication during aggressive interactions under anthropogenic noise.Significance StatementHuman activity has an enormous effect on wildlife, including on their social behavior. Animals living in urban areas often tend to be more aggressive than those living in rural areas, which may be due to urban acoustic noise making communication between individuals more difficult. In a study with a common songbird, the European robin, we investigated the role of urban acoustic noise in aggression and territorial communication. Urban robins were more aggressive than rural robins, and additional noise in the territory increased aggression in rural but not urban robins. Robins increased their singing effort but did not increase visual signals in acoustic noise. These results suggest that noise can indeed make animals behave more aggressively although the effect may depend on how noisy it is already. These results further our understanding of how human-made noise changes animal communication and social behavior.

Anthropogenic noise may disrupt signals used to mediate aggressive interactions, leading to more physical aggression between opponents. One solution to this problem is to switch signaling effort to a less noisy modality (e.g., the visual modality). In the present study we investigate aggressive behaviors and signaling in urban and rural male European robins (Erithacus rubecula) in response to simulated intrusions with or without experimental noise.First, we predicted that urban birds, living in noisier habitats, would be generally more aggressive than rural birds. We also predicted that during simulated intrusions with experimental noise, robins would increase their physical aggression and show a multi-modal shift, i.e., respond with more visual threat displays and sing fewer songs. Finally, we expected the multi-modal shift in response to noise to be stronger in urban birds compared to rural birds. The results showed that urban birds were more aggressive than rural robins, but an increase in aggression with experimental noise was seen only in the rural birds. Urban but not rural birds decreased their song rate in response to noise. Contrary to the multi-modal shift hypothesis, however, there was no evidence of a concurrent increase in visual signals. These results point to a complex role of immediate plasticity and longer-term processes in affecting communication during aggressive interactions under anthropogenic noise.

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Anthropogenic noise interrupts the acoustic communication between animals living in urban habitats. Accumulating evidence suggests that animals can evade this interruption using various strategies such as shifting frequencies upwards or increasing the duration of their signals. In this study, we compared the time and frequency-related characteristics of songs and rain calls of common chaffinches (Fringilla coelebs) inhabiting rural forests and an urban park in Turkey. Most of the song phrases and rain calls did not differ in any of the characteristics measured between urban and rural chaffinches. Terminal flourish phrases of the songs, however, had lower minimum frequencies and broader bandwidth in urban territories, contrary to our predictions. We discuss this finding in relation to its potential adaptive significance.

Animals can evade interference from anthropogenic noise using strategies such as shifting frequencies upwards or increasing the duration of their signals. In this study, we compared the time and frequency-related characteristics of songs and rain calls of Common Chaffinches Fringilla coelebs inhabiting rural forests and an urban park in Turkey. Most of the song phrases and rain calls did not differ in any of the characteristics measured between urban and rural Chaffinches but, contrary to our predictions, terminal flourish phrases of the songs had lower minimum frequencies and broader bandwidth in urban territories. These results suggest either that noise has limited effect on the Chaffinch vocalizations or that Chaffinch vocalizations in urban habitats are less well adapted for efficient transmission.