Bird song variations along an urban gradient: The case of the European blackbird ( Turdus merula)

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Species that use vocal communication have difficulties in transmitting their messages in noisy habitats. The capacity to adapt songs to prevent them from being masked by noise could be a key factor for reproductive success in such environments. Previous research has demonstrated changes in several song parameters between rural and urban areas. In the present study we were prompted to check vocal adjustment along an urban gradient. The survey was carried out in the city of Salamanca (Spain), which owing to its small size has a strong physical gradient over a short distance. We recorded the songs of 27 common blackbirds ( Turdus merula) and ambient noise at 13 sites. The choice of sites corresponds to different areas along this gradient: urban, periurban and rural zones. Anthropogenic noise increased from the rural to urban recording sites. Differences in vocalizations were found in both the maximum and minimum frequencies among the three areas. The blackbird can shift its entire vocalization to avoid masking by ambient noise, which occurs mainly at low frequencies. Song frequencies showed a graded response to ambient noise. Urban gradients in small cities are steep and strong, but the degree of isolation is low. In this context, vocal adjustment is more likely to be possible by means of behavioural plasticity although environmental pressure could lead to genetic changes in more isolated areas. Other changes along the urban gradient modify both ecology and population parameters of the blackbirds and could also be another possible explanation for song variation.