A coo story: Both oscine and non-oscine birds adjust their songs in noisy cities
Urbanization is a growing cause of habitat loss, a major driver of global biodiversity decline. Urban habitats differ from the habitats they replace, altering the ecological and selective environment for urban dwelling species. Low frequency traffic noise in cities potentially masks animal communication signals, interfering with important functions such as mate choice and territory defense, possibly decreasing fitness. To avoid signal masking, species might adjust their vocal signals to higher frequencies. We tested this hypothesis by examining the relationship between background noise and the frequency of bird songs. We recorded songs of five bird species along a noise/urbanization gradient in Hong Kong and predicted that species with low frequency songs most likely to be masked by traffic noise would increase their song frequency in noisy areas. It is thought that learning allows songbirds to make rapid adjustments to their songs in response to changing noise environments; it is not clear whether species which acquire their songs innately can also adjust their songs in response to noise. We included a non-learning species with low frequency songs (spotted doves Streptopila chinensis) in our study to test the hypothesis that non-learning species also increase the frequency of their signals in response to noise.
We found a strong correlation between background noise and minimum frequency, with four of the five species singing at higher minimum frequencies in noisier areas. The species which did not adjust its song sings at frequencies higher than the frequency of background noise, and thus presumably was not under selection pressure to adjust its song. These results support the hypothesis that birds adjust their songs in noisy areas to avoid signal masking. Spotted doves, which do not learn their vocalizations, also used coos with higher minimum frequencies in noisier areas, suggesting that signal adjustments in response to changing environmental conditions do not necessarily require learning. Spotted doves also used shorter, simpler coos in noisier areas, another potential adaptation to increase signal efficacy in noisy conditions. The consequences of such song modifications are not yet well understood and future studies will focus on the fitness effects of these urban dialects. Understanding which species are able to adjust to noisy city environments is important for maintaining biodiversity in an urbanizing world.