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 signals, interrupting important fitness-related functions including mate choice and territory defense. Birds are predicted to sing at higher frequencies to avoid such masking, with birds singing at frequencies overlapping the dominant frequencies of background noise predicted to be most likely to change their songs. We tested these predictions by examining the relationship between background noise and bird song frequencies in Hong Kong. We recorded songs of five bird species along an urbanization and noise gradient. We chose species with low, medium and high minimum frequencies and predicted that species with medium frequencies most likely to be masked by traffic noise would sing with higher minimum frequencies in noisy areas. We included a non-learning species with low frequency songs in order to test whether non-learning species are also able to adapt their signals in response to noise.
We found a strong correlation between background noise and minimum frequency in four of the five species, supporting the hypothesis that birds adjust their songs in noisy areas to avoid signal masking. The species which did not shift its frequency in noisy areas was the species with the highest frequency songs. The minimum frequency of this species was higher than the frequency of background noise, and thus presumably was not under selection pressure to adjust its song. Spotted doves, which do not learn their vocalizations, also used higher frequencies in noisier areas, suggesting that signal adjustments in response to changing environmental conditions do not necessarily require learning. In a related study, we found that songs are also affected by high frequency background noise (i.e. cicada choruses), but the results do not provide clear predictions of how birds might alter their songs in the presence of both high and low ambient noise. Future studies will explore this interaction further as well as investigating the fitness consequences of such urban dialects. Understanding which species are able to adjust to noisy city environments is important for maintaining biodiversity in an urbanizing world.