Urban noise and male density affect songs in the great tit Parus major

Since ambient noise interferes with sound transmission, urban noise can affect the acoustic structure of birdsong. Previous studies have examined song features (e.g. high minimum frequency) in urbanised areas; however, these studies did not exclude the effects of other factors. For example, the intensity of male-male competition is known to affect song structure. In 22 urban parks in Tokyo, Japan, we measured the effects of noise level and male density, an indicator of the intensity of competition among males, and examined the relationship between these factors (noise and male density) and song structure of the great tit, Parus major. We found that males in noisier parks sang songs with higher minimum frequencies, more phrases and longer durations. The frequency shift appeared to mitigate the acoustic masking of songs by low-frequency background noise, as has been shown in previous studies. Songs with repeated phrases and of longer duration are likely to be more detectable for receivers under noisy conditions. In addition, we found that male density tended to affect the minimum frequency and the number of phrases in, but not the lengths of, their songs. Overall, both noise and male density affected the song structure of great tits in urban habitats.