A behavioural mechanism explaining noise-dependent frequency use in urban birdsong

Acoustic signals are usually very effective in long-distance communication. However, in many habitats animals suffer more and more from signal interference caused by traffic-generated low-frequency noise. Recent observations suggest that birds are able to change the pitch of their song to reduce masking interference, but we still lack experimental evidence. Theoretically, some bird species, when confronted with increased noise levels, may be able to switch to song types in their repertoire with higher frequencies. We tested this hypothesis in the great tit, Parus major, by exposing singing males to low-frequency ‘city’ noise in their natural territories and comparing frequency characteristics of songs before and after song type switching. We also exposed birds to high-frequency, ‘inverse’ city noise, as well as to white noise as a control. Great tits adjusted temporal switching behaviour in response to noise exposure. Song types that were less masked by the noise treatment were sung for longer durations. As a result, all five birds that switched during the low-frequency noise treatment switched to song types with a higher minimum frequency. Similarly, seven of nine birds that switched while exposed to high-frequency noise switched to song types with lower maximum frequencies. These results provide experimental evidence for a short-term behavioural mechanism explaining noise-dependent frequency use in birdsong.