Birdsongs keep pace with city life: changes in song over time in an urban songbird affects communication

Animals in urban environments often must adjust their vocalizations to be heard over the din of anthropogenic ambient noise. Most studies of this process use current comparisons of environments that vary in ambient noise; here, we evaluate over time the effects of the urban environment on signal structure and salience. We show that white-crowned sparrow, Zonotrichia leucophrys, song increased in minimum frequency between 1969 and 2005 in San Francisco, California, U.S.A., coincident with rising levels of traffic noise. A higher minimum frequency is less likely to be masked by low-frequency ambient noise and thus is more likely to be effective as a communication signal. To test whether this change in song affects receiver response, we presented territorial males current (2005) and historical (1969) local songs. Males responded more strongly to current than to historical songs, suggesting that current songs communicate more effectively in the current local environment. The results suggest that behavioural adjustment to anthropogenic ambient noise over time can affect cultural evolution and communication.