Song sparrows fail to discriminate between current and historical songs

Song evolves rapidly in songbirds, as has been proven for a substantial number of songbird species by demonstrating acoustic differences between current songs and "historical" songs recorded 20 or 30 years previously. In two species of songbirds, white-crowned sparrows and savannah sparrows, it has been further shown that evolutionary changes over such time spans are sufficient to affect the response of receivers, with territorial males responding more aggressively to current songs than to historical ones. These two species, however, have especially low population variability in song, with most males in any population singing the same, single song type; this background of song uniformity makes it especially easy to discern temporal changes. Here we examine response to temporal change in song in a third sparrow species, the song sparrow, in which population variation in song is much higher: males sing 5-13 song types each, with low song-sharing between males, so that hundreds of distinct song types occur in a local population. We find evidence that temporal change has occurred in the songs of our study population, in that 24 song current song types share more introductory phrases with other current song types than do 24 historical song types recorded 27-29 years earlier. Nevertheless, in a song playback experiment, current males showed no difference in response to current and historical songs. The results are in accord with the hypothesis that high levels of population variability in song make temporal changes in song difficult to discern. Significance statementThe song of songbirds is shaped by social learning, making birdsong a model system for the study of cultural evolution. Because cultural evolution is thought to occur more rapidly than genetic evolution, many studies have examined changes in song over time periods thought too short to allow measurable genetic evolution. Here we show that song has evolved in our study population of song sparrows, with the relative abundance of introductory phrases changing over a period of just 27-29 years. We find, however, that the song sparrows themselves fail to discern these changes, responding just as strongly to the old ("historical") song types as to current ones. Measurable cultural evolution thus occurs but without functional consequences.