Time alters urban singing strategies in a North American songbird

Local habitats shape animal vocalizations through selection to improve transmission of signals to receivers. This process can be variable, however, when landscapes are changing continuously due to urbanization or other factors. Studies have shown that some birds alter the frequency, amplitude and structure of their songs in urban habitats. Because songbirds learn their songs from other individuals, this cultural transmission can result in substantial change over time. Urban bird populations may therefore show rapid shifts in song form and variation may result from the combined or interacting effects of time and anthropogenic habitat change. In this study, we used historic and recent song recordings to investigate changes in spotted towhee (Pipilo maculatus) songs over 45 years across an urban-to-rural gradient in northern California. We found that pre-trill and trilled song parts covaried differently with urban development at the two time-points, with more change occurring in trill phrases than pre-trills. Additionally, we found that birds in 1970 adjusted trills in urban areas by raising maximum frequencies and broadening bandwidths, while birds in 2015 narrowed song bandwidths by decreasing maximum frequencies in more urban areas. These results did not fit our prediction that urbanization would have a consistent effect on song at two time-points. We suggest that habitat, cultural evolution and cultural drift can act on song elements in complicated ways that vary over time.