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.

In this study, we documented for the first time singing on the nest (SOTN) in 74% of 65 Northern Mockingbird (Mimus polyglottos) nests that were monitored with continuous-running video cameras (8,353.9 hr sampled). As predicted, higher rates of SOTN significantly decreased daily survival rates of nests. SOTN occurred almost exclusively by females during the egg stage and in 86% (48 of 56) of nests for which we had sampling from the egg stage. While extensive at the population level, the average rate of SOTN per individual was very low (5.24 ± 1.24 s SOTN per hour of video sampled). We found mixed support for the hypothesis that SOTN functions in territory maintenance. We found no support for the hypotheses that SOTN functions to coordinate parental care, defend nests, or aid in vocal learning. Given the limited attention SOTN has received and the mostly anecdotal accounts of it, our understanding of its costs and benefits is lacking. We conclude that while individual rates of SOTN are quite low, SOTN may be more widespread in populations than previously thought and that studies specifically designed to test hypotheses regarding potential functions are critically needed.

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