New paper out in @AnimBehSociety with @meelyn_pandit, @JuncoProject, @ketterso, @mostly_waxbills, and Trevor Price. Higher frequency songs of urban juncos persist in a common garden for three years! Read on for the details. Article is free for 50 days! https://authors.elsevier.com/a/1c00CmjLu8nt 

Some background - Dark-eyed Juncos colonized the campus of UCSD in the early 1980s. Previous work by my coauthors and others has shown how these birds rapidly evolved in their novel urban environment. One of the changes was an increases in minimum song frequency (pitch).

Fast forward to 2007, @JuncoProject, @mostly_waxbills, and others caught 40 juvenile juncos at UCSD and 40 from a quiet neighboring population at Mt. Laguna and shipped them by air freight to @iu_eeb for a common garden experiment.

Common garden experiments are a powerful method for determining the cause of population differences. If the difference is induced by each local environment, it will disappear in a common garden. Differences that persist are driven by genetic and/or cultural divergence.

After THREE YEARS, we recorded songs from both captive populations (urban, mountain) and measured their frequencies. To our surprise, the frequency difference observed in the field PERSISTED, undiminished, in the quiet common garden! Urban birds still sang higher pitched songs.

This result indicates that unlike the white-crowned sparrows in San Francisco, which reduced their frequency when noise declined, the higher songs of urban juncos are not a plastic adjustment to noise and may be permanently shifted. https://science.sciencemag.org/content/370/6516/575

Now for caveats. The juncos in our study experienced ~25-40 days of life in their natal populations before capture. Thus, early exposure to noise and song learning from adults may have contributed to the differences in the common garden...

However, songs produced in the common garden that were shared with field recordings from both natal populations, and thus were likely to have been learned before capture, did not differ in minimum frequency from non-shared songs that likely developed later in the common garden...

This outcome suggests that early learning before capture cannot fully explain the differences we observed. Instead we argue that these two populations may be genetically distinct and predisposed to learn and/or sing songs at different frequencies.

Any genetic divergence could be specific to genes affecting the song production apparatus, auditory sensitivities, and/or the song template that guides learning preferences. Early exposure to noise could also permanently alter gene expression. Lots of unanswered questions!

The recent WCSP study mentioned above provided much needed hope that some noise-induced changes can easily be reversed, but our study provides an important counterpoint. Plasticity is an immediate escape from urbanization, but it is one of many potential outcomes.