How Do Sparrow Songs Evolve?

WILLIAMSTOWN -- Cultural evolution is familiar to anyone who has followed how fashions change from year to year or season to season.

What distinguishes this type of evolution is that cultural traits, such as bird songs, are learned - and so are not restricted to parent-offspring transmission via DNA. In a recent paper in the scientific journal Animal Behaviour, Heather Williams, William Dwight Whitney Professor of Biology at Williams College, and her co-authors (Iris I. Levin of the University of Missouri, Ryan Norris and Amy Newman of the University of Guelph, and Nat Wheelwright of Bowdoin College) examine how and why Savannah sparrow songs evolve over a period of 30 years.

As is the case for many songbirds, only the male Savannah sparrow sings.

Darwin predicted that evolution of a trait present only in males is driven by sexual selection - in the form of female choice (females mating with males who have sexier songs) or male competition (males intimidating other males with a more impressive song). The relative fitness of the song a male chooses to learn and sing can be assessed by measuring the number of offspring each male sires, and thus cultural evolution can be compared to fitness.

Williams and her co-authors tracked the males’ songs over three decades and their reproductive success for more than 20 years. Some parts of the song changed little if at all, while other segments

varied randomly over time, and still other song sections changed systematically across generations. The paper concludes that different mechanisms are responsible for the cultural evolution of different parts of the songs. The introductory notes and "buzz" section were stable and most probably denote species and population identity, cultural attributes that did not change over time. The changes in the middle segment of the song were not associated with fitness, and "drifted," or changed randomly, in a way reminiscent of the changes in fashions for baby names=8Bsuggesting that this part of the song might serve to identify individuals. Soft click-like notes between the introductory notes were repeated more often by males in successive generations; these changes were associated with fitness and appeared to be under directional selection.

The authors suggest that only "vocal virtuoso" males were able sing several clicks in rapid succession, and that doing so demonstrates fitness to listeners. Williams, who teaches Animal Behavior, Animal Communication and Neuroscience, is interested in the learning and organization of bird song and the brain circuitry of birds. She received her B. A. from Bowdoin College in 1977 and her Ph. D. in neuroscience from Rockefeller University in 1985.

She did postdoctoral work at Rockefeller University’s Field Research Center before joining the Williams faculty in 1988.