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WebExtra Feature—supporting material for Birding articles that can be found only on the ABA website. When you come across this logo in Birding, visit these archive pages to view the extended article.
January 2011 Birding, News and Notes, WebExtra
by Paul Hess
Urban noise is well known as an agent of change in the structure and style of birdsong, but natural conditions beyond the hubbub of cities also govern variations in vocal characteristics.
For example, in 2003 Timothy Brown and Paul Handford reported contrasting effects of atmospheric conditions in open fields and closed forest areas on the structure and style of Swamp Sparrow and White-throated Sparrow songs (Ibis 145:120–129; see Birding, April 2004, p. 129). Handford additionally explained how acoustic features of various habitats can promote different song dialects in the Rufous-collared Sparrow (Zonotrichia capensis) in South America (Birding, September/October 2005, pp. 510–519).
Elizabeth Derryberry now demonstrates in a series of papers how song variations in differing natural environments and in periods several decades apart may affect crucial aspects of White-crowned Sparrows’ territorial and mating behavior—and perhaps even influence the species’ evolutionary future.
Her results suggest that increased differences among local populations’ songs may become barriers to gene flow among these populations. Conceivably, an eventual result of such barriers could be speciation, although Derryberry does not speculate directly about that potential outcome.
In her initial paper of 2007 (Evolution 61:1938–1945), Derryberry compares White-crowned Sparrows’ responses to songs recorded by the late Luis Baptista at California’s Tioga Pass in 1979 with responses to songs she recorded at the same location in 2003. Acoustic features of these vocalizations in this population of the oriantha subspecies changed during the quarter-century in three ways. The whistled portion decreased in dominant frequency, and the trill decreased in rate and increased in frequency bandwith. Hear these differences in sound files and see them in sound spectrograms (“sonograms”) provided by Derryberry for Birding to accompany this WebExtra.
Current females’ mating response and current males’ territorial response to the contemporary songs differed appreciably from their responses to the historical songs. Females gave more solicitations for copulation during playback of the current songs, indicating a preference for males that sing the contemporary version. Males approached a playback speaker more closely when the current song was played, suggesting a stronger territorial reaction. The old songs were clearly not so meaningful as the new ones.
The second paper in 2009 (American Naturalist 174:24–33) evaluates mechanisms that might drive changes in song. One result of Derryberry’s study associates differences in White-crowned Sparrows’ trills with differences in density of vegetation in the birds’ habitat.
Derryberry compared songs of the oriantha, pugetensis, and nuttalli subspecies recorded at five locations in 1969–1971 with songs she recorded in 2005 at the same locations. Trills in the recent songs were slower and had lower minimum frequencies. Meanwhile, measurements from aerial photographs taken at those locations in approximately the same two time periods showed that scrub vegetation had increased in density. Based on principles of sound transmission, the changes in trill rate and structure were what Derryberry had predicted for optimum transmission of the signal in more densely vegetated surroundings.
Her third paper, published online in 2010 in Biology Letters , explores male White-crowned Sparrows’ response to territorial song variations from both a historical and a geographic perspective. She compared males’ responses to current and historical songs in their local area at Tioga Pass, to current songs in another oriantha population about 50 kilometers north at Sonora Pass, California, and to songs in a population about 600 kilometers away at Manzanita in coastal northwestern Oregon.
Males at Tioga Pass responded most strongly to current local songs. They responded less strongly to songs recorded one to several decades previously in their local area and to current songs at Sonora Pass. They responded least strongly to songs of the distant population in Oregon. The analysis suggests that males’ different levels of response are based on how a song’s frequency, tempo, and duration differ from songs they currently experience in their local population. Once again, the study indicates how differences in birds’ learned songs and corresponding responses in behavior can evolve rapidly over time spans as short as several decades.
Derryberry’s studies of White-crowned Sparrow song continue, this time in collaboration with David Luther, whose recent paper on the effect of urban noise on the species’ song in San Francisco is described in Birding (January 2011, pp. 27–28). Luther compared current urban songs with those Luis Baptista had famously recorded there three decades previously. The recent urban songs were higher in minimum frequency than the historical versions, whereas songs’ frequencies studied by Derryberry in rural areas changed in the opposite direction from songs Baptista had recorded in those same areas.
During the 2010 breeding season, Luther and Derryberry tested how males in San Francisco’s Presidio neighborhood responded to these changes in song. They expect their results to be published soon.
This sound spectrogram (“sonogram”) and corresponding audio files provided to Birding by Elizabeth Derryberry demonstrate how White-crowned Sparrows’ songs have evolved between 1979 and 2003 in a population at Tioga Pass, California.
The sonogram compares a song recorded by Luis Baptista at that location in 1979 with a song recorded there by Elizabeth Derryberry in 2003. The first segment is a whistle, and the last segment is a trill. Note in the sonogram how the whistle has decreased in frequency and how the trill has both decreased in rate and increased in bandwidth between 1979 and 2003. The two sound files bring these changes to life, and the difference is clearly evident.
song recorded in 1979 at Tioga Pass, California.
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song recorded in 2003, also at Tioga Pass.
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Here are graphical depictions (“sonograms”) of the two sound records: