The year was 1970. Simon and Garfunkel topped the charts, floppy disks were brand-new, and California white-crowned sparrows (Zonotrichia leucophrys) sang fast machine-gun trills. Just a few decades later, the sparrows sing noticeably slower songs, and a new study reveals the reason. The birds' habitat has gotten scrubbier, and their melodies have evolved to better penetrate the thickets.
Ecologists have argued for decades that habitat influences the evolution of bird song. Slow songs and low-pitched sounds transmit better through dense vegetation, whereas high notes carry farther in open environments. And overall, grassland birds do have faster, shriller songs than those from leafy surroundings have. But researchers discovered this by comparing modern populations or species from different habitats. They didn't know how long it would take a species' song to adapt to a new environment.
Now they do, thanks to a collection of historical recordings of white-crowned sparrows. Evolutionary ecologist Elizabeth Derryberry, now of Louisiana State University Museum of Natural Science in Baton Rouge, knew that, beginning in the late 1960s, renowned ornithologist Luis Baptista had spent decades studying the birds' chattery, buzzy songs. But she couldn't find his tapes. Finally, in 2003, 3 years after Baptista's death, Derryberry tracked down the sparrow songs in the professor's old office at the California Academy of Sciences in San Francisco. Among the boxes and reprints, she uncovered the songs of 170 male sparrows from 15 locations on the West Coast.
Thirty-five years after Baptista's visits, Derryberry embarked on a monthlong road trip to the same sites. She found that today's white-crowned sparrows sing the same basic song as their ancestors did, but the trill at the end is lower pitched and noticeably slower--on average only 10.3 trills per second instead of the historical 11.8. That difference was big enough for the sparrows to notice: The birds reacted more strongly to recordings of modern songs than they did to old ones, with males moving toward the sound and females doing a little dance. Derryberry wondered if a more densely vegetated habitat might be responsible for the decelerating trill.
Finding out meant tracking down another set of historical data: aerial photographs of the birds' habitats. Derryberry reports in the July issue of The American Naturalist that she found historic and modern photos for five of the 15 recording sites, and they showed that the birds' habitat had gone from being mostly grassy, with about 11% scrub, to 26% scrub. The slower modern songs would better penetrate the new scrubby habitat, which Derryberry attributes to reduced livestock grazing.
That bird song can adapt so rapidly to habitat change is "an extremely exciting finding," says Manuel Leal, an ecologist at Duke University in Durham, North Carolina. Ecologist Matthew Betts of Oregon State University in Corvallis adds that "it's something people have expected should happen, but the data really haven't been there" until now.
Whether the changes in the sparrows' songs were cultural or genetic is still an open question. Betts also wonders whether other bird species would adjust their songs as quickly. These questions may not be out of reach. It took Derryberry 6 years to assemble her epic historical data set, but many more people are recording bird song now than 30 years ago, Betts says. He expects to see a bloom of similar studies.