The changing natural world at our doorsteps | Illustration and text by Patterson Clark
April 20, 2010
Song sparrows adjust their songs to fit in with urban noise
In a community garden beneath the flight path of jets roaring out of Reagan National, a song sparrow repeatedly shouts its claim to territory and enticement to mates. A nearby leaf blower joins the chorus. Song sparrow songs occupy a prominent spot in the city's spring soundscape, adding bright icing to a thick cake of background noise.
"Suitable breeding habitat is not simply comprised of space and food, but also an auditory opening," says Stephen Yezerinac, an ornithologist at Bishop's University in Quebec. "Humans are altering all of the above in urbanized areas, and birds are being affected."
While at Reed College in Oregon, Yezerinac and student William E. Wood examined the effects of urban noise on the song sparrow songs of Portland. They recorded 28 birds in areas with various levels of background noise, carefully measuring frequency (pitch) and amplitude (volume; the amount of energy invested in making a sound).
Birds in the noisiest spots were more likely to cede some of their lower frequencies to background noise, while their songs' higher-frequency notes remained constant.
Listen to a local song sparrow compete with an airliner and a leaf blower:
SOURCE: "Song sparrow (Melospiza melodia) song varies with urban noise," The Auk, 2006
A high-pitched future?
Song sparrows are notable for numerous regional variations in plumage, size and song. Individual birds seem to embody that plasticity with a repertoire of five to 13 songs.
Wood and Yezerinac suggest that urban sparrows may be evolving higher-pitched songs. Many factors are at work:
- ♦ Birds may be choosing to sing songs that contain higher notes, or are raising pitches to stand out above the noise.
- ♦ The regions of the birds' brains associated with song undergo partial renewal each spring, which may cement into the bird's head certain songs tailored for a noisy territory.
- ♦ Birds may learn modified songs, or may not be able to hear low notes, or they may simply drop low notes that aren't effective.
- ♦ Females may be more likely to choose males who sing in higher frequencies, and are known to select males that show a proficiency for learning.
- ♦ If birds sing more loudly in response to noise, they will use more energy, which could lead to either shorter, more efficient songs or diminished vigor.