Song Sparrow / USFWS Photo
When I hear urban Song Sparrows singing, I often feel that their songs are missing something. It turns out that there is a very good reason for that. The lower pitches in their songs may be drowned out by ambient noise. In fact, some urban birds have mostly abandoned the lower pitches to expend more effort on the higher ones.
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).The link has an audio sample and spectrographs that illustrate the Song Sparrows' challenge quite well.
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.