As it turns out, white-crowned sparrows have their own dialects, their own dictionary of regional sparrow, if you will. In the 1960's, researchers in the San Francisco Bay Area discovered differences in the songs of white-crowned sparrows. Based on neighborhood, the sparrows had markedly different songs, and held a fidelity to those areas with their dialect. Young white-crowned sparrows do not learn directly from parents but rather from the general acoustic environment where they are raised, the researchers later proved. Thus, because the sparrows learn from their surroundings and because they have a restricted geographic range within the city, dialects form.
The content of the white-crowned sparrow's song carries with it other messages beyond the varying spectograms (pictures of sound) of the San Francisco Bay Area. In Colorado, researchers found that white crowned males that are infected with blood parasites will have different songs with fewer trill notes than uninfected males. These parasites can reduce brood success by 15-20%, and thus females can determine which mates will be successful based on the fitness of their song.
Back in San Francisco, researchers have followed up studies from the 1960's with current data on song dialects in white-crowned sparrows, with surprising results. One of the dialects has vanished. Even more surprising was the likely reason: traffic. The San Francisco dialect, with its highest minimum frequency, was able to out-compete other dialects. Those birds with the San Fran dialect were singing their high pitched song over the bustle of a growing city with ever-increasing traffic loads, and successfully attracting a mate because of it.