Geographic variation

Click on the songs to hear them (6 kB RealAudio files)

One common consequence of vocal learning is the formation of vocal dialects. These two songs were recorded from Puget Sound white-crowned sparrow populations along the Washington coast approximately 40 km apart.

White-crowned sparrow populations differ in whether or not neighboring males sing similar songs. The size of the vocal dialect that results is determined in part by the amount of suitable habitat. We are currently comparing patterns of song sharing in local populations of Nuttall's, Mountain and Gambel's white-crowns to test Nelson et al.'s (1995) prediction that neighbor-neighbor song sharing should be more pronounced in sedentary nuttalli. They suggested that the longer sensitive phase in nuttalli would enable males to imitate the neighbor they settle next to in their first calendar year, while sharing in migratory populations would be only approximate because males do not settle on territories until their second calendar year, after the sensitive phase is closed. If migratory males visited, and learned, the local dialect in their first summer, they would be able choose that song during selective attrition and conform to the local dialect, but the probability is small that they would be able to settle next to the same male they imitated the year before.

Using the Mantel test to examine several hypotheses about geographic patterns in song sharing, we have found that neighboring male nuttalli at Bodega Bay do have more similar songs than would be expected by chance. In contrast, neighboring males in migratory oriantha at Sonora Pass and Tioga Pass, and migratory gambelii at Churchill, Manitoba do not have more similar songs than do non-neighbors in the local population. These results agree with the predictions outlined above.

In another project, Heidi Harbison and Tom Hahn of Princeton University and Nelson recorded male oriantha in 1995/1996 in several of the Sierra Nevada populations recorded by Orejuela & Morton in 1969 and 1970. Songs remained remarkably stable over the 25 period in the larger dialects. This work was made possible because Orejuela and Morton archived their recordings in the collection of the Florida Museum of Natural History. All bioacousticians should do the same, and ensure that their recordings are archived in an institution that can properly care for them.