White-crowned sparrow song learning

Doug Nelson is studying several aspects of vocal development in the white-crowned sparrow. This species has been studied intensively for over 30 years now, because of the fascinating degree of variation in its song in space. Peter Marler's classic studies demonstrated that males form local song dialects, in which neighboring males sing very similar songs which differ from the songs of males recorded several kilometers away. He also demonstrated that young males learn their songs, so that songs are passed from one generation to the next in a system of cultural inheritance. Males readily imitate tape-recorded songs in the laboratory, making them ideal subjects for the study of song development. Adult males sing a single song type, but prior to "crystallizing" this song juveniles often sing 2-8 different song types. Nelson's current research focuses on different aspects of this "song overproduction" process.

1) Comparative studies of vocal learning--The five recognized subspecies of white-crowned sparrow differ in several aspects of their natural history that might influence how and when songs are learned (Table). Results from laboratory studies of song learning by hand-reared males, conducted in collaboration with Peter Marler, have been published in Animal Behaviour in 1995 and 1996.

Males of the migratory Puget Sound and Mountain subspecies learn more tutor song types than do males of the sedentary nuttalli race. The great majority of song learning is concentrated in a senstive period in the first 2-3 months of life. Mountain white-crowned sparrows have a significantly shorter and earlier sensitive phase than Nuttall's males. We suggest that vocal dialects in the migratory subspecies result not from yearling males imitating adult males in their first breeing season, but from a process of selective attrition of song types during matched countersinging. In contrast, males of the sedentary nuttalli subspecies may rely more upon imitating the song of the neighbor they settle next to after dispersing. These differing processes of vocal learning may have consequences on the pattern of geographic variation observed in wild populations.

 Table

 Nelson is currently studying song learning in Gambel's white-crowned sparrow. This subspecies is interesting because, in contrast to the first three subspecies listed in the Table, males do not form local song dialects. Instead, there are several different song types in a given location, and neighboring males do not share similar songs. He is currently studying a group of hand-reared males to discover how they may differ from the first 3 subspecies in the first year of vocal development.

2) Selective attrition of songs and matched counter-singing

Male sparrows learn more songs ("overproduce") than they need for their final repertoire. In the early stages of practice singing, termed plastic song, hand-reared males of some subspecies may sing up to 8 different imitations of different tutor songs. They then gradually discard all but the one song type they retain for the rest of their lives as their "crystallized" song. This attrition process is selective--it is guided by auditory input during the plastic song stage. If a male hears playback from a loudspeaker of one song type that matches one of the songs in his overproduced repertoire, he will retain that song as his crystallized song, and discard the other song types. Males presented with playback of a novel song did not learn it, but rather crystallized one of the songs they had learned 6-9 months earlier during the sensitive phase. We suggest that instances of song change in yearlings and adults that have been assumed to reflect memorization of novel songs, and hence, an open-ended sensitive phase, may in fact be the result of overproduction of songs memorized early in life, followed by selective attrition due to matched counter-singing.

There are several reports of overproduction and selective attrition of songs in wild populations of sparrows. Nelson is initiating a detailed study of the Puget Sound white-crowned sparrow to determine whether this behavior commonly occurs in wild birds settling upon territories for the first time.

3) A new assay for vocal learning

Fledgling sparrows, hand-reared in the laboratory, can recognize their own species' song with little or no prior experience with adult song when first presented with it in playback tests. Males and females give more "chirp" calls to playback of white-crowned sparrow songs than to the songs of other species. We have recently extended this test by tutoring males and females for five 10-day-long periods, and then conducting playback tests after each period. Males called more to tutor songs than to novel playback songs after the first 3 periods, but females did so only after the first 2 periods. Neither sex discriminated between novel and tutor songs presented in the winter, outside of the normal sensitive period for song learning. Males called more to tutor songs they subsequently imitated the next spring when they came into song. Thus, the pattern of calling when young predicted which songs were held in long-term memory for song production. This work is in press in Animal Behaviour.

Future studies using this assay will explore how songs are represented in memory during the 6 month storage interval that intervenes between when songs are memorized during the sensitive phase and when they are subsequently reproduced. Also, females hand-reared in the laboratory do not sing (usually), and so do not inform us whether they have learned song. We hope to use an alternative bioassay for song learning in adult females, the copulation solicitation assay, to establish whether the juvenile preferences for tutor songs displayed by chirp calls correlates with preferences for songs in adult females.

4) Geographic variation

One common consequence of vocal learning is the formation of geographic song dialects. The sonagrams to the left were sampled from mountain white-crowned sparrow populations in the Sierra Nevada of California, approximately 200 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 (Table). 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, Nelson recorded male oriantha in 1995 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.