SONG DIALECTS AND GENETIC DIFFERENCES IN WHITE-CROWNED SPARROWS (ZONOTRICHIA LEUCOPHRYS).

Abstract

Song dialects have been described in several bird species (Marler and Tamura, 1962; Lemon, 1966; Nottebohm, 1969). Clear examples are found in the resident white-crowned sparrows (Zonotrichia leucophrys nuttalli) of coastal California and in populations of the chingolo (Zonotrichia capensis) of Argentina. Where dialects occur in these species an area of breeding habitat is occupied by territorial males that with few exceptions sing a very similar song. Contiguous and allopatric with this area males of another population sing a different song type. It has been shown (Marler and Tamura, 1964; Marler, 1970) that in the white-crown (Z. 1. nuttalli) the dialect is learned during the first few weeks of life. The contiguity of the populations with discretely different vocalizations could be related to several interesting questions in population genetics, vocal communication, and evolutionary processes. On a general level, however, the dialect phenomenon leads to two central issues: (1) How do dialects arise? and (2) What role, if any, do they play in the biology of the birds? In this paper I provide a speculation on the first question and some data that may clarify the second. Following a suggestion by Marler and Tamura (1962), Nottebohm (1969) proposed that females might preferentially mate with males of their own dialect, thereby constraining a locally adapted gene pool. This hypothesis, that dialects reduce gene migration, came from the observation that Z. capensis on altitudinal gradients in Argentina exhibits a series of dialect populations possibly corresponding to different selective regimes, whereas a single dialect is found over vast areas of lowland pampas where selection may be spatially homogeneous (Nottebohm, 1969; Nottebohm and Selander, 1972). There, are other possible explanations of song dialects in birds. First, song dialects, a consequence of vocal learning, may play no important role. Perhaps during the early learning phase, young birds acquire population specific calls that promote flocking, foraging efficiency, predator avoidance, or individual recognition, that are important for winter survival. Thus, the selective advantage of vocal learning would be explained by winter ecological conditions and not territorial breeding behavior. Second, dialects could maximize outcrossing in small populations. Females would mate with males having a song type unlike that of their early experience. Third, dialects may be a result of intrasexual selection and bear no direct relationship to mate selection. Perhaps vocal convergence bestows some advantage in gaining and holding a territory but females do not use dialect information for mate choice. Using an electrophoretic analysis of alloenzymes, I investigated populations of Nuttall's white-crowned sparrow (Z. 1. nuttalli) in California and populations of the mountain white-crown (Z. 1. oriantha) in Colorado to determine if song dialects were correlated with genetic differentiation.