Geographical Variation in Song and Song Dialects of Montane White-Crowned Sparrows

Abstract

-We sampled the songs of 18 populations of montane Whitecrowned Sparrows (Zonotrichia leucophrys oriantha) in order to define their dialect groups, if any, and to explore vocal affinities with other western subspecies of Z. leucophrys. We found a clear-cut regional differentiation of song primarily on the basis of syllabic morphology and secondarily on the sequence of elements in the song. The birds of the Sierra Nevada and the San Bernardino Mountains of California constitute a fairly homogeneous dialect group related to another distinct group in the Warner Mountains, California, which are separated from the Sierra Nevada to the north by habitat unsuited to breeding oriantha. Those in two nearby but isolated desert ranges share a unique song type resembling that of Z. 1. gambelii. Oriantha in the Wallowa Mountains, Oregon, to the northeast of the Steens Mountains, are allied with the dialect region of the northern Rocky Mountains. Syllabic morphology and the sequence of song elements also suggest certain vocal affinities of oriantha with other western subspecies of Z. leucophrys. For instance, the songs of oriantha in the Sierra Nevada, Warner Mountains, and San Bernardino Mountains have elements in common with those of nuttalli in central California and pugetensis north of the Columbia River. The songs of oriantha at Hart Mountain and in the Steens Mountains are very similar to those of gambelii in Alaska and the western Canadian Rockies. The songs of oriantha in the San Bernardino Mountains (a population founded after 1907) are identical with those of the central Sierra Nevada, and thus trace the origin of the founding group. We postulate that these and other data are consistent with Rand's interpretation of the subspecific differentiation of these sparrows in Pleistocene refugia. Local sharing of song themes or parts of themes has been described in many avian taxa (e.g., Thielcke 1969, Marler and Mundinger 1971, Lemon 1975), and produces what are commonly called local dialects. The degree of sharing may differ between subspecies or populations within a subspecies (Williams and MacRoberts 1977, 1978, Eberhardt and Baptista 1977). Before we can hope to assess why song dialects develop in some populations and not in others and the adaptive significance of dialect formation, we must continue to describe dialect systems across phylogenetic groups and ecological types (Avery and Oring 1977). The songs of the crowned sparrows (Zonotrichia spp.) are the most thoroughly studied of any group (reviews in King 1972, Nottebohm 1975, Baptista 1977). Theme sharing in White-crowned Sparrows (Z. leucophrys) appears to be absent in the migratory subspecies gambelii (DeWolfe et al. 1974), but shared themes (dialects) range over large areas in the migratory form pugetensis (Baptista 1974, 1977) and the montane breeding populations of oriantha in Colorado (Baker 1975). Both of these forms appear to occupy more or less continuous habitat. Song dialects are very local in the sedentary subspecies nuttalli (Marler and Tamura 1962, Baker 1975, Baptista 1975), and in populations of oriantha in the Sierra Nevada of California (Orejuela and Morton 1975). The highly localized dialect system in nuttalli may result in part from the relative sedentariness of this subspecies (Blanchard 1941), whereas the local dialects in migratory Sierran oriantha may be attributed to patchiness of suitable habitat, each isolated meadow being an ecological island (Baker 1975, Orejuela and Morton 1975). Between 1970 and 1978, we sampled songs of montane White-crowned Sparrows from 12 populations (Fig. 1, Table 1). Colleagues provided recordings from six additional localities. Two populations (Tioga Pass, Mount Lassen) were sampled earlier by Orejuela and Morton (1975), and allow