Vocal dialect and genetic subdivisions along a geographic gradient in the orange-tufted sunbird

At least four hypotheses have been suggested to explain the formation and maintenance of song dialects among birds: historic processes (epiphenomenon), genetic or local adaptation, acoustic adaptation, and social adaptation. We studied spatial and temporal distribution of dialect in the orange-tufted sunbird (Nectarinia osea), a small nectarivorous bird that expanded its breeding range in Israel during the past 100 years from the southern part of Rift Valley to the entire country. Sunbird range expansion was concurrent with the establishment of many small settlements with an ethos of gardening, which introduced many ornithophilous plants. We recorded songs and genetically screened individual sunbirds in 29 settlements distributed across a 380 km north-south gradient along the Rift Valley. We show that dialects cluster together into geographical regions in 70% of cases, a moderate concurrence to geography. Settlement establishment date, geographical position, and genetic distance between local populations (i. e., settlements) were all poor predictors for the variance among song dialects. The specific effect of habitat was not tested because all sampled localities were similar in their physical and acoustic properties. Using a network analysis, we show that dialects seem to aggregate into several network communities, which clustered settlement populations from several regions. Our results are best explained by either the epiphenomenon hypothesis or the social adaptation hypothesis, but at present our data cannot state unequivocally which of these hypotheses is better supported. Last, we discovered a negative association between network centrality and genetic diversity, a pattern that requires further examination in other systems.