Variation in song structure along an elevation gradient in a resident songbird

Abstract

Heterogeneous environments can create differential selection pressures among populations, which may result in the evolution of local adaptations. Premating isolation mechanisms may emerge due to limited movement among locally adapted individuals, thus further enhancing such adaptations. The song of male songbirds is one potential premating isolation mechanism that has long been of interest because of its implications for mate choice, local adaptation, and speciation associated with environmental heterogeneity. We tested whether locally adapted mountain chickadees exhibit variation in song structure along an elevation gradient and whether song structure shifts with the distinct change in winter climate severity around the snowline. These birds exhibit large elevation-related differences in numerous behavioral and neurological traits, including variation in spatial cognition associated with fitness consequences. We recorded song along two continuous elevation gradients and detected significant differences among the sampling sites along an elevation gradient and between “high” and “low” elevation songs. Our current results, coupled with previously reported winter climate-related differences are generally consistent with the local adaptation hypothesis explaining geographic variation in song, which suggests that variation in song structure could serve as an indicator of local adaptation. Our study shows that song differences can evolve on a small spatial scale along a continuous species distribution when there is a rapid change in environmental conditions also favoring the evolution of local adaptations.

Significance statement

Local adaptations may evolve when heterogeneous environments result in differential selection on fitness-related traits. If locally adapted populations experience reduced movement, additional traits may also change over time and serve as indicators of origin. The ability to identify and mate with local males, which are likely adapted to the prevailing environment, should procure a fitness advantage such that offspring produced with a local individual will inherit the genetic material that made that individual successful. Temperate male bird song is initially learned near the nest, varies geographically, and is commonly used by females to assess male quality. Using mountain chickadees that inhabit an elevation gradient associated with large phenotypic differences, we found significant variation in song structure along an elevation gradient and that male song also changes significantly at an important ecological threshold, consistent with high- and low-elevation songs that may be used by females to identify and mate with local males.