Understanding the patterns and processes driving population structure is crucial for conservation planning. Spatial conservation units (CUs) for one or more species have been defined using similarities in communities, habitats, and resource management concerns. Additionally, CU boundaries can be informed by population genetic structure. Here, we determined the population structure of the Bobolink (Dolichonyx oryzivorus), a migratory grassland bird. Its breeding range spans longitudinally across most of North America in the northern U.S. and southern Canada, and its population has declined by 59% from 1971 to 2014. We sampled blood from Bobolinks at seven breeding sites and used a ddRAD sequencing approach to identify 3236 SNPs for population genetic analyses. Using the Bayesian clustering approach, STRUCTURE, we found low levels of genetic differentiation across the breeding range. Fₛₜ values ranged from 0.002 to 0.036 among all population pairs, and genetic structure followed an isolation by distance model. Despite low levels of genetic differentiation, we found evidence for four genetic groupings—breeding populations in Oregon and British Columbia were distinct from each other as well as from Central and eastern North American breeding populations. Investigating the demographic history of the populations using approximate Bayesian computation, we found evidence that western Bobolink populations are not relict populations but instead were founded during a recent westward range expansion, possibly enabled by agricultural expansion. We identify four genetics-based CUs that may serve as a complementary spatial framework, broader in scale compared to the commonly-used Bird Conservation Regions, for defining and achieving population objectives.
