Fig. 1

Map depicting Saltmarsh and Nelson’s Sparrow sampling locations and admixture plot from STRUCTURE. The map on the right has orange dots as allopatric Saltmarsh Sparrow sampling locations, while blue dots are allopatric Nelson’s Sparrow sampling locations. White stars on the map indicate coastal and inland sites where this study was conducted, while white dots indicate previous study locations (not included in the admixture plot) that provided broad scale comparisons to our two sites as representative of the southern hybrid zone for this study. The hybrid zone extent is shown shaded grey on the map. The admixture plot on the left depicts the central inland and coastal study locations and allopatric populations, where each vertical bar represents the genetic makeup of an individual, blue representing the Nelson’s Sparrow alleles, and orange representing the Saltmarsh Sparrow alleles. Sparrows of pure ancestry have a bar of a solid color, while sparrows of mixed ancestry have bars comprised of both colors