Genetic Sample Preparation
Our final dataset (n = 29) included five S. v. virenssamples from New York that were sequenced in a previous study (Baiz, Wood, Brelsford, Lovette, & Toews, 2021). These samples were prepared with an identical method but sequenced separately from the new samples included in the current study (n = 24). For phylogenetic analyses we also included a single golden-cheeked warbler (Setophaga chrysoparia ) as an outgroup, also published in Baiz et al., (2021). Genomic DNA was extracted from blood using the Qiagen DNeasy Blood and Tissue kit following the nucleated blood spin-column protocol with these minor modifications: we used 75 μL of blood and the 56°C incubation was performed overnight. All samples were normalized to 2ng/μL and DNA was sheared with a Covaris S220 to a target insert size of 350bp, according to the TruSeq Nano protocol. We then prepared sequencing libraries using the Illumina TruSeq Nano DNA kit and submitted them to the Pennsylvania State University Genomics Core Facility for sequencing (150nt, paired-end) on a single NextSeq High Output lane.