Bird data
We collated data for 6 North American migratory species for which analysis has been previously conducted to identify genetically distinct populations: Willow Flycatcher (Empidonax traillii ; WIFL), Yellow Warbler (Setophaga petechia ; YEWA), Wilson’s Warbler (Cardellina pusilla ; WIWA), Common Yellowthroat (Geothlypis trichas ; COYE), American Redstart (Setophaga ruticilla ; AMRE), and Painted Bunting (Passerina ciris ; PABU). A total of 3326 genetic samples were available and distributed widely across the breeding and wintering ranges of these species (Figs. S1–S6).
Migratory connectivity was estimated using high-resolution genetic markers data compiled by the Bird Genoscape Project (www.birdgenoscape.org). Data and methodology for mapping migratory connectivity using genetic markers is described in detail in a set of species-specific papers (Bay et al. 2021; DeSaix et al. 2023a; Rueda-Hernández et al.2023; Ruegg et al. 2014, 2020, 2021, Bobowski et al. in review ). Briefly, for each species, DNA was sampled from individuals across the species’ breeding ground and RAD-seq (WIFL, YEWA, PABU and WIWA) or whole genome sequencing data (COYE and AMRE) were used for an initial estimate of population structure. For all species except AMRE, a subset of 96–158 SNPs were identified for each species that could accurately assign individuals to breeding population using population assignment tests (Moran & Anderson 2018). These subsets of SNPs were used to genotype additional birds from the breeding grounds as well as individuals sampled across the wintering grounds. For the whole genome sequencing, genome-wide SNPs were identified that could accurately assign individuals to breeding populations and the genotype likelihoods of these data were used for population assignment (DeSaix et al.2023b). Taking advantage of the population structure and isolation by distance signals on the breeding ground, we estimated breeding location for each wintering sample.
This procedure resulted in 26 genetically distinct breeding populations, whose seasonal distributions are shown in Figs. S1–S6. We removed populations with low quality data (i.e. with fewer than 10 sampled individuals, or fewer than 3 distinct locations for sampled individuals, or fewer than 2 occupied ecoregions during at least one season), thus removing 4 populations: YEWA East, COYE California, AMRE Mid-West and PABU Louisiana. Analyses were performed using the remaining 22 genetically distinct populations.