We analyzed phenotypic distribution, nesting success, and genetic purity of Golden-winged Warblers (Vermivora chrysoptera) and Blue-winged Warblers (V. pinus) in two ecologically distinct nesting habitats: early-succession uplands, and swamp forests. The proportion of phenotypically pure Golden-winged Warblers in swamp forests (94%) differed significantly from uplands (53%) as did the proportion of Golden-winged Warbler pairs (93%) in swamp forests compared to uplands (48%). Only 1% of the phenotypically pure Golden-winged Warblers in swamp forests paired with either a hybrid or a Blue-winged Warbler, but 7% of the phenotypically pure Golden-winged Warblers in uplands formed a hybrid pair. The probability of nesting success for Golden-winged Warbler pairs in swamp forests (65%) was significantly higher than in uplands (37%). Mitochondrial DNA analyses indicate that all 10 phenotypically pure Golden-winged Warblers sampled from swamp forests had the ancestral Golden-winged Warbler haplotype, while 10 of 25 Golden-winged Warblers from uplands had the ancestral Blue-winged Warbler haplotype (P = 0.033). Swamp forests may provide a source habitat for Golden-winged Warblers with a high phenotypic and genotypic purity even in sympatry with Blue-winged Warblers.