We present field observations and genetic evidence of interspecific mate-selection and nesting resulting in successful hybridization between the Warbling Vireo (Vireo gilvus) and the Red-eyed Vireo (V. olivaceus). Two interspecific nesting pairs were found in western North America, each of which produced two fledglings. We examined the DNA of a salvaged fledgling from the nest of a mixed-species pair in Reno, Nevada, using mitochondrial and nuclear DNA analyses. The mitochondrial DNA of this individual indicated a Red-eyed Vireo mother, while its nuclear DNA was heterozygous and consistent with a hybrid origin. Phenotypic traits of the fledglings from the nest of a second mixed-species pair near Juneau, Alaska supported heterospecific parents for those offspring as well. We discuss these findings in the context of reproductive barriers that appear to result in little or no admixture across broad areas of sympatry in these two species.