Acknowledgements. The data for the analyses of this manuscript came from studies that were published previously. This compilation was not directly supported by any grant. The 47 years of field studies required the effort of a large number of dedicated and much appreciated field assistants. J Gillis, W MacCabe, L. Stenzler, T. Schat reviewed versions of the manuscript. The senior author thanks his spouse for 40 years of support during studies of Golden-winged and Blue-winged warblers. Contributions J. Confer initiated the effort to pool several studies in order to create a data base sufficient to analyze uncommon but significant events with hybrids. C. Porter was responsible for the analyses of reproductive isolation. D. McNeil was responsible for all illustrations and statistical analyses relating phenotype differences to frequency of mating. All authors contributed to the analyses, data interpretation, and manuscript writing. All authors contributed to the sometimes lengthy and vigorously debated revisions of the manuscript. All authors approved this version as submitted for review.