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.