We introduce a phenotypic method to test for excess mortality in hybrids of the heterogametic sex, as expected from Haldane's rule, and apply this method to the unusually narrow hybrid zones between Hermit Warblers (Dendroica occidentalis) and Townsend's Warblers (D. townsendi) in the Pacific Northwest. Our test requires establishing comparable hybrid indices for male and female warblers. The hybrid index that we developed for females produced age-corrected distributions for phenotypically pure reference samples that closely matched those used by Rohwer and Wood (1998) for males. The similarity in these distributions enabled us to compare the relative frequency of males and females in hybrids and parentals. We detected no deficiency of hybrid females and thus no inviability in the heterogametic sex. Our failure to find evidence of the inviability component of Haldane's rule is not unexpected given the close relationship between these taxa; nonetheless, our methods should be generally useful for studies of hybrid zones.