BEHAVIORAL ISOLATION

Across all studies, primary hybridization occurred with 0.9% (n= 14) of the 1,680 paired Golden-winged Warblers and 2.4% (n = 14) of the 583 paired Blue-winged Warblers, substantially less primary hybridization than expected by chance for either species (Table 2; Chi-square with Yates correction: χ2 = 7.47, p= 0.006). Collectively, 1.2% of the birds with a pure phenotype in these sympatric populations paired with the alternative pure phenotype (Table 2). These patterns reflect high levels of behavioral isolation across all studies, which ranged from 0.864 to 1 (mean: 0.958; lower 95% CI: 0.917; upper 95% CI: 0.983). See Fig. 2. We found no evidence for a relationship between the strength of behavioral isolation and the difference in relative abundance of Golden-winged and Blue-winged warblers across sites (r 2 = 0.03, df = 7,P = 0.67), suggesting that the strength of behavioral isolation (the observed rate of interbreeding, corrected for random expectations) is unaffected by variation in local relative abundance of the two lineages.