RELATIONSHIP BETWEEN PLUMAGE DIVERGENCE AND PAIRING FREQUENCY

Of 849 paired, male Golden-winged Warblers, 4.9% (n = 42) paired with a female Brewster’s Warbler (Table 4). This backcross frequency is 5.4 times greater than the rate of primary hybridization (χ2 = 24.29, p < 0.0001). For the 288 paired, male Blue-winged Warblers, 1.7% (n =5) paired with a Brewster’s Warbler. Backcross frequency by male Blue-winged Warblers was less than but not significantly different from their frequency of primary hybridization (χ2 = 3.48, p = 0.062). The frequency of backcrossing by male Golden-winged Warblers with Brewster’s Warblers was 2.9 times greater than the rate for male Blue-winged Warblers (χ2 = 4.81, p = 0.028). Sample sizes for Lawrence’s Warblers were too small for statistical analyses.
Of 834 paired, female Golden-winged Warblers, 4% (n = 33) formed a backcross with a male Brewster’s Warbler (Table 5), which is three times the rate of primary hybridization (χ2 = 16.90,p = 0.012). For the 309 paired, female Blue-winged Warblers, 3.6% (n =11) formed a social pair with a Brewster’s Warbler. Backcross frequency by female Blue-winged Warblers was not significantly different than their frequency of primary hybridization (χ2 = 0.888, p > 0.10). The frequency of backcrossing by female Golden-winged Warblers with Brewster’s Warblers was not different than the rate for female Blue-winged Warblers (χ2 = 0.0001, p> 0.10). Sample sizes for Lawrence’s Warblers were too small for statistical analyses. From the pooled social pairing data (Table 6), there was a significant, negative relationship between the extent of plumage divergence between males and females and their frequency of pairing corrected for random expectations (Fig. 3;observed number of pairs – expected number of pairs = 141.72 - 7.28*plumage divergence ; df = 7, r 2 = 0.66, p = 0.014).