Sex determination and differences in plumage coloration
Using PCR and bioinformatics approaches, we identified 44 males and 35 females across the contact zone. The bioinformatics approach involved comparing the mean depth on 1532 SNPs on the Z chromosome with 103,401 other SNPs for each individual. We expected comparatively higher Z chromosome to autosome depth ratios for males compared to females, and we indeed found a clear separation. Male Z:autosome ratios ranged from 0.83 – 1.19, while female ratios ranged from 0.43 – 0.67 (see supporting information). The results of the bioinformatics method corresponded 100% with the PCR and gel electrophoresis method of Fridolfsson and Ellegren (1999).
The GLMs found no differences between the sexes in either forecrown hue or chroma, nor in forecrown, supercilium, or wing bar scores. Hybrid index, on the other hand, was significantly related to all colour measures for crown colour (Table 2) and other plumage patches (Table S1), confirming species differences in those traits.