Data analysis
We tested for an effect of sex on plumage coloration characters using
generalized linear models (GLM) in the R package lme4 (Bates et al.,
2015). We included the hybrid index score as a fixed factor in the
model. In particular, we tested the extent to which differences in
plumage scores for forecrown, supercilium, and wing bar, as well as the
calculated hue and chroma from spectrophotometry of forecrown feathers,
were associated with sex. False discovery rates were calculated to
account for the five comparisons (Benjamini & Hochberg, 1995). We also
tested for the direction of asymmetric backcrossing by comparing the
hybrid index of the whole genome and the Z chromosome for individuals
with a hybrid index between 0.2 and 0.8 using a paired t-test in R. All
analyses were performed in R version 3.6.2 (R Core R Core Team, 2019).