One of the first hypotheses based on this was Bob Triver's Resource Accrual Hypothesis to explain large racks of antlers in males. House finches and red breast. Red coloration determined by good diet with lots of caratinoids. Males that are red are eating well. Doesn't work only in males.
Epigamic traits are common in both sexes in monogamous birds, in females with sex-reversal.
However, we sometimes see it as well in females in polygynous mating systems. Discuss recent baboon paper. Males prefer females with largest swellings. Lots of attention recently. Only males with the best genes will develop with symmetry.