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. If you can't identify the best genes, at least you can identify genes that are compatible with your own. MHC genes influence immune recognition by coding cell-surface glycoproteins. Up to 60 alleles and show extreme heterozygosity.