Why do male (but not female) goldfinches have bright yellow
feathers?

A. The color is a warning to predators not to attack; males with
yellow feathers therefore survive well and pass on their alleles
for yellow feathers to the next generation.

B. The yellow chemicals help the males to fight off diseases and
parasites; males with yellow feathers therefore survive well and
pass on their alleles for yellow feathers to the next
generation.

C. The color helps males blend in with the background; most
predators perceive yellow and green to be the same, so males with
yellow feathers are camouflaged in the trees.

D. The color is produced because males have so much energy left
over from reproduction; if females had sufficient energy available,
they would also produce yellow feathers.

E. The color is attractive to females; males with yellow
feathers are likely to be chosen as mates and pass their alleles
for yellow feathers to the next generation.