Male American Goldfinches have finished molting into their screaming yellow summer suits and sharp black caps and soon will be selected by the more prudently-dressed females of their species.
Evolution has dictated that the brighter the male’s yellow, the healthier and more robust he is. And, the female Goldfinches instinctively know that – male color brightness is a major criterion for them when they choose a mate, according to researchers. Here's a female:
As for the new black caps, they’re used to scare off other males – studies show that the blacker and longer the cap feathers are when raised in anger, the more dominant the bird.