One of the great dilemmas of nature is why some bird species display vivid or bold colors. Take the Summer Tanager for example. While the female is a soft yellow (as shown here by the superb photo recently recorded by David from Cary, NC using his Wingscapes BirdCam), the male is a vivid red.
Ornithologists tell us that producing such red plumage is physically taxing on the birds. Not only does it require more energy to do so, it also makes the bird more visible to predators; however red also indicates the male's good health and thus likely attracts females.