When I was at Hawk Ridge on Wednesday, the banding station sent four Sharpies to the main overlook at once so we could compare immature and adult plumages and see how eye color changes over time. First, here's an immature. Notice how yellow his eyes are, and the vertical streaking on the breast.
The next birds are all adults, with horizontal, rustier streaking. Over time, the eyes get orange and then red. The progression isn't a precise year-by-year evolution, but as birds mature, the eyes get redder.
This bird appears to be the oldest and so the most attractive to the opposite sex.
Quite a few species show changes in plumage and/or eye color as birds age. Mockingbirds and Brown Thrashers acquire new songs throughout their lives, and the more songs a male sings, the older and more experienced he appears, making him increasingly desirable for females. These clues about age give birds of the opposite sex a better picture of just how old, and thus experienced, an individual is. In nature, the more mature a bird is, the more attractive it is to the opposite sex, because maturity = experience = greater likelihood of successfully nesting and rearing young.