I've read that the male bird coloration is a function of what Darwin called "sexual selection." The females of a particular species prefer the males with colors or tails or head tufts or whatever, and the males that are preferred father the (similarly outfitted) offspring. In many cases, there's a tradeoff between what the females want and what predators find makes their life easier. E.g., peacocks' tails make them more vulnerable, cardinals' brilliant red makes it more visible, etc. There's a point of diminishing returns, I guess. But it's pretty well established that the females want the features. E.g., if you cut off 15% of a peacock's tail feather "eyes", that bird gets no peahens.
Turns out that the birds (or fish or bucks or whatever) with the desireable features also tend to be healthiest, so the females' choices in decor are reinforced with good providers and healthier offspring.
Turns out that the birds (or fish or bucks or whatever) with the desireable features also tend to be healthiest, so the females' choices in decor are reinforced with good providers and healthier offspring.

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