I think that there is at least the potential of a book in this question.

I would further suggest that the majority of United States birds that are very colorful are migratory species, who spend their winter in the tropics. I think that if we made a subset of birds that are our resident birds that we could describe their colors as "muted." (I am trying to describe their colors in a non-pejorative way.)

My only stab at an answer at this intriguing question is to paraphrase one of David Snow's points in The Web of Adaptation (this is stretching back a fair ways so I may be inaccurate) that the food in the tropics is fairly plentiful and the birds were able to utilize their time for developing more and more complicated, fancy, and glittzy courtship patterns. This process also brought about more colorful feathers. Now of course as you mention, there are times when the male Scarlet Tanager could probably do without its bright colors, but in the competition to breed there is nothing like a little black against red.

Jim Rosso
Arlington, Va
jlrosso at aol.com
703-685-2426
http://birdcentral.net/