Looking at ads in the newspaper, magazines, junk mail, and television, I notice that when the ads include adult people, the guys, especially young ones, haven’t shaved for four days, have a wrinkled shirt with an undershirt peeking out below it, crumpled pants, and untied tennis shoes or scuffed sneakers. Women, on the other hand, are dressed nicely and their makeup is impeccable. It all depends on what the sponsors are pitching, of course, but generally the women are appealing and the men rather plain and unkempt. In the bird world, it is mostly the opposite. The male, having the responsibility of attracting a female and defending a territory, tends to be distinct and splashy in his dress. Why else would a female mate with him and why would an intruding male be hesitant to, ah, intrude?
Female birds, on the other hand, tend to be dull and often cryptically colored because they tend to the nest and want to remain inconspicuous.
Now, I don’t know what a female human finds attractive in a male (it’s always been a mystery to us guys), but female birds aren’t just attracted simply to the male’s looks. In some birds, like the House Finch, the deeper the (red) color, the more appealing the male because he is clearly good at finding red berries, an important diet item. Red-winged and Yellow-headed Blackbirds set up territories in marshes in the spring before the females arrive from their winter quarters. See this. The better the territory, the more attractive the male is to the females. In this case, the more aggressive male gets the most lady birds. Bower Birds build a U-shaped nest (a bower) and collect colored objects; the best collector gets the gal. See this. Male manikins and birds-of- paradise, although spectacularly colored, are judged more by their courtship display and calls. See an incredible video here.
Then what about the birds with no sexual dimorphism where the sexes look alike? In that case, the male’s behavior and ability to sing come to the fore. I don’t think we have any human comparisons here as you can almost always tell men from women regardless of their behavior and both can sing (except in my case.) Of course, there are those senior citizens who dress very similarly; I guess after many years of marriage a couple tends to meld their fashion tastes. ( I don’t know why some people look like their dogs, though.)
What it’s all about is reproduction. Bird species have evolved to replicate themselves in the form of offspring in the most efficient manner possible, as all organisms have. So you don’t see birds with their head crests on backwards and pants exposing their paisley underwear and buttocks with one wing trying to hold them up.