The experience you have looking at the way birds of paradise attract mates is the same experience an alien would have seeing the way humans dress.

It's a social construct believe it or not, the young male birds in that species and many similar ones learn the dances from watching adults and making their own adjustments, their own interpretations.

My own crazy speculation about what's being communicated in a bird of paradise dance is that it's more akin to artistic expression than "taking on the sexiest shape/colors." (Not to say art can't be that too)