Imagine a community where you could guess a person's intelligence by glancing at a yellow stripe on their shoulder.

Welcome to the society of Siskins.

Many birds use brightly-colored plumage as an index of reproductive fitness, but these sergeant stripes may have an extra meaning.  Experiments with Eurasian Siskins have shown birds with longer yellow stripes are better at solving puzzles to obtain seeds protected by thickets of toothpicks. The smartest birds had the longest stripes -- even after allowing for the bird's size and social standing.

Apparently the female siskin has come to recognize that the yellow stripe indicates superior qualities, and is reinforcing the trait with her choice of mates.  But why haven't dumb birds evolved that mimic the successful flag?  Perhaps mimic birds who haven't "earned their stripes" are are shunned altogether by mates. It could be that mimics exist at a certain low frequency -- just as certain non-poisonous snakes gain a little advantage by imitating their poisonous cousins. Alternatively, it could be that imitators haven't had time to catch up, and the yellow stripes may be a temporary signal that will be replaced by a new one in future decades.  Yet -- for now -- the two characteristics of yellow stripes and high intellect have been pushed together by the force of female choice, forging a powerful evolutionary tool for honing the intelligence of the species.  But that raises a new question -- why don't humans females respond in a similar way to horn-rim glasses and pocket-protectors?