Blue Tit crown feathers reflect ultraviolet light. What does this signal? To find out, experimenters dulled the crowns of females tending nestlings with an oily compound. Males continued to guard the nest, but reduced their food-bearing trips. Apparently, females maintain attractiveness (and keep mates returning to the nest) through preening that retains the blue luster often muted by dust, pollution and parasites.
Interestingly, when delivering bottled milk to households was the norm, a number of Blue Tits began piercing the shiny foil cap and sipping. The behavior was picked up by other Blue Tits and spread from the U.K. throughout Europe. Attraction to UV and shiny foil suggests sexual selection on the one hand and social learning that might have led to cultural evolution on the other.