Disruptive sexual selection for plumage coloration in a passerine bird

The theory of sexual selection was developed to explain the evolution of hi ghly exaggerated sexual ornaments(1). Now supported by vast empirical evide nce(2), sexual selection is generally considered to favour individuals with the most extreme trait expression(2-4). Here we describe disruptive select ion on a sexual ornament, plumage coloration, in yearling male lazuli bunti ngs (Passerina amoena). In habitats with limited good-quality nesting cover , the dullest and the brightest yearlings were more successful in obtaining high-quality territories, pairing with females and siring offspring, than yearlings with intermediate plumage. This pattern reflects the way that ter ritorial adult males vary levels of aggression to influence the structure o f their social neighbourhood. Adult males showed less aggression towards du ll yearlings than intermediate and bright ones, permitting the dull yearlin gs to settle on good territories nearby. Fitness comparisons based on pater nity analyses showed that both the adults and dull yearlings benefited gene tically from this arrangement, revealing a rare example of sexually selecte d male-male cooperation(5,6).