Function of breeding plumage in a monogamous tropical fairy-wren

Conspicuous or colourful plumage ornaments are important sexual signals in male birds. Sexual selection theories assume that such ornaments improve reproductive success of the bearer, whereby males with more elaborate or extreme ornaments are more successful in obtaining mates or warding off rivals. Because production and maintenance of ornaments are costly, they are generally assumed to indicate the quality of an individual, as a competitor or potential mate. For seasonal plumage ornaments, where individuals alternate between colourful breeding plumage and dull non-breeding plumage each year, the nature and magnitude of benefits and costs are not well understood, particularly for species that form monogamous, year-round and long-term partnerships. In many such species, both sexes will develop conspicuous ornaments, challenging current sexual selection theories.