As the days begin to lengthen and winter recedes for yet another year, many birds dust off their most glamorous plumage to attract a mate. Males often opt for ostentatious displays while females tend to the dowdier end of the spectrum. But what factors regulate the dramatic differences in avian regalia between males and females? Willow Lindsay from the University of Gothenburg, Sweden, explains that the hormones that regulate sexual differences were thought to play a key role in defining plumage patterns, but the precise role of individual hormones was less clear. As testosterone starts flowing in spring, male red-backed fairy-wrens (Malurus melanocephalus) shed their dreary brown feathers in favour of a vivid scarlet and glossy black coat, while the females retain their dingy brown and white plumage. So, Lindsay and Douglas Barron headed south to Queensland, Australia, to find out how the plumage of red-backed fairy-wrens altered after they received a testosterone implant.
Initially, the males and females both looked drab; however, soon after the birds received the implant, the females moulted and began sprouting shorter male-like feathers. The females also produced a vibrant patch of red feathers on their backs, similar to the bright red streak on the males’ backs. In addition, the feathers on the crown, belly and breast of the females turned a shade of pinky-orange, which Lindsay said was: ‘Never previously documented on a red-backed fairy-wren’. Only one female went on to produce black feathers like those of the males, although the beaks of all of the females became darker. And when Lindsay and Barron injected some females with gonadotropin-releasing hormone, in a bid to naturally raise the birds’ testosterone levels, the females were unable to increase the levels of the hormone in their blood.
So, it seems that testosterone is the hormone that regulates the production of the carotenoid pigments that produce the males’ scarlet plumage; however, the hormone was unable to elicit production of the jet-black melanin pigment that produces their distinctive dark feathers. It seems that the female fairy-wrens still maintain the essential hormone mechanism that allows the males to put on their gaudy display, although the females are unable to produce sufficient testosterone to trigger the masculine transformation.