Testosterone treatment of female Superb Fairy‐wrens Malurus cyaneus induces a male‐like prenuptial moult, but no coloured plumage

When selection strongly favours a testosterone‐dependent trait in males, and this trait is not beneficial to females, a correlated response to selection in females, which also circulate some testosterone, could slow the rate of evolution in males. Here I investigate whether experimental testosterone treatment in female Superb Fairy‐wrens Malurus cyaneus can induce the testosterone‐dependent sexually selected moult into nuptial plumage, as it does in males. Silastic testosterone implants in females rapidly induced a moult akin to the male prenuptial moult, involving all body areas that are colourful in nuptial males (head, upper back, ear tufts, breast). Moreover, the newly moulted feathers had a similar glistening appearance and morphology as male nuptial feathers. However, the treatment failed to induce production of the blue and black structural colours, and the breast and ear tufts were lighter than the normal grey‐brown feathers of females and males in eclipse plumage. Microscopic and ultrastructural analyses of these unusual feathers could further our understanding of structural colour production.