Male Red-backed Fairywrens appear to enhance a plumage-based signal via adventitious molt

Phenotypically plastic signals that can be altered in response to changing environmental conditions provide animals with the ability to dynamically signal their current condition or status. Such flexibility might also provide a means of avoiding potential trade-offs between signal components. Among birds, for example, both the timing of expression and the coloration of nuptial plumage are often thought to be honest signals of condition. However, because plumage is a relatively static signal type, birds that express condition-dependent plumage signals may face a trade-off between timing of signal production and signal quality, in that signals produced relatively early may be of lower quality because of seasonal constraints. A related cost may be increased fading or wear of plumage associated with extended duration of signal expression. Male Red-backed Fairywrens (Malurus melanocephalus) exhibit asynchronous development of nuptial red-black plumage, with some individuals molting into nuptial plumage months earlier than others. We report that male Red-backed Fairywrens that molt into nuptial red-black plumage early during the nonbreeding season appear to increase their plumage coloration by replacing feathers outside of normal molt periods (i.e. adventitious molt). In this way, some male Red-backed Fairywrens may be able to molt into nuptial plumage in the nonbreeding season, which is likely to increase access to mates or resources, and to subsequently enhance the red hue of a plumage-based sexual signal to a putatively more attractive state. We suggest that adventitious molt may be a currently underappreciated mechanism that birds use to improve or maintain the quality of plumage-based signals over time, between periodic full-body molts.