Changes in breeding status are associated with rapid bill darkening in male red-backed fairy-wrens Malurus melanocephalus

In many animals, an individual’s social environment and status can change quickly and unpredictably, suggesting an advantage for signals capable of responding to these changes. However, many signaling traits are fixed for relatively long periods. In birds, for example, most plumage-based signals are constrained by molt and unable to respond to sudden changes in social environment. Soft parts, conversely, are vascularized and may provide an alternative signaling system that can be updated relatively quickly. In this study, the effects of changes in breeding status are documented for a putative plumage-based signal, plumage brightness, and a putative soft part signal, bill darkness, in the red-backed fairywren Malurus melanocephalus. Males that switched from unpaired helper to paired breeder within a breeding season developed significantly darker bills within three weeks of the switch relative to males that did not change status. There was no clear corresponding change in plumage brightness, though the data suggests that changes in plumage brightness are at least possible. These results suggest that bill color is a socially mediated trait capable of responding rapidly to changes in breeding status, and it is proposed that the importance of soft parts as labile signals in birds may be currently under-appreciated.