Alula size signals male condition and predicts reproductive performance in an Arctic‐breeding passerine

While studies of achromatic plumage signaling are scarce relative to chromatic ornaments, achromatic ornaments have the potential to act as an efficient form of visual communication due to the highly conspicuous contrast between black and white body regions. Recently, achromatic plumage reflectance has been shown to indicate condition, yet the condition‐dependence of achromatic patch size remains unstudied. Here we show the first evidence that alula size, an achromatic plumage patch, has the potential to signal a male’s condition and predict reproductive performance. In Arctic‐breeding snow buntings Plectrophenax nivalis , the size of the alula simultaneously predicted pre‐breeding physiological health and the number of offspring produced, through an intermediate variable (lay date). Snow buntings appear to pair assortatively; males and females arriving earlier pair together, and changes in body condition over the breeding season are positively related within pairs. We suggest that simple achromatic plumage patches, like alula size, have the potential to act as condition‐dependent signals. Consequently, females may benefit from assessing these signals to reliably evaluate a male’s condition and reproductive potential as a means of maximizing their reproductive success.