White eye patches of female wood ducks, Aix sponsa, vary markedly in size and may reflect individual status or condition

Abstract

Research into plumage ornaments and plumage signals of individual state has focused primarily on males, while variation among females has received far less attention. White patches of plumage around the eye (eye patches) are a female-specific plumage trait in the wood duck, Aix sponsa, a sexually dimorphic species of cavity-nesting waterfowl in North America. We observed considerable variation in the size of these patches among females and sought to evaluate whether this trait might indicate individual quality (‘quality cost hypothesis’) or the influence of stress or past investment (‘stress indicator hypothesis’). We measured the area of female eye patches in five populations of wood ducks during three breeding seasons and examined whether eye patch area varied in relation to age, nest success, future breeding, activity and/or body condition. Eye patch area was highly variable among individuals but repeatable within individuals, varied across years and study sites, correlated strongly with age but weakly with an index of body condition. For individuals greater than 4 years of age, eye patch area was negatively related to the likelihood of returning to breed in the following year. Eye patch size was positively correlated with nest initiation date and this was more pronounced in older birds, but there was no correlation of eye patch area with clutch size or hatch success. Our results suggest that this unique female-specific plumage trait reflects individual age and future reproduction potential. Larger eye patches in older females may be an indicator of senescence or terminal investment, and we hypothesize that eye patch size could act as a signal of individual state or social status in this species.