The meaning of purely structural colour: white plumage reflectance indicates feather condition

White plumage areas are widespread among birds but the mechanisms generating individual differences in white feather reflectance are poorly known. Additionally, the proximate mechanisms of within‐season feather colour changes in general are also poorly known. Here we explored within‐individual changes in macrostructure and reflectance of the white wing‐patch of female Collared Flycatchers Ficedula albicollis using data from three stages within a breeding season. Changes in brightness and UV chroma were related to changes in the neatness and size of reflective surfaces, respectively. This suggests that white feather reflectance traits may act as indicators of feather intactness and preening activity.