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.

Selection may favor greater investment into sexual ornaments when opportunities for future reproduction are limited, for example, under high adult mortality. However, predation, a key driver of mortality, typically selects against elaborate sexual ornaments. Here, we examine the evolution of sexual ornaments in killifishes, which have marked contrasts in life-history strategy among species and inhabit environments that differ in accessibility to aquatic predators. We first assessed if the size of sexual ornaments (unpaired fins) influenced swimming performance. Second, we investigated whether the evolution of larger ornamental fins is driven primarily by the pace of life-history (investment into current vs. future reproduction) or habitat type (a proxy for predation risk). We found that larger fins negatively affected swimming performance. Further, males from species inhabiting ephemeral habitats, with lower predation risk, had larger fins and greater sexual dimorphism in fin size, compared to males from more accessible permanent habitats. We show that enlarged ornamental fins, which impair locomotion, evolve more frequently in environments that are less accessible to predators, without clear associations to life-history strategy. Our results provide a rare link between the evolution of sexual ornaments, effects on locomotion performance, and natural selection on ornament size potentially through habitat differences in predation risk.