Factors that determine the condition of migratory birds at their wintering sites are poorly known. Age, sex, and morphological characteristics of birds may have an influence on their winter condition by affecting their foraging and competitive abilities. Winter body condition could have long-term consequences on the reproductive success of migratory birds during the subsequent breeding season. Using 3 yr of data from Collared Flycatchers (Ficedula albicollis), we examined the characteristics of winter-grown tail feathers, as indicators of winter body condition, in relation to sex, age, morphological traits, and future breeding variables. Tail feather mass was highly repeatable between years, but feather growth rate was not repeatable, which suggests that the latter trait mainly indicates environmental circumstances during molt, whereas feather mass may more strongly reflect genetic effects. Tail feathers of males and adults showed better quality than those of females and juveniles, possibly because of differences between age classes and sexes in individual quality and foraging skills or because of winter habitat segregation. Birds with longer wings produced better-quality tail feathers, which suggests that wing and tail feather characteristics are similarly affected, presumably by individual genetic quality. Smaller Collared Flycatchers grew their tail feathers faster during the winter molt, possibly because they had better foraging ability due to better flight maneuverability. Tail feather quality showed no relationship with laying date; however, females that had produced heavier tail feathers during winter laid larger clutches during the following breeding season, which suggests that tail feather mass potentially reflects intrinsic individual quality.