Wing patch size predicts assortative mating but not reproductive outcomes in the Daurian Redstart ( <i>Phoenicurus auroreus</i> )

Assortative mating, where individuals select partners based on phenotypic similarity or dissimilarity, is a common phenomenon in natural populations and plays an essential role in individual reproductive success. Although it has been studied for many years, how assortative mating influences fitness remains unclear. The Daurian Redstart Phoenicurus auroreus is a socially monogamous passerine species that exhibits pronounced sexual size dimorphism (SSD). Although both sexes possess white wing patches, males have significantly larger wing patch sizes (WPS) than females. In this study, based on 5‐year data collected from a Redstart population in northeastern China, we aimed to investigate: (1) whether WPS serves as a principal trait of assortative mating in Daurian Redstarts, i.e. whether individuals prefer mates with similar or dissimilar WPS; and (2) whether the SSD of white wing patches in Daurian Redstarts affects their reproductive outcomes. We found a significant positive correlation between WPS among paired Daurian Redstarts, i.e. individuals tend to select mates with WPS similar to their own. However, we found no significant association between WPS and clutch size, brood size or nestling body condition. This suggests that WPS‐based assortative mating has no effects on reproductive outcomes in Daurian Redstarts.