Divergent ornamentation within a single population of the barn swallow

Differential migration strategies favour different sets of characteristics, including sexually selected ornamentation. Such phenotypic variation is particularly evident in a population with partial migration, where migrants and nonmigrants co-exist. Partial migration provides insights into the link among migration, local environment, and ornamentation, although empirical studies remain scarce. Here, we studied the plumage traits of barn swallows (Hirundo rustica) in southern Japan, where both winterings and migrants breed sympatrically. We further examined this relationship with multiple isotopes (δ2H, δ13C, δ15N, and δ34S), which provides insight into their moulting habitat. Among males, winterings and migrants differed in their morphological traits: wintering males had shorter wings, which suggests the high demand for flight apparatus in migratory birds. Moreover, wintering males had larger white tail spots and less colourful throat patches than migratory males, indicating ornament divergence between them. Wintering males had a significantly smaller isotopic space when examining the combinations of δ34S with the other isotopes compared to migratory males, which indicates a differential geographic range between them, perhaps because of the limited variation in the distance to the sea in wintering males. As in males, wintering females had a significantly smaller isotopic space than migrant females, but there were few morphological differences between migratory and wintering females. Instead, some morphological traits were related to isotope values in females. These results indicate sex-specific linkage among migration, local environment, and ornamentation.