Isotopic evidence of arthropod-based diet shaping feather ornamentation in saffron finches ( Sicalis flaveola , Linnaeus 1766)

Dietary variation at the individual level plays a fundamental role in carotenoid-based ornamental plumage coloration, which often influences sexual and social signaling. This study investigated how dietary differences shape the coloration of two ornamental feather types (breast and forehead) in males of the saffron finch. This small neotropical bird is considered granivorous, although it frequently incorporates arthropods into its diet. We used reflectance-based color measurements and carbon and nitrogen stable isotopes (δ¹³C, δ¹⁵N) to infer dietary patterns and their association with color variation. A total of 29 males were sampled across seven field campaigns conducted between January 2017 and March 2018 in a farm located in the Central Brazilian savannas. δ¹³C values showed no association with breast or forehead coloration, indicating that variation in carbon sources was not a major driver of color expression. In contrast, δ¹⁵N values were significantly associated with hue variation in both feather types. Higher δ¹⁵N values corresponded to a yellower-shifted hue in breast feathers and red-shifted hues in forehead feathers. Taken together, these findings suggest that saffron finches may selectively forage on high-trophic level prey that are relatively rich in carotenoids and associated with improved nutritional conditions. This dietary variation may be linked to differences in carotenoid processing and allocation across feather patches, contributing to different color phenotypes. In addition, such diets are associated with the production of high-quality feathers and promote more elaborate ornamental coloration. Although δ¹⁵N values may also be influenced by baseline variation in nitrogen sources, the observed patterns are consistent with trophic differences and highlight the role of diet in shaping plumage coloration. Our results highlight the importance of individual dietary strategies in shaping carotenoid-based color phenotypes through both direct (carotenoid acquisition) and indirect (nutritional condition) pathways, with distinct effects across different ornamental feather regions.