Diet shapes color plumage in Saffron Finches

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. 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.