Conditions during development, including nutrition and immune challenges, can have long-lasting effects on subsequent physiological parameters. Many of these organizational effects are mediated by hormones, including corticosterone (CORT), a hormone involved in nutrient mobilization, immune response modulation, and responses to stressors. While the effects of exogenous CORT during development on later-life physiology have demonstrated the important role this hormone plays in developmental plasticity, relatively less attention has been paid to other factors capable of driving a CORT-mediated cascade of phenotypic effects. We tested the effect of nonpathogenic immune challenges during different stages of development and used levels of CORT in feathers (CORT_f) to investigate whether CORT levels during development (when the feathers were grown) could predict variation in traits at adulthood. Mallards (Anas platyrhynchos) exhibit a variety of traits as adults that could be influenced by CORT, including a carotenoid-pigmented beak used in mate choice and melanin-based patterning in the speculum of the wing that has no known signaling function. We found that nonpathogenic immune challenges during development had no detectable effects on duckling CORT_f, which suggests that more robust immune challenges are required to create a sustained increase in circulating CORT levels. However, duckling CORT_f predicted multiple traits at adulthood: higher levels of CORT_f were associated with increases in circulating carotenoid levels, degree of carotenoid-based beak coloration, and mass gain. We failed to demonstrate a link between melanin-based speculum patterning and CORT at time of trait development, although speculum patterning was both affected by immune challenge treatment and correlated with adult immune function. These results support the “quality hypothesis,” which states that variation in CORT could be positively correlated with traits reflecting individual quality under certain conditions, and also suggest that the speculum in Mallards may be able to serve as an honest signal.