Colorful traits can vary in response to neural control, hormone levels, reproductive state, or abiotic factors. In birds, colorful plumage traits are generally considered static ornaments that only vary irreversibly due to abrasion, bacterial degradation, or wear. However, in this work it is shown that iridescent feather color varies rapidly and reversibly in response to changes in ambient humidity. Based on optical models and sorption experiments, these changes appear to be caused by a swelling of the outer keratin cortex following water absorption. To our knowledge, this is the first study describing dynamic color changes in any keratinous biophotonic nanostructure.