Theory predicts that physiological costs of producing elaborate phenotypes assure the honesty of sexually selected traits. It is generally assumed that these physiological processes drive sexually selected displays. However, a recent study by Safran and colleagues demonstrates that the manipulation of plumage ornaments in barn swallows alters the temporal course of circulating androgens, thus rejecting the scenario of a static, unidirectional relationship between physiology and sexual displays. Instead, these results suggest that dynamic feedbacks between physiological, morphological and behavioural costs underlie the development and maintenance of sexually selected ornaments.