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Many vertebrates use carotenoid‐based signals in social or sexual interactions. Honest signalling via carotenoids implies some limitation of carotenoid‐based colour expression among phenotypes in the wild, and at least five limiting proximate mechanisms have been hypothesized. Limitation may arise by carotenoid‐availability, genetic constraints, body condition, parasites, or detrimental effects of carotenoids. An understanding of the relative importance of the five mechanisms is relevant in the context of natural and sexual selection acting on signal evolution. In an experimental field study with carotenoid supplementation, simultaneous cross‐fostering, manipulation of brood size and ectoparasite load, we investigated the relative importance of these mechanisms for the variation in carotenoid‐based coloration of nestling great tits (Parus major). Carotenoid‐based plumage coloration was significantly related …