Abstract
Research on carotenoid as the pigments responsible for the brightest colouration of potentially sexually-selected traits in birds has been intense in the last years. For all animals, carotenoids must have a dietary origin as only photosynthetic organisms are able to synthesize them. The incorporation of carotenoids before final deposition in feathers depends on access to carotenoid-rich foods, foraging ability and specific abilities to absorb them through the gut. Carotenoids, none-the-less, have been assumed to be generally limiting in the environment. As a refinement of this potentially limiting status of the carotenoids, a trade-off between their ornamental properties and their physiological functions, particularly an anti-oxidant role, has also been assumed by most authors. Although this assumption mainly stems from medical literature and there is little actual evidence for this in wild birds, it is replicated in a majority of papers dealing with carotenoids in birds. It has been well established, particularly through experiments in captivity, that birds need to ingest a minimal amount of carotenoids to colour their plumage or other integumentary parts. However, demonstrations that carotenoids are utilized for strictly physiological functions are scant. Current evidence derives from captive-bird trials. As an example, female birds significantly reduce their circulating levels of carotenoids at a time when they form carotenoid-rich egg-yolks. The questions whether carotenoids are mobilized from storing organs or the blood during food shortages or in stressful situations, and how many carotenoids remain available in these situations remain contentious.