8 pages, 3 figures.-- PMID: 16191594 [PubMed].-- Printed version published on Sep 22, 2005.Several avian species show a bright carotenoid-based coloration during spring and following a period of duller coloration during the previous winter, despite carotenoids presumably being fully deposited in feathers during the autumn moult. Carotenoid-based breast feathers of male linnets (Carduelis cannabina) increased in hue (redness), saturation and brightness after exposing them to outdoor conditions from winter to spring. This represents the first experimental evidence showing that carotenoid-based plumage coloration may increase towards a colourful expression due to biotic or abiotic environmental factors acting directly on full-grown feathers when carotenoids may be fully functional. Sunlight ultraviolet (UV) irradiation was hypothesized to denature keratin and other proteins that might protect pigments from degradation by this and other environmental factors, suggesting that sunlight UV irradiation is a major factor in the colour increase from winter to spring. Feather proteins and other binding molecules, if existing in the follicles, may be linked to carotenoids since their deposition into feathers to protect colourful features of associated carotenoids during the non-breeding season when its main signalling function may be relaxed. Progress towards uncovering the significance of concealment and subsequent display of colour expression should consider the potential binding and protecting nature of feather proteins associated with carotenoids.Peer reviewed