Carotenoids cannot be synthesised by vertebrates and must be derived from the diet. A growing amount of research has focussed on the role of carotenoids in immune function, as well as the signalling function of carotenoids in the colour of the integument (skin, bill and legs) and feathers. We used an experimental approach to examine the effects of experimental reduction and supplementation of dietary carotenoids on phytohaemagglutinin (PHA) response (as an estimate of cell-mediated immune response) and colour of the bill in the Diamond Firetail (Stagonopleura guttata). To demonstrate the biological relevance of our study, we first show that carotenoid concentrations of our captive experimental birds did not differ significantly from those of wild birds. We found that carotenoids circulating in the plasma of captive Diamond Firetails dropped to 50% of the original concentration in less than 5 days when on a carotenoid-free diet. The mean level of total carotenoids in the plasma stabilised at 6% of the original concentration after 26 days. Finally, Diamond Firetails showed a significant increase in PHA response and increased bill-hue wavelength while on either a lutein or a zeaxanthin supplemented diet. Our results show rapid signalling of carotenoid concentration in the integument (as measured here in the bill) that we suggest complements the longer term signalling potential of feathers.